CANNIBALISM, FOOD LIMITATION, INTRASPECIFIC ...

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Cannibalism, food limitation, intraspecific competition,

and the regulation of spider populations

Article in Annual Review of Entomology · February 2006

DOI: 10.1146/annurev.ento.51.110104.150947 · Source: PubMed

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2 Nov 2005 13:14 AR ANRV263-EN51-19.tex XMLPublishSM(2004/02/24) P1: KUV10.1146/annurev.ento.51.110104.150947

Annu. Rev. Entomol. 2006. 51:441–65doi: 10.1146/annurev.ento.51.110104.150947

Copyright c© 2006 by Annual Reviews. All rights reservedFirst published online as a Review in Advance on September 22, 2005

CANNIBALISM, FOOD LIMITATION,INTRASPECIFIC COMPETITION, AND

THE REGULATION OF SPIDER POPULATIONS

David H. WiseDepartment of Entomology, University of Kentucky, Lexington, Kentucky 40546-0091;email: [email protected]

Key Words Araneae, intraguild predation, population regulation, trophic cascades

■ Abstract Cannibalism among generalist predators has implications for the dy-namics of terrestrial food webs. Spiders are common, ubiquitous arthropod generalistpredators in most natural and managed terrestrial ecosystems. Thus, the relationshipof spider cannibalism to food limitation, competition, and population regulation hasdirect bearing on basic ecological theory and applications such as biological control.This review first briefly treats the different types of spider cannibalism and then focusesin more depth on evidence relating cannibalism to population dynamics and food webinteractions to address the following questions: Is cannibalism in spiders a foragingstrategy that helps to overcome the effects of a limited supply of calories and/or nu-trients? Does cannibalism in spiders reduce competition for prey? Is cannibalism asignificant density-dependent factor in spider population dynamics? Does cannibalismdampen spider-initiated trophic cascades?

INTRODUCTION

Terrestrial food webs have a high diversity of generalist predators (32, 102, 105),among which spiders are abundant and ubiquitous (143). Cannibalism occurs in awide range of generalist predators (43, 100) and is perceived by many researchersto be common among spiders. Types of cannibalism can be classified accordingto the life-history stages and relatedness of cannibal and victim. Among spiderssexual cannibalism has been the most extensively studied, yet Elgar (31) pointsout that among invertebrates “. . .sexual cannibalism is surprisingly uncommonrelative to the other types of cannibalism.” The current review focuses on the less-studied types of cannibalism in spiders. This emphasis is not meant to suggesta clear dichotomy, because some of the selective pressures molding both sexualand nonsexual cannibalism are similar. Thus, aspects of sexual cannibalism arereviewed briefly. Emphasizing research findings on nonsexual forms of spidercannibalism hopefully places in perspective what is currently known of the roles

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442 WISE

that cannibalism plays in the ecology of a major generalist predator, with a primarygoal of identifying research questions deserving future scrutiny.

Spiders are frequently generalized as highly cannibalistic, which could meanthat most spider species exhibit cannibalism and/or that killing and eating con-specifics occurs frequently in those species that are cannibalistic. Given the gapsin our knowledge about the frequency of cannibalism in nature, generalizationsabout rampant spider cannibalism are sometimes too glibly stated, but it is truethat spiders and other arachnids display a wide range of cannibalistic behaviors.Elgar & Crespi (32) list five types based upon the life-history stage of cannibal andvictim: adults cannibalizing adults, adults cannibalizing juveniles, juveniles can-nibalizing juveniles, adults cannibalizing eggs, and juveniles cannibalizing eggs.Among invertebrates only arachnids exhibit all five types, and all five occur in spi-ders (32). However, it does not follow that cannibalism occurs frequently in mostspider species. Sexual cannibalism has been documented in many spider families;however, Elgar (31) points out that we have scant data on the actual frequency ofsexual cannibalism in natural populations. It appears that sexual cannibalism isrelatively rare among the therophosids (22, 61). Jackson (59) suggests that sex-ual cannibalism may not be as widespread in spiders as commonly believed andthat many of the courtship behaviors attributed to the avoidance of sexual can-nibalism have alternative explanations. Elgar (31), however, suggests that sexualcannibalism in spiders is widespread. Relatively few investigators have attemptedto quantify the frequency of sexual cannibalism among spiders in nature.

We do not have a good idea of the extent to which nonsexual cannibalism is amajor spider foraging behavior. Predation by spiders on other spider species hasbeen documented for numerous families in both nature and laboratory (143), andauthors (30, 41, 44) frequently echo Bristowe’s (14) assessment that puts “spiderseasily at the top of the list of spider enemies.” Even if it is true, widespread predationby spiders on other spider species does not necessarily implicate cannibalismas a major factor in spider ecology. Furthermore, it does not necessarily followthat cannibalism is frequent in nature if conspecific individuals confined in thelaboratory kill and eat each other. A review of the evidence indicates that, despitethe absence of extensive data on the frequency of cannibalism in many families innature, available data from field surveys, from accumulating evidence from fieldexperiments, and from laboratory studies in which habitat structure and densitiesof conspecifics and alternative prey are similar to natural conditions suggest thatnonsexual cannibalism may be a significant foraging behavior in some spiderfamilies, especially the families of wandering spiders, and among them, the wolfspiders (Lycosidae) in particular.

THEORETICAL CONSIDERATIONS

Cannibalism and Intraguild Predation

The ecological roles of spider cannibalism in population and food web dynamicsare understood best when compared with intraguild predation (IGP) because both

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CANNIBALISM IN SPIDER POPULATIONS 443

involve generalist predators feeding on other predators that share one or more non-predacious prey species. Such a comparison also highlights how costs and benefitsinteract in shaping the evolution of cannibalistic behaviors in spiders. Polis (101)defined IGP as “the killing and eating of other species that use the same resourcesand are therefore potential exploitative competitors.” Many authors view cannibal-ism as a form of IGP, but the definition proposed by Polis excludes cannibalism.Separating IGP and cannibalism recognizes a crucial difference between the twobehaviors, i.e., that costs of the two can be different because cannibalism can incurdirect genetic costs that are impossible in IGP. On the other hand, their bene-fits overlap extensively, including the advantage gained by eliminating a potentialcompetitor, an indirect effect that should be even greater for cannibalism than forIGP. The benefits ascribed to cannibalism are acquisition of a high-quality foodsource and elimination of a potential exploitative competitor. The possible costsare injury or death, contracting pathogens or parasites, lowering of inclusive fitnessby killing relatives, and loss of sperm.

