Ambrosia Beetle Host Selection Among Logs of Douglas Fir, Western Hemlock, and Western Red Cedar...

Journal of Chemical Ecology, Vol. 23, No. 4, 1997

AMBROSIA BEETLE HOST SELECTION AMONG LOGSOF DOUGLAS FIR, WESTERN HEMLOCK, AND

WESTERN RED CEDAR WITH DIFFERENT ETHANOLAND a-PINENE CONCENTRATIONS

RICK G. KELSEY1'* and GLADWIN JOSEPH2

'USDA Forest Service, Pacific Northwest Research Station3200 Jefferson Way, Corvallis, Oregon 97331

^Department of Forest Science, Oregon State UniversityCorvallis, Oregon 97331

(Received April 4, 1996; accepted November 28, 1996)

Abstract—Logs from Douglas fir, Pseudotsuga menziesii; western hemlock,Tsuga heterophylla; and western red cedar, Thuja plicata, were left in theforest through winter. In April, segments from these logs were removed andrandomly positioned adjacent to one another allowing ambrosia beetles toselect their preferred host. In early June the tissues of Douglas fir and westernhemlock logs contained significantly higher ethanol concentrations and ambro-sia beetle (Trypodendron lineatum and Gnathotrichus spp.) densities than logsof western red cedar. Low beetle attack densities in western red cedar wereprobably a consequence of low ethanol concentrations. Although Douglas firtissues produced significantly higher ethanol concentrations than western hem-lock, the beetles did not effectively discriminate between these two coniferspecies. Ethanol and a-pinene were significant covariates for the ambrosiabeetle densities. a-Pinene concentrations were highest in the phloem of west-em red cedar, intermediate in Douglas fir, and nearly absent in western hem-lock. a-Pinene did not synergize the beetle's response to ethanol or to elhanol+ pheromone during host selection, and it may have functioned as a deterrent.

Key Words—Host selection, kairomones, ambrosia beetles, Trypodendronlineatum, Gnathotrichus spp., Pseudotsuga menziesii, Tsuga heterophylla,Thuja plicata.

INTRODUCTION

Host selection by the ambrosia beetles Trypodendron lineatum (Oliv.), Gnath-otrichus sulcatus (LeConte), and G. retusus (LeConte) (Coleoptera: Scolytidae)

*To whom correspondence should be addressed.

1035

0098-0331/97/0400-1035$l2.50/0 © 1997 Plenum Publishing Corporation

1036 KELSEY AND JOSEPH

in the spring or summer is characterized by a distinct preference for logs thathave aged through the winter over logs freshly felled in the spring (Prebble andGraham, 1957; Bletchly and White, 1962; Dyer and Chapman, 1965; Pulliainen,1983; Lindelow et al., 1992; Kelsey, 1994a). Graham (1968) demonstrated thatT. lineatum were attracted to fresh western hemlock, Tsuga heterophylla (Raf.)Sarg., sapwood only after subjecting it to anaerobic conditions. Ethanol wasfound to accumulate in live conifer tissues under anaerobic conditions and wasidentified as a kairomone in aged logs under attack by T. lineatum (Moeck,1970) and G. sulcatus (Cade et al., 1970).

Trypodendron lineatum is attracted to ethanol released alone from artificialsources (Moeck, 1970, 1971; Nijholt and Shonherr, 1976; Klimetzek et al.,1986; Schroeder, 1988; Liu and McLean, 1989; Schroederand Lindelow, 1989;Salom and McLean, 1990), and in Europe the trap catches increase as the eth-anol release rates increase (Klimetzek et al., 1986; Schroeder, 1988; Schroederand Lindelow, 1989). A comparable response has not been observed in NorthAmerican populations of this species (Moeck, 1970; Salom and McLean,1988).

Also in Europe, the release of ethanol with a-pinene causes a synergisticincrease in the trap catches of T. lineatum compared to each compound releasedalone (Bauer and Vite, 1975; Nijholt and Shonherr, 1976; Borden et al., 1982;Schroeder, 1988; Schroeder and Lindelow, 1989). A synergistic response wasobserved when the ethanol-a-pinene release rate was 10:1 but not at 1:1(Schroederand Lindelow, 1989). In western North America, Nijholt and Schon-herr (1976) are the only investigators to observe a synergistic response from T.lineatum with the binary release of ethanol + a-pinene.

