Ethanol and Ambrosia Beetles in Douglas Fir Logs Exposed or Protected from Rain

Journal of Chemical Ecology, Vol. 25, No. 12, 1999

ETHANOL AND AMBROSIA BEETLES IN DOUGLASFIR LOGS EXPOSED OR PROTECTED FROM RAIN

RICK G. KELSEY1,* and GLADWIN JOSEPH2

1PNW Research StationForestry Sciences Laboratory

3200 Jefferson Way, Corvallis, Oregon 973312Department of Forest Science

Oregon State UniversityCorvallis, Oregon 97331

(Received March 30, 1999; accepted August 15, 1999)

Abstract—Logs from the base of Douglas fir (Pseudotsuga menziesii) trees cutin October 1993 were randomly assigned to one of three treatment groups: (1)wet logs—cut from the fallen tree and left exposed to rain, (2) dry logs—cutfrom the fallen tree, placed on blocks, and protected from rain under a plastictent, and (3) crown logs—left attached to the fallen tree with its branches intactand exposed to rain. The following May, ethanol concentrations were highest inthe phloem and sapwood of wet logs (0.24 and 0.35 Umol/g fresh wt, respec-tively). Ethanol concentrations in tissues from dry and crown logs were similarto each other (ranging from 0.002 to 0.03 Umol/g fresh wt), but were signifi-cantly lower than in wet logs. It appears that rain absorbed by the outer bark ofwet logs creates a barrier to gas exchange between living tissues and the atmo-sphere, which facilitates the development of hypoxic conditions necessary forethanol synthesis and accumulation. Branches on crown logs exposed to rainhelp maintain low ethanol concentrations in the log tissues; we discuss severalpotential mechanisms to explain this response. By early September, the densitiesof Gnathothrichus spp. gallery entrance holes were high on wet logs (21.5/m2)and low on dry (2.5/m2) and crown logs (5.8/m ), indicating their preferencefor logs with higher ethanol concentrations. Protecting logs from rain will sig-nificantly reduce ethanol concentrations and the density of ambrosia beetle gal-leries. Leaving branches attached to logs will produce similar results, but itseffectiveness may vary depending on the environmental conditions. Host selec-tion by secondary scolytid beetles that use ethanol as a kairomone can be manip-ulated and possibly managed by controlling the production of ethanol in the hostresource.

Key Words—Pseudotsuga menziesii, Gnathothrichus spp., ethanol, hostselection, anaerobic respiration, fermentation, Coleoptera, Scolytidae.

*To whom correspondence should be addressed.

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0098-0331/99/1200-2793$ 16.00/0 © 1999 Plenum Publishing Corporation

INTRODUCTION

Insects that colonize dying or recently dead coarse woody debris are ecologi-cally important in initiating decomposition and nutrient cycling, thus promotinglong-term productivity and stability of forest ecosystems (Harmon et al., 1986;Schowalter et al., 1992; Schowalter and Filip, 1993). However, these insectscan also become pests of economic importance. For example, ambrosia beetlesare detritivores, but their attacks can greatly decrease the value of commerciallogs or lumber (McLean, 1985). Bark beetles and weevils can cause damage toseedling and sapling regeneration or can vector spores of pathogens from dis-eased stumps to nearby healthy trees (Harrington et al., 1985; Witcosky et al.,1986a,b; Ciesla, 1988; Bedard et al., 1990; Nevill and Alexander, 1992a-c). Var-ious forms of woody residues are being retained in forests in greater quantitiesthan in the past to maintain and facilitate important ecological processes andfunctions (Schowalter and Filip, 1993). Stand thinning is being used more fre-quently to achieve desired vegetative patterns and management objectives, butit may also provide woody residues that can aggravate insect and disease prob-lems.