Asymmetries, Ecological Factors,and Population Consequences

One can make several generalizations about factors that should influence rates ofcannibalism (28, 32, 43, 100). Certain asymmetries influence the risk involved:Cannibals should prey on smaller conspecifics in order to avoid the risk of retalia-tion; they should avoid killing kin; and they should avoid killing a mate unless theenergy or nutrients acquired outweigh the cost of losing sperm. Because cannibal-ism is most frequently a predator-prey interaction, its frequency should respondto the ecological factors of cannibal density, alternative prey (both abundance andrelative food quality), and habitat structure, all of which can act separately ortogether to determine the frequencies of encounters with potential cannibals orpotential prey (conspecific and heterospecific), the spectrum of foraging choices,hunger level, and the strength of exploitative competition. Many of these generalattributes lead to the prediction that certain types of cannibalism should act as astrong density-dependent factor regulating population density, which has conse-quences for spider-initiated trophic cascades.

As a way to organize this review, I first discuss findings for several types ofcannibalism between nonsolitary and related spiders (two or more spiders thatare together because of behaviors separate from cannibalism or because they arerelated genetically): sexual cannibalism, sibling cannibalism, filial cannibalism,matriphagy, and cannibalism in social groups. Cannibalism between solitary spi-ders can be viewed more directly as foraging for prey (cannibalistic foraging) oras an agonistic interaction (interference cannibalism and cannibalistic territorial-ity). Effects of size asymmetries, hunger levels, and habitat features are discussedprimarily under cannibalism among solitary spiders, although these factors alsocan influence cannibalism between nonsolitary and related spiders. Thus, as insolitary spiders, cannibalism between nonsolitary spiders also can be explainedas an adaptation to food limitation. It is convenient, however, to separate types

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444 WISE

of cannibalism into these two broad categories because relatedness (genetic orotherwise) between cannibal and potential victim adds a unique disadvantage tothe cost/benefit ledger.

CANNIBALISM AMONG NONSOLITARYAND RELATED SPIDERS

Sexual Cannibalism

Sexual cannibalism is the killing and eating of a courting, copulating, or post-mating male by the female. Rarely do courting males cannibalize the female. Thefemale spider is almost always the cannibal, sometimes killing her potential matebefore copulating (31). The evolution of precopulatory sexual cannibalism can beexplained most directly in terms of the relative value of the male as food versushis current worth as a sperm donor (92). Postcopulatory cannibalism could beexplained as male investment in the young (18), yet males often attempt to escapebefore being cannibalized, so this cannot be a universal explanation. An excellentexample of the complexity of the problem is the highly stereotyped behavior of theAustralian redback male (Latrodectus hasselti), for which research has revealedlikely selective advantages to both sexes (2–5). A different theory, not based uponsexual selection and parental care, postulates that the killing of courting malesbefore copulation is the result of “aggressive spillover,” i.e., that selection amongjuveniles for aggressive foraging behavior due to severe food limitation has se-lected for genes that favor indiscriminate foraging behavior in the female, eventhough female fecundity supposedly is not limited by foraging as an adult (7). Themost problematic feature of this theory is the postulation that severely food-limitedjuvenile females suddenly escape food limitation once they have matured, eventhough energy demands should increase, not decrease, because of the need to pro-vide yolk to the eggs (41). Recent studies fail to support the basic assumptions andpredictions of the aggressive spillover hypothesis for fishing spiders in the genusDolomedes, the spider for which the theory was originally proposed (62, 63, 69).Most studies of sexual cannibalism indicate that the female’s interests are almostinvariably the acquisition of calories or nutrients; nevertheless, it is reasonable toseparate the treatment of other types of spider cannibalism from that of sexualcannibalism, for which the selective factors involved are related more to problemsof parental investment than to the environmental conditions favoring nonsexualcannibalism (18). The reader is directed to the original seminal reviews of sexualcannibalism in spiders (18, 31, 92) and recent research publications on the topicthat contain comprehensive reviews of the literature (3, 5, 22, 42, 84, 89, 98, 120).

Sibling Cannibalism

Because of the possible genetic costs of eating close relatives, natural selectionshould favor preferential cannibalism of non-kin, unless the gain in energy or

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nutrients is substantial. The evolution of kin discrimination in solitary spidersshould be strongest among the youngest instars before they have finished dispers-ing and thus have a high probability of contacting each other. Cannibalism amongrecently dispersed (second instar) Hogna helluo (Lycosidae) wolf spiderlings washigher among pairs of non-kin than among siblings (111). That size differenceswere greater between than within broods could explain the result, but even if can-nibalism rates differed solely because of size differences related to kinship and notbecause of direct recognition of kin, the potential for discrimination exists withinthe population because rates were lower among siblings (i.e., there is the poten-tial for differential treatment of relatives independently of whether the individualrecognizes the degree of genetic relatedness; see Reference 139 for the distinctionbetween “kin discrimination” and “kin recognition”). A study with another speciessuggests that when kin discrimination in rates of cannibalism among wolf spidersdoes occur, it is due to size asymmetries: Cannibalism rates did not differ betweenpairs of siblings and non-kin Pardosa pseudoannulata spiderlings if the degree ofsize asymmetry was similar in both groups, but size asymmetry itself did have alarge effect (58). Iida (58) argues that kin discrimination occurs within the first 24 hof dispersal owing to differing coefficients of variation in spiderling size withinand between broods. The results of Hvam et al. (57) support the hypothesis thatkin discrimination among newly emerged lycosid spiderlings is most likely dueto differences in size asymmetries. They paired sibling and non-sibling Pardosaamentata spiderlings that differed in weight by <1% and found no evidence thatkin recognition (i.e., the ability to respond to cues indicating genetic relatedness)affected the rate of cannibalism.

Avoidance of cannibalism among siblings of nonsocial spiders might occur mostoften in species in which siblings remain in close contact when they are feedingand growing, i.e., among the subsocial solitary spiders that show extended mater-nal care. For example, the rate of cannibalism was much lower in experimentalgroupings of spiderlings of a subsocial burrowing wolf spider than in groupings ofspiderlings from nonburrowing, more vagile lycosid species (1% versus 67%) (79).Kin recognition has been demonstrated in the subsocial spider Stegodyphus linea-tus (Eresidae) (11). Differences between kin and non-kin pairings were marginallysignificant statistically (P = 0.048), but kin recognition was demonstrated whencannibalism was measured in groups (P < 0.001).

Filial Cannibalism and Matriphagy

Filial cannibalism occurs when females consume their own eggs or young. In thelaboratory, female spiders consume eggs they have laid (65), but usually theseeggs appear to be unfertilized. Most cannibalism of fertilized eggs by adults isnot filial cannibalism but instead occurs when females invade the nests or webs ofconspecific females and cannibalize their eggs, as happens with the web-buildingjumping spider Portia labiata (Salticidae) (20). Male S. lineatus invade nests anddestroy egg sacs guarded by females, but they do not cannibalize the eggs (119).