In North America and Europe the combined release of ethanol + phero-mone, or the ternary combination of ethanol + a-pinene + pheromone, maytrap more T. lineatum than traps releasing only the pheromone (Vite and Bakke,1979; Borden et al., 1982; Shore and McLean, 1983). Similar responses havebeen observed for the Gnathotrichus spp. (Borden et al., 1980, 1982; Shoreand McLean, 1983, 1985). In some experiments, however, the combination ofpheromone + kairomone(s) did not enhance the beetle capture over the phero-mone alone (Borden et al., 1980, 1982; Salom and McLean, 1988). In Europe,increasing the release rate of a-pinene from low to high in ternary mixtures,without changing the release rates of the pheromone (lineatin) or ethanol, causeda decrease in trap catches of T. lineatum (Bakke, 1983).

Aside from the initial studies by Moeck (1970) and Cade et al. (1970),only a few experiments have focused on the kairomone chemistry of logs duringhost selection by these ambrosia beetles. Ethanol and acetaldehyde concentra-tions accumulate in logs of Norway spruce, Picea abies (L.) Karst., felled inNovember and left on the ground through winter (Lindelow et al., 1992). Injured(tops removed) Scots pine trees, Pinus sylvestris L., attacked by T. lineatum

AMBROSIA BEETLE HOST SELECTION 1037

contained more acetaldehyde and ethanol than injured trees that were not attacked(Sjodinetal., 1989).

Recently it was shown that ambrosia beetle preference for Douglas fir,Pseudotsuga menziesii (Mirb.) Franco, logs felled on different dates is relatedto the tissue ethanol concentrations (Kelsey, 1994a). Logs felled in Novembersynthesized and accumulated more ethanol than those felled in January andMarch, and subsequently they were more heavily attacked. In a related study,delimbed Douglas fir logs produced more ethanol and were more heavily attackedby beetles than logs with branches attached (Kelsey, 1994b).

These beetles also prefer to attack particular conifer species. For instance,logs of Douglas fir and western hemlock are preferred over those of western redcedar, Thuja plicata, Donn ex D. Don, and Pacific silver fir, Abies amabilisDougl. ex Forbes, (Prebble and Graham, 1957; Chapman, 1961, 1963; Johnson,1958; Zhong and Schowalter, 1989). In Europe, T. lineatum attacks Scots pinelogs more heavily than Norway spruce, (Pulliainen, 1983). However, very littleis known about interspecific kairomones and their relationships to ambrosiabeetle host preference.

Considering the effect of ethanol on ambrosia beetle behavior it seemsreasonable to suspect that their preference for certain conifer species may becaused, in part, by the synthesis or release of different quantities of ethanolfrom each conifer species. Under laboratory conditions, ethanol synthesis amongconifer species varied significantly when foliage (Kimmerer and MacDonald,1987) or stems (Kelsey, 1996) were subjected to anaerobic conditions. Theobjective of this study was to measure and compare ethanol, a-pinene, andacetaldehyde concentrations with ambrosia beetle attack densities in Douglasfir, western hemlock, and western red cedar logs exposed to similar environ-mental conditions in the field.

METHODS AND MATERIALS

Study Site. This study was conducted on the Oregon State University,College of Forestry, Blodgett Tract (46°05'30"N, 123°20'06"W) in the CoastRange of northwestern Oregon, near the Columbia River. The experiment wasset up on a ridge top, adjacent to the forest edge at the upper boundary of a 23-ha clear-cut. The cutting unit faced north with some components facing eastand west. Elevation of the unit ranged from 311 to 390 m. Logs for this exper-iment were taken from trees harvested in the clear-cut during September andOctober 1992.

Lag Preparation. After felling and limbing, the logs (approximately 10 mlong) were yarded and decked near the experimental site on the ridge top. InFebruary 1993, nine logs from each species were removed from the decks, sorted


by species and set aside. On April 13, log segments 1.83 m long, with minimalbark damage and similar diameters, were cut from each 10-m log. Their meandiameters were 30.2 ± 1.5 (SE), 32.2 ± 1.0, and 30.9 ± 1.7 cm (P = 0.685)for Douglas fir, western hemlock, and western red cedar, respectively.