Tissues in dying or recently dead woody residues will typically synthesizeand accumulate some ethanol as they age (Moeck, 1970; Sjodin et al., 1989;MacDonald and Kimmerer, 1991; Kelsey, 1994a,b; von Sydow and Birgersson,1997; Kelsey and Joseph, 1999). If it escapes into the atmosphere, ethanol canfunction as a primary host attractant for various insect species that colonize andreproduce in the woody substrate (Moeck, 1970; Klimetzek et al., 1986; Nord-lander et al., 1986; Witcosky et al., 1987; Chenier and Philogene, 1989; Liuand McLean, 1989). For many conifers, the simultaneous release of A-pinene,or mixtures of their terpenes, and ethanol may further enhance or synergizeinsect responses compared to ethanol alone (Tilles et al., 1986; Nordlander, 1987;Raffa and Hunt, 1988; Chenier and Philogene, 1989; Schroeder and Lindelow,1989; Phillips, 1990; Lindelow et al., 1993). If the insect produces a pheromone,ethanol may also enhance its attractiveness, with or without the presence of A-pinene (Pitman et al., 1975; Borden et al., 1980; Shore and McLean, 1983; Liuand McLean, 1989).

Ethanol accumulation in woody tissues is dependent on the rates of synthe-sis, movement, and possibly metabolism into other compounds when the supplyof O2 is adequate (MacDonald and Kimmerer, 1993). Each of these processesmay be influenced by internal and external factors, thus causing the amount ofethanol in woody debris to vary significantly. Under similar environmental con-ditions ethanol concentrations may vary among tree species or among positionswithin logs (Kelsey 1996; Kelsey and Joseph, 1997). When branches remainattached to logs, the ethanol concentrations will be lower than in branchlesslogs (Kelsey, 1994b). Season of harvest can affect subsequent ethanol concen-

2794 KELSEY AND JOSEPH

trations. For example, logs cut in November produced more ethanol in their firsttwo months after harvest than logs cut in January (Kelsey, 1994a).

Since ethanol is a primary attractant or kairomone for the various speciesof bark and ambrosia beetles described above, a better understanding of the fac-tors regulating ethanol accumulation in woody tissues could lead to alternativemanagement strategies to help mitigate undesirable behavior of these insects. Ina previous experiment, rain was identified as an environmental factor that couldpotentially influence ethanol synthesis and accumulation in logs (Kelsey, 1994a),but it required further verification. The objectives of this study were to determinewhether exposure to rain affects ethanol concentrations in aging log tissues andwhether the ethanol concentrations influence which logs ambrosia beetles willselect and colonize as a host.

METHODS AND MATERIALS

Study Site. The study was located in the McDonald Experimental Forest(Oregon State University) near Corvallis, Oregon (44°39'N; 123°16'W), in aDouglas fir [Pseudotsuga menziesii (Mirb.) Franco] stand on the northwest sideof a ridge at 1240 m elevation and slope ranging from 15 to 32%. This areahad been harvested in the late 1920s or early 1930s, replanted with Douglasfir in 1936, and thinned in the 1970s and 1980s. Our study was set up duringa commercial thinning in September and October 1993. After thinning, therewere 203 dominant or codominant Douglas-fir per hectare with a diameter of25.4 cm or greater, with 55% having a diameter of 40-56 cm. Canopy closurewas estimated at 78%.

Treatments. Eight blocks were located along a 200-m line, with variabledistances between blocks. Three trees per block were cut on October 20, 1993.Ends of logs were examined and any portion with damaged or missing bark inthe first 2 m was removed. Each log was tagged and randomly assigned to oneof three treatments: (1) wet log—a log cut from the base of a felled crown andleft exposed to rain through winter; (2) dry log—a log cut from the base of afelled crown and placed under a plastic tent through winter to protect from rain;and (3) crown log—a log that remained attached to the base of a felled crown(with branches intact) and exposed to rain through winter. Wet and dry logs (183cm in length) were removed from the base of cut trees and repositioned closeto the unmovable crown logs. They were placed lengthwise along the contourwith their top side level. Most crown logs were also perpendicular to the slope.Within blocks, the final distance between logs varied from 2 to 10 m. Unusedcrown portions were removed from the site.

Dry logs were placed on wooden blocks to eliminate direct contact withthe soil and a plastic tent was constructed over them on October 26 or 27, 1993.