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Females of the wandering (i.e., nonweb building) spider Clubiona cambridgei(Clubionidae) (106) and the funnel-web spider Coelotes terrestris (Agelendidae)(56) guard their egg sac against female cannibalistic intruders, a behavior thathas been observed in the field and proven in laboratory experiments to increasethe number of surviving young. Females of the latter two species cannot distin-guish their egg sacs from those of another female; P. labiata can do so, but onlyif the sac has been moved to another web. A female wolf spider often adopts theegg sac of another female if given to her and carries it attached to her spinnerets(136). Absence of strong discriminatory faculties in most of these examples is notsurprising, given the small probability of a female solitary spider losing contactwith an egg sac that she is guarding. The probability that a female encountersrecently dispersed spiderlings is greater. The female wolf spider carries hatchedspiderlings on her back for several days before they disperse. She might frequentlyencounter her own young during and immediately after the dispersal phase. Sev-eral studies have uncovered a general inhibition against consuming spiderlings,even heterospecifics, that is highest when female lycosids carry the egg sac anddisappears completely 1 to 2 weeks after the spiderlings have dispersed (6, 29, 48,136). Filial cannibalism can occur at low rates within the first few days of disper-sal. Female Pardosa milvina discriminate, although not perfectly, between theirown progeny and those of other females during this period, but this discriminationdisappears by 3 days postdispersal (6).

In some spiders juveniles eat their mother before dispersing from the com-munal nest (33, 34, 66, 67, 118, 128–130) [termed gerontophagy (121), or morecommonly matriphagy (36)]. In Amaurobius ferox (Amaurobiidae) the female isphysiologically capable of producing a second egg sac; experiments show thather net reproductive output, calculated as the number of surviving midinstar juve-niles, is maximized by matriphagy versus the alternative strategy of abandoningher progeny early in order to lay a second clutch (67). The amount of food pro-vided to spiderlings through matriphagy in the Australian social crab spider, Diaeaergandros (Thomisidae), is positively correlated with the number of surviving spi-derlings, probably because of a reduction in sibling cannibalism (36).

In solitary species that exhibit maternal care, any tendencies for the female tocannibalize her young, and for her progeny to cannibalize each other, are sup-pressed during the period when the young remain with their mother. In Tegenariaatrica (Agelenidae) changing levels of tolerance and cannibalistic behavior of boththe female and juveniles are correlated with alterations in the amounts of severaltypes of chemical compounds in the cuticles of both adult and juvenile spiders(131, 132).

Social Spiders

Social behavior in spiders is rare. It ranges from facultative aggregations of con-nected but individually maintained webs to groups in which adults share webbingand cooperate in brood care, and ranges from groups of genetically unrelated

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individuals to species with highly inbred colonies (9, 17, 23, 134). A major as-pect of sociality is toleration, so it is not surprising that cannibalism (other thanmatriphagy) in spider social groups is rare and, when it does occur, appears tobe related to food scarcity (23). Although many social spiders accept individualsfrom other groups (23), cannibalism can occur when individuals from anothergroup enter the colony. Females of D. ergandros exhibit extended maternal careand accept unrelated juvenile immigrants into the nest in areas where nest densityis high (34), although they recognize their own offspring and preferentially providethem with prey and trophic eggs. Juveniles accept unrelated immature spiders intothe nest but can recognize them as nonsiblings and cannibalize them if sufficientlyhungry (35). Subadult females that have been starved cannibalize unrelated immi-grant females and their own brothers, but they will not cannibalize unrelated maleimmigrants, possibly to maximize outbreeding (35).

Relationship to Population Dynamics

In most types of cannibalism among nonsolitary and related spiders, the cannibalkills to obtain food, presumably because the supply of food is limited. The death ofthe victim, and also the degree to which cannibalism improves the growth, survival,and fecundity of the cannibal, potentially influence the population dynamics of thespecies. However, few, if any, comprehensive field studies have addressed theseconsequences for the types of cannibalism just discussed, although there is someresearch related to the topic, e.g., the effect of food supply on group size andemigration rates in colonial and social spiders (9, 35, 134). Most studies that relatecannibalism directly to food limitation, competition, and population dynamics havedealt with the more generalized types of cannibalistic behavior characteristic ofsolitary spiders that are uncomplicated by potential genetic or mating relationshipsbetween the interacting individuals. The next section focuses on these types—cannibalistic foraging, and the related behaviors of interference cannibalism andcannibalistic territoriality—but also reviews some findings for the other types ofcannibalism that relate specifically to factors such as size asymmetry and hungerlevel.

CANNIBALISM AS FORAGING FORFOOD AMONG SOLITARY SPIDERS

Food Limitation Among Spiders

In principle, killing conspecifics could have evolved as an adaptation to removecompetitors for any limited resource. However, because such behavior is poten-tially dangerous and the results would benefit all members of the population (100),killing a conspecific primarily as a means to remove a competitor will evolve onlyunder extensive local resource competition. In almost all cases in which a spiderkills a conspecific, the victim is eaten. Extensive evidence suggests that spiders

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frequently are food limited, i.e., a relative shortage of prey limits their growth, de-velopment, reproduction, and/or survival (earlier evidence reviewed in Reference143; for more recent evidence for food limitation in spiders, see 19, 47, 54, 69,74, 80–83, 85, 86, 88, 103, 115, 125). Thus, it is reasonable to hypothesize thatspider cannibalism has evolved primarily as a means of foraging for calories andnutrients in limited supply.

Size Asymmetry

Field observations of spider cannibalism most often involve juveniles feeding onother juveniles or adults feeding on juveniles, with fewer examples of adults feedingon each other (30, 48, 149). In laboratory studies the probability of cannibalismincreases with increasing size difference between paired juvenile spiders (16, 58,78, 114). Among Pardosa agrestis (Lycosidae) juveniles that had been starvedfor 14 days, the cannibal was always the heavier spider (114). Cannibalism neveroccurred if the ratio was <2:1 and always occurred if it was >4:1. These resultsmight appear to contradict findings of high rates of cannibalism among recentlydispersed instars of the wolf spider Schizocosa ocreata (Lycosidae) (137, 138).However, Samu et al. (114), who used older instars of P. agrestis, argue that thesmallest instars, because of their relatively low energy reserves, may be morerisk-prone and hence more likely to attack a similar-sized spiderling.

Hunger Level

Laboratory experiments with paired juvenile spiders show that starvation levelstrongly affects rates of cannibalism (11, 77, 78, 111, 114). For example, Samuet al. (114) paired juvenile P. agrestis with weight ratios within the range in whichstarved spiders showed a mixture of tendencies to cannibalize and found ratesof cannibalism of 10%, 70%, and 100% for 0, 14, and 28 days of starvation,respectively. Hunger level affects the propensity to cannibalize most clearly insolitary spiders. In subsocial spiders hunger can affect rates of cannibalism (11),but not always (79), and in some social spiders cannibalism is absent even if thespiders suffer extreme hunger (23).