The top of each log was marked to ensure it remained upright and thenplaced into one of three blocks. Within blocks, the logs were randomly posi-tioned 0.5-1.5 m apart, with their sides parallel and the cut ends facing east/west. Each block contained three logs from each species. The blocks wereparallel to one another with the log ends separated by about 1-2 m. The remain-ing pieces of the original 10-m logs were left on site less than 50 m from theblocks.

Tissue Sampling. Log tissues were sampled for chemical analysis on June2, 1993. Separate samples of phloem and sapwood (1.0 cm depth) were removedwith an increment borer (5 mm ID) along the top of each log at 30, 91, and152 cm from the west end. The north and south sides of each log were sampledin the same manner, yielding a total of nine samples of each tissue per log.Each sample was sealed in a small vial (15 X 45 mm), frozen with Dry Ice fortransport to the laboratory, and stored at —36°C until prepared for analysis.After thawing for a few minutes on ice, each sample was weighed into a pre-weighed headspace vial (22 x 75 mm, 22 ml volume) and sealed with a septum.The sealed vials were heated for 30 min at 100°C to deactivate the enzymes.Sample analysis was completed within three weeks after collection.

Headspace Analysis. The chemical analysis was performed with a PerkinElmer HS40 headspace autosampler connected to a Hewlett-Packard 5890gas chromatograph with a J&W DB-Wax column, 0.32 mm ID X 30 m, and0.25-^em film thickness. Other operating conditions and calculations were thesame as previously reported (Kelsey, 1994b), except for a sample thermostattemperature of 90°C instead of 100°C on the HS40. Furthermore, chemicalconcentrations were expressed on a fresh weight basis rather than dry weight,because ethanol is water-soluble and nonionic so it can mix and move with waterthrough the tissues. Fresh weight concentrations may represent more accuratelythe actual ethanol concentrations experienced by ambrosia beetles excavatingthe tissues.

Ambrosia Beetle Measurements. All logs were free of beetle attacks onApril 13 when arranged into blocks. Ambrosia beetle attack densities wereestimated on June 3 and 4, 1993, by removing the bark and counting galleryentrance holes in a 15- X 15-cm quadrat centered around each tissue samplinghole (nine quadrats per log). If the bark was damaged, the quadrat was movedto the nearest undamaged position. Entrance holes of T. lineatum were separatedfrom those of Gnathotrichus spp. by checking the diameter of each hole withthe end of a No. 53 wire gauge drill bit (Kinghorn, 1957a). When sampling


was complete, a few beetles were excavated from their galleries for identificationby the Entomology Extension Service at Oregon State University.

Statistical Analysis. The data were analyzed as a split-plot design withrandomized complete blocks in the whole plot and strip-plots in the subplot.Tree species was the whole plot treatment, whereas tissue and position on logs(north side, south side, and top) were the subplot treatments.

For the analysis of variance (ANOVA), concentrations of each chemicalcomponent were averaged across the three logs of a species to give a singlespecies value for each tissue and position within each block. The averages wereweighted by the number of observations because values from one of the Douglasfir logs were discarded as outliers. Its tissues contained no ethanol, low a-pineneand acetaldehyde concentrations, and a water content about half the amount inthe other Douglas fir logs. This log might have been taken from a dying tree atthe time of felling. The SAS (SAS Institute Inc., 1989) General Linear Model(GLM) procedure with type HI sums of squares was used for the ANOVA ofeach chemical constituent, the attack densities of T. lineatum, and the combinedattack densities of the Gnathotrichus species. Fisher's protected LSD test (a =0.05) was used for preplanned comparisons of means. The values for beetledensities, acetaldehyde, and a-pinene required natural log transformation toensure normality and homogeneity of variances. Geometric means with back-transformed standard errors are presented for the transformed variables.