ETHANOL, AMBROSIA BEETLES, AND DOUGLAS FIR 2795

Two rectangular frames (3.66 x 1.52 m) made with 2.54 m (outside diam.) plasticPVC pipe were bound together with plastic bands and positioned lengthwise overthe log as an A-frame. A sheet of clear plastic (6 mil) was stretched over theframe and secured with waterproof tape. On both sides of the tent there was a 30-cm opening between the bottom edge of the plastic sheet and the frame to allowfor air movement along the log sides. The tent extended 92 cm beyond each endof the log so that air or insects could enter, yet the logs remained dry. Tents werestaked to the ground and left in place through winter. They were removed onMay 6, 1994, three days after the first ambrosia beetle holes were found in a wetlog. Tents were replaced during periods of rain from May 15-18, May 31-June 9,June 13-17, and then removed permanently on June 17. Temperatures in the tentswere not measured, but probably did not increase much above ambient becausethe tents were shaded under the canopy and received limited solar radiation witha northwest exposure.

Rainfall Measurements. On November 8, 1993, one rain gauge (15.5 cmdiam. and 16.5 cm deep) was placed in each block near the wet log treatment.A small amount of oil was placed in each gauge to float over the water andminimize evaporation. The water was collected periodically, measured with agraduated cylinder, and converted into centimeters of rainfall.

Analysis of Volatiles and Water. Log tissues were sampled twice during theexperiment to measure ethanol concentrations and water contents. Samples werefirst collected on May 24 and 25, 1994, three weeks after the initial ambrosiabeetle gallery holes were observed in a wet log. The second set of samples wastaken on August 25, 1994, about two weeks before the densities of ambrosiabeetle galleries were counted. Three cores were taken along the top of each logwith an increment borer (5 mm diam.), one at 36-40 cm from each end, orthe equivalent distance on crown logs, and another at the log center. Phloemwas separated from the sapwood (1.0 cm depth) and the three pieces of eachtissue type combined in a sealed vial (13 x 45 mm) as a single sample. Thiswas repeated along both sides about midway between the log top and soil level,yielding three samples of phloem and three samples of sapwood per log. Vialswere frozen with Dry Ice, returned to the laboratory, and stored at -36°C untilanalyzed. Core holes were plugged with corks. August cores were taken at adistance of 4-6 cm from previous holes.

Ethanol and A-pinene were analyzed as previously described (Kelsey,1994b; Kelsey and Joseph, 1998) by a multiple headspace extraction proce-dure with a Perkin Elmer HS40 headspace autosampler connected to a HewlettPackard 5890 gas chromatograph. Ethanol was quantified with a standard dilutedin water. A-Pinene was quantified in May tissue samples only, using a standarddiluted in methanol. After analysis, the vials were uncapped and samples diredat 102°C for 16 hr, desiccated for 30 min, and weighed to determine dry weightsand water content.


Carbohydrate Analysis. Samples of log tissues were collected on October25 and 26, prior to constructing the tents, and then again on May 26 and 27 todetermine concentrations of starch and soluble sugars. One core (19 mm wide)was taken about 31 cm from the cut ends, or the equivalent distance on crownlogs with a specially adapted bit connected to a power drill. Phloem and sap-wood (1.0 cm depth) were separated and each tissue type combined in plasticbags, frozen with Dry Ice, and stored at -36°C until analyzed. Core holes wereplugged with corks. May samples for carbohydrates were taken near the coresremoved for ethanol analysis.

Concentrations of starch and soluble sugars were measured for dupli-cate samples of ground (60 mesh) and dried (100°C) tissues. Soluble sugarswere extracted with methanol-chloroform-water (2:5:3) (Haissig and Dickson,1979; Rose et al., 1991), then measured colorimetrically (Blakeney and Mutton,1980) after hydrolyzing sucrose. The extracted tissue was redried, the starchhydrolyzed enzymatically, and then quantified colorimetrically as described byRose et al. (1991, enzyme method 2). Additional details for this procedure arepresented in Kelsey et al. (1998).

Densities of Ambrosia Beetle Gallery Holes. From September 7 to 9 barkwas peeled from logs and the entrance holes to ambrosia beetle galleries in thesapwood counted in a 25- x 128-cm quadrat positioned lengthwise along the logtop starting 30 cm in from the base end. A similar quadrat was positioned andcounted on each of the two sides. The diameter of each gallery entrance hole waschecked with the end of a No. 53 wire gauge drill bit to separate Trypodendronlineatum galleries from the Gnathotrichus spp. galleries (Kinghorn, 1957).