Alternative Prey

Heterospecific, nonpredacious prey tend to be safer victims than similarly sizedconspecifics. Thus it is not surprising that the presence of alternative prey, whoseconsumption would decrease hunger level, decreases rates of cannibalism in lab-oratory experiments in which spiders are maintained in groups of more than twospiders (11, 112, 113, 137, 138). This expected confirmation of a seemingly simpleprediction is deceptive, however, because the addition of habitat complexity, i.e.,an element of realism, complicates the pattern (see below); and one experiment ina simple arena with paired juvenile Hogna helluo revealed an unexpected effectof alternative prey (111). Adding collembolans did not reduce the high rate of

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cannibalism observed for starved spiders (∼90%), and increased, by almost three-fold, rates of cannibalism among recently dispersed (second instar) spiderlings.Roberts et al. (111) proposed several explanations for their finding; one of whichis that hunger in the non-fed treatment caused the spiderlings to be lethargic andhave lowered fighting ability, which agrees with findings of relatively low ratesof cannibalism among starved second-instar wolf spiderlings in another lycosid,Pardosa lugubris (96), but is opposite of the effect of added collembolans oncannibalism in groups of second-instar S. ocreata (137, 138).

Habitat Complexity

Increasing habitat complexity in a simple laboratory arena can lower rates ofcannibalism of spiderlings by lycosid females (30, 107). Thus it would appearthat increasing habitat complexity should decrease cannibalism between spiders,perhaps by providing hiding places for potential victims and/or by decreasingrates of contact between potential cannibals. The situation is not so simple, asexperiments with immature lycosids demonstrate (107, 137). In one study, recentlydispersed second-instar Schizocosa spiderlings were released into 0.3-m2 arenaswith either a simple plaster-of-paris bottom or with leaf litter, crossed with a no-prey treatment or one with collembolans and pinhead crickets (137). The density ofspiderlings reflected the high end of patches of S. ocreata spiderlings in nature. Inthe absence of alternative prey, habitat complexity had no effect on mortality fromcannibalism (∼65%). In the presence of heterospecific prey, the incorporation ofnatural habitat structure increased cannibalism from ∼15% to 35%. The growthrate of spiderlings with leaf litter and alternative prey was 15% less than that in thesimple arenas with alternative prey. The leaf-litter habitat may have provided thecollembolans and crickets some protection from predation by Schizocosa, causingspiderlings in the complex habitat to be hungrier than those in the simple habitat.Thus, spiderlings in the complex habitat were more likely to cannibalize otherspiderlings. The absence of a litter effect in the no-prey treatment fails to supportan additional hypothesis, that leaf litter made cannibalistic ambushes more likely.

Rickers & Scheu (107) conducted an experiment with Pardosa palustris (Ly-cosidae) spiderlings that had first been kept together 10 to 17 days without preyand allowed to cannibalize. Because only the largest spiderlings, i.e., the mostsuccessful cannibals, were selected for the experiment, the behaviors uncoveredmight differ from experiments with S. ocreata (137) owing to the possible presenceof behavioral cannibalistic morphs (57, 78). The 2 × 2 × 2 factorial experimentconsisted of a no-prey/collembolan treatment crossed with two levels of habitatcomplexity (0.3 or 1.0 g of moss to mimic the range in nature) and two spiderdensities. Differences in habitat structure did not affect rates of cannibalism, butgrowth rates were higher in the treatment with the most moss when alternativeprey was absent. The investigators hypothesize that increased habitat complexitydecreased the energy lost in agonistic interactions, an effect not observed with S.ocreata (137). These two studies are not directly comparable because the one with

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P. palustris lacked a no-complexity treatment. Nevertheless, both failed to find anyevidence that habitat complexity decreases the rate of cannibalism among youngjuvenile lycosids.

Increased structural complexity of the habitat could reduce cannibalism amongweb-building spiders by providing more sites for building webs, thereby reducingthe frequency of web invasions and consequent exposure to cannibalism, as wasobserved in a cage experiment (112). Such an effect will be most apparent infamilies in which spider density exceeds the number of suitable web sites andspiders leave their webs to forage (such as erigonid Linyphiidae) (50, 51), but theeffect is likely to be minimal for most web-building families because cannibalismappears to be rare among most web spiders.

Foraging Mode

A strong argument for cannibalism as foraging for prey is the apparent correlationbetween the frequency of cannibalism and foraging mode, i.e., whether or not thespider uses a web to capture prey. Cannibalism accounts for 5% of the prey itemsin the diet of fishing spiders (Pisauridae) (150). Rates of cannibalism for wolfspiders range from 10% to 20% (an estimate, because conspecific and congenericjuveniles are difficult to distinguish) (30, 48, 149). Reported rates of cannibalismamong web-building spiders are much lower; for example, in a review of theprey of spiders, Nentwig (91) gives no examples of cannibalism for web-buildingspecies. Because it is difficult to distinguish cannibalism from predation on closelyrelated species, an approximate surrogate for differences in rates of cannibalismis differences in rates of predation on all spiders. A survey of the prey of spidersin cranberry bogs (10) revealed that 1% (1 of 73) of the prey of the web builderswas other spiders; for wandering species the rate was 10% (12 of 115). In a reviewof studies reporting the prey of spiders in agroecosystems, Nyffeler (94) foundoverall average rates of predation on other spiders to be <1% for the web-buildingfamilies and 15% for the families of wanderers. In rice fields, rates of cannibalismand IGP among lycosids were 5.5% and 3.4%, respectively; corresponding rateswere 0% and 2.5% for tetragnathid orb weavers and 1.6% and 1.6% for a theridiidweb spinner (68).

This dichotomy can be explained by the fact that most web builders rarelyleave their webs in search of prey. When they do leave, it usually is to search fora new web site or, in the case of mature males, to search for a mate (41). Thusconspecifics rarely come into contact, unless they are invading a web, in whichcase the invasion is more related to a web takeover than it is to predation on theowner and may rarely result in cannibalism. For example, in induced web invasionsin the araneid Metepeira labyrinthea, the rate of cannibalism was small (3 of 88encounters); most interactions involved web shaking and eventual retreat of onespider, usually the smaller one, without cannibalism (141). Another study foundthat web invasions involving tropical orb weavers rarely resulted in the death andeating of the loser (55). The outcome is often different when wandering spiders

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meet, because when they come into foraging range of a conspecific, they are liableto be preyed upon. Signaling behaviors may reduce rates of cannibalism (discussedbelow), but web builders have an advantage because web vibrations alert them toan intruder.