To determine the relative influence of individual chemical constituents onthe variation in beetle densities, the data were subjected to an analysis of co-variance (Steel and Torrie, 1980) run on mean values for each log using theGLM procedure of SAS (SAS Institute Inc., 1989). Means for each log wereobtained by averaging across tissues and the side positions, but excluding valuesfrom the tops. Data from log tops were not included because beetle attacks onthe top and sides were probably confounded by a photonegative response (Gra-ham, 1959) (see Discussion).

To identify the significant covariates explaining beetle densities, an iterativeGLM procedure was used (Thomas Sabin, Oregon State University, personalcommunication). Each of the covariates was independently included in an initialmodel of the form: y (beetle density) = covariate (ethanol, etc.) X treatment(species). Ethanol was identified as the only significant covariate at P < 0.05with the corresponding type III sums of squares. Ethanol was then paired witheach of the other covariates and analyzed as separate models using GLM pro-cedures (type I sums of squares) to determine the significance of the othercovariates after adjusting for the effect of ethanol. Finally, ethanol and the othercovariates (in the order of significance from the paired analyses with ethanol)were combined in a final model (with type I sums of squares) to determine theirrelative contributions to the variation in beetle densities.


RESULTS

Ethanol concentrations were dependent on the species, tissue, and position onthe log (Table 1), illustrating how heterogeneous the source of this kairomoneis in logs. Nevertheless, for each tissue and position, the ethanol concentrationswere significantly different among species (Figure 1). On average, the ethanolconcentrations in tissues of Douglas fir were 2.1 times greater than in westernhemlock and 3.3-4.0 times greater than in western red cedar. Comparing tissueswithin species, ethanol concentrations in the phloem were significantly greaterthan in the sapwood on the sides of Douglas fir and western hemlock logs,whereas the opposite was observed in western red cedar (Figure 1). Westernhemlock was the only species with a significant difference in ethanol concen-tration between sapwood and phloem on the tops of logs. Overall, within eachspecies the differences in ethanol concentrations between tissues were not largeat any position. Comparing positions within species, the tissues with the highestethanol concentrations were usually from the tops of log, although they werenot always significantly greater than the concentrations in the correspondingtissues from the sides (Figure 1). Western hemlock exhibited the greatest dif-ference between tops and sides, for both tissue types. Tissues from the northside of Douglas fir contained greater ethanol concentrations than the tissuescollected from the south side, whereas the opposite was observed for westernred cedar (Figure 1). In general, ethanol concentrations in a tissue differed moreamong positions than they did between tissues from the same position.

Acetaldehyde concentrations were dependent on the species and tissue(Table 1), but only in western red cedar were the tissue concentrations signifi-cantly different (Figure 2). Concentrations among species exhibited a trend sim-ilar to ethanol, with highest quantities in the sapwood and phloem of Douglasfir and the lowest quantities in the sapwood from western red cedar. Theacetaldehyde concentrations, however, were much lower, at only 1-4% of theethanol concentrations (Figures 1 and 2). There were no significant differencesin acetaldehyde concentrations among positions (Table 1).

a-Pinene concentrations were dependent on the species and tissue (TableI) because of large concentration differences between the phloem and sapwoodwithin western red cedar, but no differences in concentrations between thesetissues for the other two species (Figure 2). a-Pinene concentrations were high-est in the phloem of western red cedar, but not significantly greater than theconcentrations in Douglas fir sapwood (Figure 2). The phloem and sapwood ofwestern hemlock contained the lowest quantities of a-pinene (Figure 2).a-Pinene was detected in only two of the nine hemlock logs, at very low con-centrations in three tissue samples of one log and in four tissue samples ofanother. There were no differences in a-pinene concentrations among positionson the logs (Table 1).


FIG. 1. Mean (±SE) ethanol concentrations in the phloem and sapwood at three differentpositions on logs of Douglas fir, western hemlock, and western red cedar that were beingattacked by ambrosia beetles. For each tissue and position, the differences between eachof the species were significant (P < 0.05). Within each species and tissue, log positionswith different letters are significantly different. Asterisks indicate a significant differencebetween the phloem and sapwood at a particular position within each species.