Statistical Analysis. The data were analyzed with SAS software (SAS Insti-tute, 1989, 1996). Phloem and sapwood were each analyzed separately forethanol, water, and carbohydrate concentrations. Ethanol and water concentra-tions were analyzed as a split-plot design with treatment as the main plot andmonth as subplot. Soluble sugars and starch concentrations were analyzed sep-arately for October and May as a one-way ANOVA with treatment as the maineffect. Changes in concentrations of soluble sugars and starch between Octoberand May were analyzed as a one-way ANOVA. Most May samples containedno starch and therefore were not analyzed statistically. A-Pinene concentrationswere analyzed as a strip-plot with treatment as the main plot and tissue as a strip(Petersen, 1985). Data were tested for homogeneity of variance and normalitybefore analysis and natural log-transformed when necessary. Back-transformedmeans and standard errors are presented for transformed data. Significant differ-ences among means were identified by Fisher's protected LSD at A = 0.05.

Regression analyses were performed separately for wet and crown logs toexamine the relationships between rainfall and tissue water content, rainfall andtissue ethanol concentrations, water contents between tissues, ethanol concen-trations between tissues, and the relationship between water content and ethanol


concentrations within tissues. Regression analysis also was performed for beetlenumbers as a function of ethanol concentrations in May and August across alltreatments. Phloem and sapwood ethanol concentrations were averaged for thisanalysis.

RESULTS

Ethanol concentrations in the phloem and sapwood were dependent onlyon the treatment, with no interaction between months (Figure 1A and B). Theethanol concentration in phloem from wet logs was 13.3 and 13.7 times higherthan in dry or crown logs, respectively (both P values < 0.001), with no dif-ference between the latter two (P = 0.972). Similarly, the ethanol concentrationin sapwood from wet logs was 31.3 times higher than in sapwood of dry logs(P < 0.001) and 6.9 times higher than sapwood of crown logs (P = 0.021). Sap-wood of dry logs and crown logs had similar quantities of ethanol (P = 0.084).

FIG. 1. Ethanol concentrations and water contents in the phloem and sapwood of wet,dry, and crown logs sampled in May and August 1994. Values are means + SE. Insertlists P values for the ANOVA, M = month, T = treatment.


ETHANOL, AMBROSIA BEETLES, AND DOUGLAS FIR

FIG. 2. A-Pinene concentrations in wet, dry, and crown logs sampled in May 1994. Valuesare means ± SE. Insert lists P values for the ANOVA, T = treatment, Ti = tissue.

A-Pinene concentrations in May were dependent only on the tissue type and notthe treatment (Figure 2). Sapwood contained 2.8 times more A-pinene than thephloem (P < 0.001). Mean log diameters measured at mid-length were 45.7,46.6, and 48.5 cm for dry, wet, and crown logs, respectively, with no significantdifference among treatments (P - 0.475).

Rainfall through the canopy from November 8, 1993, to May 23, 1994, was222.3 ± 18.3 cm (mean ± SE) near the wet logs. From May 24 to August 24,the rainfall was 17.2 ± 15.7 cm. Water content in the phloem was dependenton the treatment and time of year (Figure 1C), because of changes associatedwith crown logs. In May, the phloem of crown logs contained 24.8 and 20.9%less water than phloem of wet (P = 0.002) or dry logs (P = 0.011), respectively,with no difference between the latter two (P = 0.491). Water in the phloem ofcrown logs increased by about 20% between May and August (P < 0.001) withno changes occurring in the other treatments (both P values >0.400). Conse-quently, by August phloem water contents were not different among the threetreatments (all P values >0.063). Sapwood water contents were dependent onlyon the treatment, with no interaction between months (Figure 1D). Sapwoodfrom crown logs contained 68.4 and 73.1% of the water in sapwood from wetor dry logs, respectively (both P values <0.001). Wet and dry logs had similarsapwood water contents (P = 0.252).