By no means have all families been thoroughly investigated with respect to theincidence of either IGP or cannibalism; and some families show a mix of foragingstrategies. In a rock-face system, two species of web spiders showed high per-centages of other spiders in their diets (20% to 45%) (110), although the authorsimply in their discussion that these numbers reflect IGP and not cannibalism. Amajor exception to proposed low rates of cannibalism in web-building familiesmay be the erigonid Linyphiidae, which use webs close to the ground for preycapture but also capture prey by foraging off the web (49–51). Laboratory studiesreveal high rates of cannibalism among groups of the erigonid Oedothorax gibbo-sus (Linyphiidae) housed with collembolans (135); this may reflect comparablyhigh rates of cannibalism in nature, or alternatively, the high incidence of canni-balism may reflect the inability of these small spiders to prey on the collembolanprovided, either because they are too large, as was observed in attempting to rearother genera of small linyphiids (147), or because they are unsuitable prey forother reasons (127). One field survey of several small linyphiids, including Oe-dothorax spp., failed to find evidence of predation on other spiders (1), althoughanother study uncovered low rates of spider predation by Oedothorax insecticeps(68).

Cannibalism among the young juveniles of these very small spiders would bedifficult to detect in the field without intensive, systematic observations. Becauseadult Oedothorax species do not build webs (1) and may use lines of silk to detectprey (J. Harwood, personal communication), high rates of cannibalism in Oe-dothorax spp. would actually confirm the postulated association between foragingmode and cannibalism. More research is needed on the frequency of cannibalismamong small linyphiids in nature. Linyphiid species that are both wanderers andweb-spinners will likely show intermediate levels of cannibalism.

Prey Quality: Calories, Nutrients, and Pathogens

One view of the nutritional demands of spiders and other generalist predators isthat a shortage of calories, not nutrients, limits their populations (143). Othersargue that nutrients, particularly nitrogen, are also limiting, perhaps even moreso than calories (140). Spiders tend to have a higher level of nitrogen than otherarthropod predators (24, 37), which means that predation on other spiders, in-cluding cannibalism, would be a particularly rich source of a potentially limitingnutrient. The balance of nutrients may also be important for spiders, which ex-hibit elevated rates of survival, growth, development, and fecundity on mixed dietscompared with single-species regimens (96, 122, 127, 133); a mixed diet, how-ever, can be unsatisfactory, depending on the toxicity and nutrient contents of theprey involved (12, 13, 122). If nutrient balance is important, conspecifics should

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provide the highest quality prey, because their nutrients most closely match thoseof the cannibal.

Surprisingly, two studies with lycosid spiderlings fail to confirm this predictionbut instead suggest that conspecifics are exceptionally low-quality prey. Toft &Wise (127) paired recently dispersed Schizocosa spiderlings with no alternativeprey. During the first four weeks survival rate of the successful cannibals was high(>90%), but no spiders molted more than twice, all spiders had died by week 11,and at best the cannibals had only doubled their weight, compared with a fourfoldincrease in weight for spiders fed collembolans, fruit flies, or both. Oelbermann &Scheu (96) found a similar pattern for second-instar Pardosa lugubris. Spiderlingcannibals doubled their weight in four weeks, but 90% died before reaching thenext instar. These findings are not inconsistent with the results of the two experi-ments with lycosid spiderlings discussed in the section on habitat complexity (107,137), because those experiments lasted 14 to 18 days, not long enough to have re-vealed the detrimental effects of a solely cannibalistic diet on survival and growth.The paradox persists—Why is cannibalism (and predation on closely relatedspecies) apparently common among lycosids if conspecifics are such low-qualityprey?

One likely explanation is that lycosids rely on cannibalism when other prey arescarce but eventually diversify their diets with heterospecific prey. An additionalintriguing explanation comes from two recent experiments indicating that con-specifics may be high-quality prey, but the propensity for cannibalism among youngspiderlings is a polymorphic trait. In an initial 11-week experiment, D. Mayntz& S. Toft (78) raised individual spiderlings of Pardosa prativaga on low-qualityfruit flies, high-quality flies (reared on an enriched medium), and conspecifics(spiderlings in this treatment had cannibalized once before being included in theexperimental design, thereby possibly overestimating the cannibalistic tendenciesin the population). Overall survival of the cannibals was lower than for spidersreared on the other two diets but was higher than that observed by Toft & Wisefor Schizocosa (127). The cannibals, however, consisted of two distinct groups:The majority grew slowly and exhibited the high mortality that was observed inearlier experiments, whereas 19% (5 individuals) survived for 11 weeks and grewfaster than spiderlings fed high-quality fruit flies. In a second experiment (78) spi-derlings that had fed on fruit flies and were then switched to a diet of conspecificsshowed a higher growth efficiency than spiders fed solely fruit flies, indicatingthat conspecifics are high-quality prey. The presence of a polymorphism in can-nibalistic tendency has been confirmed in an experiment with a closely relatedlycosid, Pardosa amentata (57). Three morphs were uncovered: Those that exhibitno cannibalism, spiderlings in which the cannibalism rate was sensitive to sizeasymmetries and had a latency period, and spiderlings displaying “sudden can-nibalism” that occurred independently of the size ratio of cannibal and potentialvictim.

In both studies (57, 78) most spiders showed a high latency to cannibalize,even if the size ratio was 2:1. Why are so many spiders reluctant to cannibalize

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if conspecifics are high-quality prey? Perhaps the threat of retaliation is too great,given that in nature nondangerous alternative prey may appear within a few days.Perhaps the threat of acquiring a pathogen or parasite helps maintain this poly-morphism (78). Contracting disease is a cost in other species (99) but has not yetbeen demonstrated in spiders. These studies can explain why conspecifics werelow-quality prey to most of the spiders in earlier research (96, 137), but whydid the earlier studies not uncover at least a few highly cannibalistic morphs thatcould grow, develop, and survive solely as cannibals? Species differences wouldnot be surprising, yet one species studied earlier was in the genus Pardosa (97).Cannibalism in spiders, even in one family, the Lycosidae, is a complex behav-ior. Understanding its evolution and ecological significance requires that simpleconcepts of the spider cannibal be abandoned.