Water content was dependent on the species and tissue (Table 1), but inall three species the sapwood contained significantly less water than the phloem(Figure 2). Among species, western red cedar phloem and sapwood containedthe highest water content of each tissue (Figure 2). The water contents of westernhemlock and Douglas fir tissues were not different. Although the water contentwas also dependent on the tissue and log position (Table 1), this was probablyof little biological significance because at all three log positions the sapwoodcontained 65.2-71.9% of the water in the phloem. The water content of phloemon the tops of logs was 90.1% of the water in phloem from the sides. Forsapwood, there were no differences in the water contents among log positions.

In addition to T. lineatum, both Gnathotrichux retusus and G. sulcatuswere extracted from galleries and identified. The attack densities of T. lineatumon Douglas fir and western hemlock (Table 2; Figure 3) were greater than onwestern red cedar. There were 1.4 times more T. lineatum attacks on Douglasfir than on western hemlock logs, but the difference was not significant (P =0.090). The densities of Gnathotrlchus spp. on both Douglas fir and westernhemlock were greater than on western red cedar logs (Table 2; Figure 3). Therewere no differences (P = 0.508) in Gnathotrichm spp. densities between Doug-las fir and western hemlock. Both beetle genera avoided attacking the tops oflogs in favor of either side, with no preference between sides (Table 2; Figure4).

To examine further the relationship between beetle densities and ethanolconcentrations, the means of both variables were calculated for each log withina species. Logs with the lowest ethanol concentrations had the lowest beetle


FIG. 2. Mean (±SE) acetaldehyde, a-pinene, and water contents of tissues from Douglasfir, western hemlock, and western red cedar logs that were being attacked by ambrosiabeetles.

densities in each species. However, at the higher ethanol concentrations, thebeetle densities were not always correspondingly higher.

Covariate analysis indicated that ethanol and a-pinene were the only twosignificant covariates (Table 3), accounting for 61 % of the variation in T. linea-tum densities and 52% of the variation in the Gnathotrichus spp. densities.


TABLE 2. ANALYSIS OF VARIANCE FOR BEETLE DENSITIES IN Loos OH DOUGLAS FIR,WESTERN HEMLOCK, AND WESTERN RED CEDAR"

Source of variation

BlockSpecies

Error (a)PositionSpecies x position

Error (b)

<ff

22424

12

T. lineatum

MS P

0.736.24 0.0130.41

17.83 0.0000.05 0.9630.34

Gnathotrichus spp.

MS

4.887.611.08

35.330.930.67

P

0.049

0.0000.293

"Data were log,,^ + 1 )-transformed before analysis. MS: mean square.

FIG. 3. Mean (±SE) densities of T. lineatum and the Gnathotrichus spp. in logs ofDouglas fir, western hemlock, and western red cedar in early June.


FIG. 4. Mean (±SE) densities of T. lineatum and the Gnathotrichus spp. in the sidesand tops of Douglas fir, western hemlock, and western red cedar logs in early June.

TABLE 3. ANALYSIS OF COVARIANCE FOR ETHANOL, B-PINENE, ACETALDEHYDE, ANDWATER AMONG CONIFER SPECIES WITH VARYING DENSITIES OF BEETLE ATTACK

Source of variation

CovariateEthanola-PineneAcetaldehydeWater

TreatmentSpecies

Error

4f

1111

219

T. linealum

MS P

53.388 0.0007.698 0.0543.566 0.179

2.594 0.249

4.481 0.3171.834

Gnathotrichus spp.

MS

27.02025.0540.3020.148

8.2673.600

P

0.0130.0160.7750.841

0.338

"MS: mean square.

1046 KF.LSF.Y AND JOSEPH

Ethanol concentrations explained more variation in attack densities of T. linea-tum than it did for Gnathotrichus spp. The effect of a-pinenc was just theopposite. Estimates of regression coefficients for each variable in the modelindicated a positive relationship between beetle densities and ethanol concentra-tions and a negative relationship between beetle densities and a-pinene concen-trations.