Phloem and sapwood water contents from wet and crown logs were notrelated to the amount of rainfall. In crown logs, sapwood water content was

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exponentially related to phloem water content (y = e(3.27+0.0057x); r2 =0.56, p =0.032), but in wet logs there was no relationship. The ethanol concentration inphloem from wet logs was exponentially related to rainfall (y = e-

2384+0.1009x;

r2 = 0.79, P = 0.003), and unrelated for crown logs. Sapwood ethanol concentra-tions were not related to rainfall in either treatment. The ethanol concentrationin sapwood of wet logs was positively related to concentrations in the phloem(y = 0.48 + 0.57x; r2 = 0.98, P < 0.001), while there was no relationship incrown logs.

Initial starch concentrations within tissues were similar for all treatments inOctober (Table 1). By the following May, the only starch remaining was in thesapwood of crown logs that retained about half the original amount. Changesin sapwood starch concentrations during the winter were greater in wet and drylogs compared to crown logs (both P values <0.001).

Total soluble sugar concentrations in October were much greater than starchin both the phloem and sapwood of all logs, with the highest quantities in thephloem (Table 1). The only difference for initial soluble sugar concentrations inphloem occurred between dry and crown logs (P = 0.028). In May, the solublesugars remaining in the phloem of crown logs were 18 and 34% higher than indry (P = 0.027) or wet (P < 0.001) logs, respectively, with no difference betweenthe latter (P = 0.063). The change in soluble sugar concentration over winter wasgreater in the phloem of wet logs than in dry (P - 0.009) or crown logs (P =0.012), with no difference between the dry and crown (P = 0.904). Sapwoodof crown logs contained 26 and 32% more soluble sugars initially in Octoberthan either the wet or dry logs, respectively (both P values < 0.006), with nodifference between the latter two (P = 0.570). By May, the sapwood from crownlogs contained 6.2 and 2.6 times more soluble sugars than the wet (P < 0.001)or dry (P = 0.052) logs, respectively, with no difference between the latter (P -0.067). The change in sapwood soluble sugar concentration during winter wasgreater in wet logs than crown logs (P - 0.033), with no difference between wetand dry (P = 0.490) or dry and crown logs (P = 0.138).

Trypodendron lineatum and Gnathotrichus retusus were excavated andidentified from pieces of logs on the study site. Although no G. sulcatus wereexcavated, our sample size was not large enough to assure their absence withconfidence, and therefore we did not designate species within Gnathotrichus.Trypodendron lineatum galleries were found only in one wet treatment log andnot analyzed statistically. In early September the density of Gnathotrichus spp.gallery entrance holes (Figure 3) in wet logs was 8.6 times higher than in drylogs (P = 0.001) and 3.7 time higher than in crown logs (P = 0.011), with nosignificant difference between the latter two treatments (P = 0.252). The den-sities of beetle galleries in September were significantly related to log ethanolconcentrations in May (Figure 4), but not the concentrations in August.



FIG. 3. Densities of Gnathotrichus spp. gallery entrance holes in the wet, dry, and crownlogs sampled in September 1994. Values are means ± SE.

FIG. 4. The relationship between log ethanol concentrations (averaged for the phloem andsapwood) in May 1994 and the densities of Gnathotrichus spp. gallery entrance holes inSeptember 1994.

DISCUSSION

Higher ethanol concentrations in wet logs than in dry logs demonstratesthat rain contributes to the synthesis and accumulation of ethanol in coarsewoody residues, which probably results from water interfering with gas exchangebetween respiring log tissues and the atmosphere. Tree tissues typically maintainconstitutive levels of anaerobic enzymes (Kimmerer and Stringer, 1988; Harryand Kimmerer, 1991) that can quickly synthesize ethanol if O2 becomes limited.For example, the phloem (with cambium) from Douglas-fir stems will producehigh ethanol concentrations (58-79 Umol/g dry wt) in 4 hr when sealed in a vialwith an anoxic N2 atmosphere (Kelsey et al., 1998). The phloem will also pro-duce ethanol (6-11 Umol/g dry wt) within 4 hr if the vial contains air, becauseaerobic respiration reduces the O2 concentration low enough to initiate ethanolsynthesis. Similarly, ethanol synthesis is initiated in the cambium of eastern cot-tonwood (Populus deltoides Bartr.) when whole logs are exposed to low oxygenlevels (3-7%) for 6 hr (MacDonald and Kimmerer, 1991). Consequently, wesuspect that rain on logs creates a barrier to gas exchange with the atmosphere,and the O2 concentrations decrease as aerobic respiration proceeds in the tissues.The more frequently that rain interferes with the gas exchange, or the longer thata water barrier remains, the higher the ethanol concentrations produced.