INTRASPECIFIC COMPETITION

Exploitative Competition

One possible indirect benefit of both IGP and cannibalism is reduction in the inten-sity of exploitative competition through elimination of a competitor (101). Eventhough prey frequently are a limited resource for spiders, exploitative competi-tion for prey has not been documented often in web-building spiders (143). Itsrarity is unlikely due to cannibalism, as cannibalism among most solitary webbuilders appears to be infrequent. Cannibalism appears to be common among wolfspiders; yet in this family there is evidence from field experiments that growthrates of litter-inhabiting lycosids are lower at higher densities (137, 148), whichlikely reflects exploitative competition for prey. This conclusion is supported byfield experiments demonstrating that wolf spiders limit densities of collembolans(15, 71, 144, 148) and that increased densities of collembolans and other microbi-detritivores increase the density and individual weight of lycosid spiderlings (19,47). There is little evidence that eliminating an interspecific competitor is themajor benefit of IGP (104), and analogously, it is reasonable to argue that anyreduction in intraspecific competition as a result of cannibalism in spiders is asecondary, “epiphenomenal” (104) consequence of predation. Cannibalistic terri-toriality, which is a form of interference competition that affects spider spacing,is an exception.

Interference Competition

Interference, or contest, competition evolves in the context of exploitative compe-tition for a limited resource. In spiders, contest competition can be for resourcesother than prey, such as web sites or the web itself. Such agonistic interactionscould lead to cannibalism, since food is also a limited resource for spiders. Manyresearchers who study spiders appear to equate defense of the web with territori-ality. Whether such behavior is territoriality depends on the definition. Despite the

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definition one might choose, defense of the web per se, and not an area in excessof the web, will not reduce exploitative competition for prey. Web builders invadethe webs of conspecifics and displace them from the web, but such interactions,which often involve a series of signaling behaviors that can escalate to contact,rarely result in cannibalism (142). Wandering spiders also engage in elaborateagonistic behaviors that often lead to contact, grappling, and sometimes canni-balism, but cannibalism is rarely the outcome and usually occurs when a spiderapproaches another from behind (8, 60, 93). Cannibalism is likely avoided becausethe behaviors give the inferior contestant an opportunity to assess its chances andescape. These studies have been conducted in the laboratory, and it is not clearhow frequently the behaviors occur in nature, although it is reasonable to assumethat they occur any time two spiders meet face to face. These agonistic behav-iors could contribute to population regulation by influencing the emigration ratefrom an area and by lowering the mortality rate owing to cannibalism. The mostthoroughly documented example of territoriality in spiders is that of the web spi-der Agelenopsis aperta (Agelenidae), which reduces exploitative competition forprey by defending an area in excess of the web (108). Agonistic interactions occurbetween territory residents and invaders, but cannibalism is not a component of ter-ritorial defense, as injuries occur in <1% of encounters (109). Spacing patterns andbehavioral observations have established that three species of burrow-inhabitingspiders exclude conspecifics from a territory in excess of the burrow (38–40, 52,53, 75, 76, 87, 90). Cannibalism appears to be a component of territoriality inthe dancing white lady spider, Leucorchestris arenicola (Heteropodidae), on thebasis of spacing patterns, cannibalism rates of 3% to 8%, and reactions of bur-row owners to intruders (52, 53; K. Birkhofer, personal communication). Directevidence of the role of cannibalism in territoriality comes from field experimentswith the Mediterranean tarantula, Lycosa tarantula (Lycosidae) (87). Manipula-tions of spiders in artificial burrows revealed that female spiders defended an areain excess of their burrows and that the winner of territorial disputes grew at a ratetwice that of control spiders that had not engaged in a territorial contest. In a sec-ond experiment (87), approximately one third of the induced encounters betweenresidents and intruders resulted in cannibalism, but the behavioral interactionsdiffered from the typical predator-prey behavior of cannibalistic foraging in thatcannibalism never occurred in the absence of escalating agonistic interactions.Whether or not an escalation resulted in cannibalism depended strongly upon therelative size of the contestants (P = 0.004), the residency status of the winner(P = 0.021), and marginally upon body condition, a measure of hunger level(P = 0.086). Thus territoriality in L. tarantula contributes to the regulation ofpopulation density through the exclusion of nonterritory owners by both agonisticinteractions and cannibalism. Sexual cannibalism also likely affects the dynamicsof L. tarantula populations via effects on the growth and eventual fecundity of fe-males; a field experiment demonstrated that strong food limitation among juvenilefemales was likely alleviated in the adult stage at least partly by killing and eatingmales (88).

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POPULATION REGULATION

Cannibalistic territoriality is a special case; in principle, all types of cannibalismshould regulate population density because rates of cannibalism are expected tobe density dependent (28, 100, 101). Two laboratory studies have demonstrateddensity-dependent cannibalism between juvenile wolf spiders (107, 138), but theirresults fail to provide convincing indirect evidence that cannibalism might beregulating wolf spider populations in nature. Increasing the density of juvenilePardosa palustris increased the percentage mortality by a factor of three (107),but densities were approximately 10 times higher than field densities, and thedensity effect may have been restricted to spiders in the presence of alternativeprey (P of interaction = 0.072). This possible interaction may have been due tothe relatively few spiders (5) in the low-density treatment and the high mortalityrate in the absence of alternative prey at both densities (∼100% after 18 days),which incidentally suggests that conspecific P. palustris are low-quality prey. Alaboratory experiment with Schizocosa ocreata employed larger arenas (0.3 m2

versus 0.004 m2), greater initial numbers of spiders per treatment (15 and 60 versus5 and 15), and densities more similar to field conditions (50 and 200/m2 versus1250 and 3750/m2, respectively) (138). Densities in the S. ocreata experimentapproximated low- and high-density treatments in a related field experiment (137).Mortality from cannibalism was high, but the overall effect of density was onlymarginally significant (P = 0.084), and there was no interaction between spiderdensity and the presence or absence of alternative prey. Furthermore, there wasno hint that cannibalism was density dependent in the presence of alternativeprey, the treatment more similar to conditions in nature. Thus these two laboratoryexperiments do not provide evidence, even indirect, that cannibalism might regulatelycosid populations.

Field experiments in small (0.25 to 2.25 m2) fenced plots have found, and havefailed to find, density-dependent mortality in lycosids (16, 137, 148). Buddle et al.(16) decreased the 7-day survival rate of adult and juvenile Pardosa milvina by17% in soybean fields by increasing spider density four times over normal lev-els. In a forest experiment, doubling densities of juvenile Pardosa moesta andP. mackenziana had no impact on survival (15), but simply doubling spider den-sity also had no effect on spider survival in soybeans (16). Establishing densitiesof young juvenile S. ocreata above (four times), below (one fourth) and equal tothe estimated mean density in the forest revealed strong density-dependent sur-vival: After 74 days numbers in the high-density and mean-density treatmentshad converged, and densities in the low-density treatment had converged partiallywith those in the mean-density treatment (from 25% to 50% of the number in themean-density treatment) (148). Some of the convergence of S. ocreata densitiesmay have been due to density-dependent emigration (138), and density-dependentsurvival in both experiments (16, 148) could have been caused by predation byother species and not cannibalism, as other arthropod predators were not removedfrom the enclosures.