DISCUSSION

During the winter and spring months the tissues of Douglas fir and westernhemlock logs synthesized and accumulated much higher ethanol concentrationsthan the tissues of western red cedar. Their ethanol concentrations were notcorrelated with their tissue water contents, which agrees with previous obser-vation for Douglas fir (Kelsey, 1994a,b). Logs of western red cedar, with lowethanol, were more lightly attacked by T. lineatum and the Gnathotrichus spp.than logs of Douglas fir and western hemlock, with higher ethanol concentra-tions. For individual logs within each conifer species, the relationship betweenlow beetle densities and low ethanol concentrations paralleled that observedamong species. Finding ethanol as the most significant contributor to the vari-ation in beetle attack densities is consistent with ethanol attracting ambrosiabeetles to artificial traps (Moeck, 1970, 1971; Nijholt and Shonhcrr, 1976;Klimetzek et al., 1986; Schroeder, 1988; Liu and McLean, 1989; Schroederand Lindelow, 1989; Salom and McLean, 1990).

Although the chemical concentrations in our logs were not measured imme-diately after the beetle attacks began, they were sampled soon enough to rep-resent accurate concentrations that most beetles would have experienced. Themajority of attacks probably occurred in May (Daterman et al., 1965) (weobserved no attacks on April 13), only 30-35 days preceding our samplingdate. Furthermore, the relative differences in ethanol concentrations amongNovember-, January-, and March-felled Douglas fir logs were the same whensampled in March (Kelsey, 1994a), or May (Kelsey, unpublished data). There-fore, in this experiment it is unlikely that relative ethanol concentrations amongspecies changed between the initial beetle attacks and the time we sampled thelogs.

These ambrosia beetles, however, showed no preference between Douglasfir and western hemlock even though the Douglas fir logs contained 2.1 times(averaged across positions) more ethanol. Zhong and Schowalter (1989) alsoobserved equal ambrosia beetle attack densities when these two conifer specieswhere positioned near one another. Explanations could be a reduced beetleresponse to ethanol or repellence above some threshold concentration. There isevidence to support these responses (Montgomery and Wargo, 1983; Kelsey,


1994a). In particular, during olfactometer bioassays, Moeck (1970) noted thathigh ethanol concentrations were strongly attractive to T. lineatum initially, butthe beetles then lost their ability to discriminate odors in subsequent runs.

It is also possible that the ethanol release rates from the exposed surfacesof Douglas fir and western hemlock logs were not as different as the ethanolconcentrations measured in their tissues, because of interfering factors. Theouter bark of these two species differs in thickness and morphology, and thismight influence both the quantity and dispersal of the ethanol released. Differ-ences in release rates could be most important during the initial phases of theattack, becoming less important as beetle entrance holes open up the tissues andenhance vaporization.

An ethanol-pheromone interaction also could have interfered with the bee-tles' response to Douglas fir and hemlock. The initial pioneering beetles mighthave responded more strongly to the high ethanol concentrations in Douglas firlogs when pheromone concentrations were low, but, as the attack densities andthe subsequent release of pheromone increased, their ability to discriminate highethanol concentrations could have been modified. In western North America, ahigh ethanol-to-lineatin release ratio (250,000:1 per day) significantly decreasedthe trap catches of T. lineatum compared to traps baited with lineatin alone,while in England, traps with a lower ethanol-to-lineatin release ratio (25,000:1per day) caught more beetles than traps with just the pheromone (Borden et al.,1982).

a-Pinene in Douglas fir did not enhance the beetles' preference for theselogs over western hemlock, so apparently a-pinene was not functioning as anessential kairomone or as a synergist with ethanol or ethanol + pheromoneduring host selection in this experiment. There are several examples where therelease of or-pinene with ethanol + pheromone either did not increase trapcatches of these ambrosia beetles or resulted in decreased trap catches comparedto traps baited with ethanol -I- pheromone (Borden et al., 1980, 1982; Salomand McLean, 1988; Liu and McLean, 1989). In Europe, a synergistic responseby T. lineatum to ethanol + a-pinene is dependent on the ratio of their releaserates (Schroeder, 1988; Schroeder and Lindelow, 1989). The relative releaserates of ethanol and a-pinene from our logs may have been influenced by thewet and cold spring weather. The volatilization of a-pinene would probably bereduced more by these conditions because of a lower water solubility and lowervapor pressure than ethanol. If the release of a-pinene from Douglas fir wasminimal, there would be no synergistic beetle response.