While ethanol synthesis occurred in the phloem, cambium, and sapwood ofwet logs, it appears that the water barrier occurred in the outer bark and not inthese underlying tissues, for two reasons. First, there was no relationship betweenrainfall and water contents in the phloem or sapwood, yet there was a relationshipbetween rainfall and ethanol concentration in the phloem. Second, the phloem andsapwood from wet logs produced substantially higher ethanol concentrations thanin dry logs, but their tissues had similar water contents in May. This suggests thatthe phloem and sapwood in wet and dry logs had the same water contents through-out the winter months because both log treatments were exposed to the same atmo-spheric vapor pressure deficits. These observations are consistent with water fill-ing the air spaces in outer bark and creating a barrier to gas exchange. Since outerbark water content can fluctuate rapidly depending on rainfall patterns and evapo-ration (Harmon and Sexton, 1995), the formation and existence of a water barrieris probably a dynamic process that could contribute, in part, to the large variationin ethanol concentrations observed among logs subjected to similar environmentalconditions in this and in other work (Kelsey, 1994a,b).

Crown logs were exposed to the same winter rainfall as wet logs, but in Maythey were drier and contained much lower ethanol concentrations than wet logs,as observed in other work (Kelsey 1994b). Water was apparently being trans-ported up the stem and evaporating through the needles. This movement proba-bly caused phloem water to move more rapidly into the sapwood, because therewas a relationship between phloem and sapwood water contents in crown logs,


but not wet logs. Between May and August the phloem water content of crownlogs increased with summer rains, probably because water movement stoppedwhen the needles dried.

Water movement in crown logs might affect ethanol accumulation in severalways. It could disrupt the formation of a water barrier or reduce its functionallongevity, thus allowing the tissues to remain aerobic and synthesize less ethanol.If rain absorbed in crown logs moves quickly through the outer bark into thephloem and then up the sapwood into the crown, the water may carry somedissolved oxygen into the tissues and help minimize hypoxia. Increased tensionin the sapwood water column, as a result of continued evaporation loss fromthe crown, also can cause the water columns to cavitate and the tracheids to fillwith air (Tyree and Sperry, 1989), which will contain oxygen if it enters fromthe cut ends of logs. This could improve the gas exchange in sapwood adjacentto the cambium and phloem, thus minimizing ethanol synthesis. Alternatively,the tissues of crown logs may have produced the same quantity of ethanol aswet logs, but the ethanol was transported with water up the stem and into thecrown where it was metabolized in needles or other tissues that are more aerobic(Jayasekera et al., 1990; MacDonald and Kimmerer, 1993).

Ethanol concentrations in Douglas fir from the McDonald Forest were verylow compared to those in logs at a site near the Columbia River (118.5 and111.8 Umol/g fresh wt in phloem and sapwood, respectively) about 175 kmnorth (Kelsey and Joseph, 1997). Such large differences in ethanol concentra-tions between sites may be caused primarily by rainfall and solar radiation.McDonald Forest is normally drier than the area near the Columbia river. Inaddition, the logs in McDonald Forest were located on the northwest side ofa ridge beneath a canopy with 78% closure. In addition to intercepting rain-fall, the canopy shaded the logs and kept them cooler. In contrast, logs near theriver were located on a ridge, at the top of a clear-cut with no cover and maxi-mum exposure to rainfall and solar radiation. The amount of solar radiation isimportant because it affects tissue temperatures and metabolic rates (Sprugel etal., 1995). As temperature and aerobic respiration rates increase, tissue O2 levelmay be depleted faster if gas exchange with the atmosphere is impaired. Further-more, once ethanol synthesis begins, the rate will also increase as temperaturesrise (authors' unpublished data). The combination of high solar radiation in con-junction with a water barrier to gas exchange probably represents the optimumcondition for maximizing ethanol synthesis and accumulation in logs.