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In a second experiment with S. ocreata, all predators found by sifting the litterwere removed before S. ocreata densities were manipulated in enclosures withmore effective barriers to emigration (137). Mortality over 2.5 months was high(∼80%) and strongly density dependent, leading low- and high-density treatments(0.75 time and 3 times the estimated natural densities, respectively) to converge to atwofold difference. The absence of complete convergence suggests that emigrationwas a component of the density-dependent response in the first study. Reducingpredator densities had no effect on S. ocreata survival, implicating cannibalism asa major component of the density-dependent mortality. The laboratory experiment(137) in which leaf litter and alternative prey affected rates of cannibalism supportsthis interpretation, because the rate of cannibalism in the leaf-litter, collembolantreatment was similar to the rate of mortality observed in the high-density treat-ment of the field experiment. Two other experiments in which IGP predators werereduced but Schizocosa spp. still displayed high mortality support the interpre-tation that cannibalism can contribute to the regulation of densities of juvenileSchizocosa spp. (73, 145). Autumn densities of juvenile Schizocosa spp. showremarkable consistency between different years and different forests (19, 73, 137,146, 148). It is tempting to speculate that strong density-dependent cannibalism,in the presence of shifting mortality from IGP from other spiders, centipedes, andpredacious insects, contributes to this apparent stability. However, none of thesefield experiments quantify the relative contribution of cannibalism and IGP to ly-cosid mortality in the presence of other predators of Schizocosa. Nevertheless, thefield and accompanying laboratory experiments do demonstrate the potential ofcannibalism to regulate wolf spider populations.

TROPHIC CASCADES

Strong density-dependent cannibalism may limit spider populations below thecarrying capacity set by resources (100), which should reduce the ability of spidersto exert strong trophic cascades. The existence of cannibalism, however, is evidenceof severe food limitation, which suggests that the cannibal could be depressingpopulations in lower trophic levels enough to initiate a significant trophic cascade.Meta-analyses of field experiments with generalist predators, many with spiders,reveal that spiders can induce trophic cascades in a variety of terrestrial systems(46, 117). Accumulating evidence from agroecosystems suggests that generalistpredators, including spiders, can act as effective biological control agents (95, 126),further evidence that spider cannibalism does not prevent at least some species fromimpacting lower trophic levels.

The question then, is not whether spiders can induce trophic cascades, butrather, to what extent does spider cannibalism dampen cascades? Rarely has thisquestion been addressed directly. Cannibalism has the potential to maintain spiderpopulations during periods of low abundance of prey at lower trophic levels (the“lifeboat strategy”) (100), thereby promoting trophic cascades in the future whenconditions change.

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I restrict this part of the review to field experiments with enclosures and/orcages that include densities and habitat complexity similar to the field situation,whether it be a natural or highly managed agricultural system, and to experimentsthat demonstrate unambiguous effects of wandering spiders or, in one case, a webbuilder that exhibits significant rates of cannibalism.

Spider-removal experiments reveal that ambient densities of wandering spiders,or spider complexes with large numbers of wanderers, can initiate cascades ofindirect effects in grazing food webs that affect productivity in vegetable crops(124), plant damage in grassland systems (116), rates of decomposition in meadows(64), and litter decomposition in forests, where the effect can be to either inhibit(71) or enhance (72) rates of decomposition. Thus, in these systems cannibalismmay dampen spider-initiated trophic cascades but does not prevent them.

Polis & Strong (105) proposed that trophic cascades in grazing webs can beincreased by subsidies from other webs that lead to elevated densities of generalistpredators, owing to alleviation of the effects of food limitation, dampening of IGP,and a reduction in the intensity of cannibalism. Several examples of such subsidiesexist for wandering spiders. Emergence of aquatic insects and their movement toadjacent terrestrial habitats can lead to increased densities of both web-buildingand wandering species (21, 115), which in one instance resulted in decreaseddamage to plants (54). Doubling the immigration rate of wolf spiders into vegetablegardens caused densities of spiderlings, but not adults, to increase, possibly becausedensity-dependent cannibalism among the spiderlings and older juveniles affectedadult numbers (123). In a subsequent experiment, adding a detrital subsidy causedall stages of wolf spiders to increase, perhaps owing to reduced cannibalism amongyoung instars in response to increased densities of microbi-detritivores. However,the elevated lycosid numbers failed to control crop pests (47). The absence of anenhanced trophic cascade was likely due to an unusually high density of one pest,and the absence of another later in the season (47), and was not due to anotherpossibility, i.e., that large wolf spiders might shift their feeding away from thecrop-based food web to the detrital web (147). Thus, enhancing densities of preyin the fungal-based food web promises to be one technique for enhancing wolfspider numbers, owing at least partly to reduced cannibalism, as a way to increasetheir effectiveness in biological control.

Wolf spiders can limit populations of some planthopper species in Spartina saltmarshes (25, 45). Laboratory mesocosm experiments suggest that spider predationmay initiate a trophic cascade affecting Spartina, but field experiments so far havenot uncovered a limitation strong enough to induce a detectable trophic cascade inthe field (26). There exists a complex interaction between plant nutrient content,amount of thatch cover, and the ability of wolf spiders to limit planthoppers. Thepresence of thatch enhances the ability of wolf spiders to limit planthopper densi-ties (26), perhaps because thatch lowers rates of wolf spider cannibalism; a meta-analysis reveals that manipulating detritus has particularly strong positive effectson spider densities (70). Laboratory studies revealed moderate rates of cannibal-ism among the web builder Grammonota trivatatta (Linyphiidae), and predator-addition experiments in the field suggest that limited web sites, cannibalism,

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or both may limit spider numbers in the marsh. However, predation rates onplanthoppers by individual G. trivatatta are much lower than those of individualwolf spiders in the system (27), which indicates that neither cannibalism amongG. trivatatta nor IGP by Pardosa littoralis, which has been documented, explainsthe inability of this web builder to control planthopper populations.

Thus, research in vegetable crops and salt marshes reveals how different en-vironmental factors can influence rates of cannibalism among spiders, which inprinciple should affect their ability to initiate trophic cascades, but the studies alsohighlight the complex interplay of other factors that affect spider predation on her-bivore populations. The extent to which cannibalism limits the strength of trophiccascades initiated by spiders remains largely unknown.

FUTURE RESEARCH

This review has uncovered several unanswered or only partially answered questionsabout the roles of cannibalistic foraging in the population dynamics and food webrelationships of spiders:

� Are the frequencies, ecological correlates, and evolved behaviors of can-nibalistic foraging in wolf spiders (Lycosidae), the group most intensivelystudied to date, representative of other families of wandering spiders?