The high concentrations of a-pinene and other terpenoids (Snajberk et al.,1974; Snajberk and Zavarin, 1976; Kelsey unpublished data) in the tissues ofDouglas fir and western red cedar might have reduced the beetles' response tothese species. In olfactory bioassays T. lineatum was repelled by a-pinene vapors(Moeck, 1970). The application of terpenoid mixtures, such as turpentine oil


(Nijholt, 1973) or pine oil (Nijholt, 1980; Dubbel, 1992), on log surfaces candelay attack and significantly reduce attack densities.

It is unlikely that acetaldehyde was functioning as a kairomone. In a pre-vious experiment, acetaldehyde concentrations were higher than ethanol con-centrations in November-felled Douglas fir logs when sampled in May (Kelsey,1994b), and the analysis suggested it might have contributed to the variation inG. retusus attack densities. Here, acetaldehyde exhibited essentially the samerelationship as ethanol among the conifer species, but the ethanol concentrationswere 26-101 times higher. Secondly, from our covariate analysis, none of thevariation in ambrosia beetle attack densities was explained by the acetaldehydeconcentrations. Furthermore, acetaldehyde failed to attract any scolytid beetlesto traps in the field (Montgomery and Wargo, 1983), or to attract T. lineatumin olfactometer bioassays (Moeck, 1970).

The logs used in this experiment were relatively short and provided thebeetles with a limited area to attack. Consequently, the beetle densities in theselogs could potentially reach saturation levels and not increase further, eventhough additional beetles were available to attack. If this were the cause ofsimilar beetle densities on Douglas fir and western hemlock logs, these valuesmay not accurately estimate kairomone-mediated preference. However, the bee-tle populations in these logs were not at saturation levels, since others havereported much higher attack densities for both T. lineatum (Kinghorn, 1957b;Dyer, 1963; Lindgren et al., 1982; Zhong and Schowalter, 1989) and Gnath-otrichus spp. (McLean and Borden, 1977; Liu and McLean, 1993; Kelsey,1994a,b).

The ambrosia beetles preferred to attack the sides of logs in each coniferspecies, although the ethanol concentrations on log tops were equal to or greaterthan on the sides. Freshly emerged T. lineatum are so strongly photopositivethey do not respond to host kairomones (Graham, 1959), but during flight theyswitch to a photonegative response and become chemosensitive (Graham, 1959;Bennett and Borden, 1971). In our experiment the photonegative response dom-inated the attraction to kairomones on the log tops. Trypodendron lineatum avoidthe tops of logs when the logs are exposed to light (Dyer, 1963; Lindgren etal., 1982), but they are less discriminating when the logs are in the shade of aforest canopy (Dyer, 1963; Kelsey, 1994a,b). Both G. retusus and G. sulcatushave been reported to avoid log tops (McLean and Borden, 1977; Lindgren etal., 1982; Kelsey, 1994a,b).

In conclusion, when Douglas fir, western hemlock, and western red cedarlogs were left in the forest through winter and spring, they synthesized andaccumulated different concentrations of ethanol. Lower ambrosia beetle attackdensities on western red cedar, compared with Douglas fir and western hemlock,were most likely a consequence of lower ethanol concentrations in western redcedar. The ambrosia beetles were unable to effectively discriminate between the


high and moderate ethanol concentrations in Douglas fir and western hemlock,respectively, for a number of possible reasons. a-Pinene did not appear tofunction as an ambrosia beetle kairomone under the conditions of this experi-ment. Beetles did not respond synergistically to logs containing a-pinene incombination with ethanol or ethanol + pheromone, and a-pinene may havefunctioned as a deterrent during host selection.

Acknowledgments—We thank Oregon State University, College of Forestry, for supplying thelogs and allowing access to the Blodgett study site. We thank M. Rector for his assistance in findingthe site and securing the logs, E. Gerson for technical assistance in the field and laboratory, T.Sabin for advice on the statistical analysis, and G. Parsons for beetle identification. We also thankDr. C. Niwa and Dr. S. M. Salom for reviewing the manuscript. The use of trade names is for theinformation and convenience of the reader and does not constitute official endorsement or approvalby the US Department of Agriculture.

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