The effectiveness of leaving branches or crowns on logs as a means ofmitigating ambrosia beetle gallery densities will probably vary with the envi-ronmental conditions. We have not examined ethanol accumulation in branchedlogs with full exposure to rain and solar radiation. Under these conditions theywould probably produce more ethanol than measured here, thus making themsusceptible to attack by a greater number of ambrosia beetles.


The importance of ethanol as a host attractant for the Gnathotrichus spp.is evident from their selection of logs with higher ethanol concentrations. Thisresponse is consistent with previous observations with logs (Kelsey, 1994a,b;Kelsey and Joseph, 1997) and the observations of others, who used chemicalbaits (Liu and McLean, 1989; Salom and McLean, 1990). Although the logscontained very low ethanol concentrations, they were still susceptible to someattack by Gnathotrichus spp., and the beetles retained their ability to discriminatebetween different ethanol concentrations. A-Pinene by itself did not contribute tothe beetles' selection because all logs had the same A-pinene concentrations inMay. In addition, western hemlock and Douglas fir logs have been observed withsimilar densities of Gnathotrichus spp. galleries, but the western hemlock tis-sues contained minimal quantities of A-pinene compared to Douglas fir (Kelseyand Joseph, 1997). Using baited traps, Liu and McLean (1989) concluded thatA-pinene was not a primary host attractant or a synergist for the aggregationpheromones of G. sulcatus or G. retusus. Our results support this conclusion.

The densities of ambrosia beetle galleries in logs from this experiment werelow compared to our other studies (Kelsey 1994a; Kelsey and Joseph, 1997),probably in part because the surrounding beetle populations were low as a con-sequence of limited host material for them to colonize in the adjacent forest priorto the experiment. Furthermore, our study site represented only a small portionof the total area thinned, so there was abundant host material for the existingbeetles to colonize, thus diluting the gallery densities. Ethanol concentrationsin the logs also may not have been high enough to attract greater numbers ofbeetles.

Carbohydrates are the substrate for both aerobic and anaerobic respiration.Prior to the detection of ethanol in aging logs (Moeck, 1970; Cade et al., 1970),ambrosia beetles were shown to be attracted to logs with a low starch or nostarch content, but not all logs without starch attracted beetles (Chapman et al.,1963; Chapman and Dyer, 1969). Our results provide an explanation for theseearly observations. If the starch and soluble sugars in log tissues are depletedprimarily by aerobic respiration, as in dry logs, then ethanol synthesis wouldbe minimal and ambrosia beetles will not be attracted. If a significant portionof the starch and soluble sugars are metabolized by anaerobic respiration, as inwet logs, then ethanol will accumulate and attract beetles. Crown logs in Mayretained more starch in the sapwood and more soluble sugars in the phloem andsapwood than wet or dry logs, suggesting that needles on the crown may havecontributed some photosynthate and reduced the net depletion of carbohydratesin log tissues.

In conclusion, rain strongly influences the accumulation of ethanol in aginglog tissues, probably by creating a barrier to gas exchange between respiring logtissues and the atmosphere. Ethanol concentrations can be minimized by keep-ing the logs dry, or leaving their branches attached, but the effectiveness of the


latter may vary depending on environmental conditions. Although log ethanolconcentrations were relatively low compared to another experiment (Kelsey andJoseph, 1997), Gnathotrichm spp. ambrosia beetles were still able to discrimi-nate between logs containing different quantities of ethanol, with a preferencefor high concentrations. Gallery densities of Gnathotrlchus spp. can be mini-mized by limiting ethanol synthesis and accumulation in logs tissues. Manipulat-ing ethanol synthesis in logs and coarse woody materials may be useful in devel-oping management alternatives for controlling undesirable behavior of ambrosiabeetles and other forest insects attracted to ethanol.

Acknowledgments—We thank Oregon State University, College of Foresty, for access to ourstudy site in the McDonald Experimental Forest. We thank Liz Gerson for her assistance in thefield and Drs. B. A. Caldwell, D. Overhulser, and R. M. Callaway for comments and review of themanuscript. The use of trade names is for the information and convenience of the reader and doesnot constitute endorsement or approval by the U.S. Department of Agriculture.

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Ethanol and Ambrosia Beetles in Douglas Fir Logs Exposed or Protected from Rain

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