� Are conspecifics high-quality prey in terms of nutrient content?� Can costs other than the threat of retaliation—such as the acquisition of

pathogens and parasites—explain the low rates of cannibalism exhibited bysome spiders?

� Are behavioral polymorphisms in cannibalistic tendency generally charac-teristic of spider cannibalism, and if not, can their occurrence or absence becorrelated with environmental variables?

� Do agonistic displays occur frequently in natural populations, and to whatextent do they reduce mortality from cannibalism?

� Is spider cannibalism in nature strongly density dependent, and does it con-tribute to population stability in spiders?

� Does cannibalism inhibit (by dampening trophic cascades) or enhance (asa lifeboat strategy) the effectiveness of spiders in the control of agriculturalpests?

� Can the properties of cannibalistic encounters, from frequency to behav-ioral mechanisms, be accurately described from individual pairings in thelaboratory?

� What is the actual frequency in nature of cannibalism in different spiderfamilies, and does the pattern confirm the hypothesized relationship betweenforaging mode and rates of cannibalism?

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ACKNOWLEDGMENTS

Many thanks to James Harwood, David Mayntz, Klaus Birkhofer, Erin Hladilek,and Janet Lensing for their helpful comments on a draft of this review.

The Annual Review of Entomology is online at http://ento.annualreviews.org

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P1: JRX

November 2, 2005 13:47 Annual Reviews AR263-FM

Annual Review of EntomologyVolume 51, 2006

CONTENTS

SIGNALING AND FUNCTION OF INSULIN-LIKE PEPTIDES IN INSECTS,Qi Wu and Mark R. Brown 1

PROSTAGLANDINS AND OTHER EICOSANOIDS IN INSECTS: BIOLOGICALSIGNIFICANCE, David Stanley 25

BOTANICAL INSECTICIDES, DETERRENTS, AND REPELLENTS INMODERN AGRICULTURE AND AN INCREASINGLY REGULATEDWORLD, Murray B. Isman 45

INVASION BIOLOGY OF THRIPS, Joseph G. Morse and Mark S. Hoddle 67

INSECT VECTORS OF PHYTOPLASMAS, Phyllis G. Weintrauband LeAnn Beanland 91

INSECT ODOR AND TASTE RECEPTORS, Elissa A. Hallem, AnupamaDahanukar, and John R. Carlson 113

INSECT BIODIVERSITY OF BOREAL PEAT BOGS, Karel Spitzerand Hugh V. Danks 137

PLANT CHEMISTRY AND NATURAL ENEMY FITNESS: EFFECTS ONHERBIVORE AND NATURAL ENEMY INTERACTIONS, Paul J. Ode 163

APPARENT COMPETITION, QUANTITATIVE FOOD WEBS, AND THESTRUCTURE OF PHYTOPHAGOUS INSECT COMMUNITIES,F.J. Frank van Veen, Rebecca J. Morris, and H. Charles J. Godfray 187

STRUCTURE OF THE MUSHROOM BODIES OF THE INSECT BRAIN,Susan E. Fahrbach 209

EVOLUTION OF DEVELOPMENTAL STRATEGIES IN PARASITICHYMENOPTERA, Francesco Pennacchio and Michael R. Strand 233

DOPA DECARBOXYLASE: A MODEL GENE-ENZYME SYSTEM FORSTUDYING DEVELOPMENT, BEHAVIOR, AND SYSTEMATICS,Ross B. Hodgetts and Sandra L. O’Keefe 259

CONCEPTS AND APPLICATIONS OF TRAP CROPPING IN PESTMANAGEMENT, A.M. Shelton and F.R. Badenes-Perez 285

HOST PLANT SELECTION BY APHIDS: BEHAVIORAL, EVOLUTIONARY,AND APPLIED PERSPECTIVES, Glen Powell, Colin R. Tosh,and Jim Hardie 309

vii

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November 2, 2005 13:47 Annual Reviews AR263-FM

viii CONTENTS

BIZARRE INTERACTIONS AND ENDGAMES: ENTOMOPATHOGENICFUNGI AND THEIR ARTHROPOD HOSTS, H.E. Roy,D.C. Steinkraus, J. Eilenberg, A.E. Hajek, and J.K. Pell 331

CURRENT TRENDS IN QUARANTINE ENTOMOLOGY, Peter A. Follettand Lisa G. Neven 359

THE ECOLOGICAL SIGNIFICANCE OF TALLGRASS PRAIRIEARTHROPODS, Matt R. Whiles and Ralph E. Charlton 387

MATING SYSTEMS OF BLOOD-FEEDING FLIES, Boaz Yuval 413

CANNIBALISM, FOOD LIMITATION, INTRASPECIFIC COMPETITION, ANDTHE REGULATION OF SPIDER POPULATIONS, David H. Wise 441

BIOGEOGRAPHIC AREAS AND TRANSITION ZONES OF LATIN AMERICAAND THE CARIBBEAN ISLANDS BASED ON PANBIOGEOGRAPHIC ANDCLADISTIC ANALYSES OF THE ENTOMOFAUNA, Juan J. Morrone 467

DEVELOPMENTS IN AQUATIC INSECT BIOMONITORING: ACOMPARATIVE ANALYSIS OF RECENT APPROACHES, Nuria Bonada,Narcıs Prat, Vincent H. Resh, and Bernhard Statzner 495

TACHINIDAE: EVOLUTION, BEHAVIOR, AND ECOLOGY,John O. Stireman, III, James E. O’Hara, and D. Monty Wood 525

TICK PHEROMONES AND THEIR USE IN TICK CONTROL,Daniel E. Sonenshine 557

CONFLICT RESOLUTION IN INSECT SOCIETIES, Francis L.W. Ratnieks,Kevin R. Foster, and Tom Wenseleers 581

ASSESSING RISKS OF RELEASING EXOTIC BIOLOGICAL CONTROLAGENTS OF ARTHROPOD PESTS, J.C. van Lenteren, J. Bale, F. Bigler,H.M.T. Hokkanen, and A.J.M. Loomans 609

DEFECATION BEHAVIOR AND ECOLOGY OF INSECTS, Martha R. Weiss 635

PLANT-MEDIATED INTERACTIONS BETWEEN PATHOGENICMICROORGANISMS AND HERBIVOROUS ARTHROPODS,Michael J. Stout, Jennifer S. Thaler, and Bart P.H.J. Thomma 663

INDEXESSubject Index 691Cumulative Index of Contributing Authors, Volumes 42–51 717Cumulative Index of Chapter Titles, Volumes 42–51 722

ERRATAAn online log of corrections to Annual Review of Entomologychapters may be found at http://ento.annualreviews.org/errata.shtml

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