Byers, J.A. and Wood, D.L. 1980. Interspecific inhibition of the response of the bark beetles, Dendroctonus brevicomis and Ips paraconfusus, to their pheromones in the field. Journal of Chemical Ecology 6:149-164. pdf

I'm preparing to climb a ponderosa pine tree and put synthetic pheromone components on the trunk (baiting).
Abstract-- Ponderosa pine logs infested with Ips paraconfusus males inhibited the attraction of Dendroctonus brevicomis in the field to either attractive logs cut from a ponderosa pine tree under attack by D. brevicomis or to their synthetic pheromones, exo-brevicomin, frontalin, and myrcene. Logs cut from trees under attack by D. brevicomis inhibited the response of l. paraconfusus to logs infested with male I. paraconfusus. Exo-brevicomin, frontalin, and myrcene did not inhibit their response but verbenone did. Verbenone was found in male D. brevicomis dissected from attractive logs under attack during the same time the response of I. paraconfusus was inhibited by these logs. Trans-verbenol and exo-brevicomin were found in female D. brevicomis while verbenone, trans-verbenol, and frontalin were found in male D. brevicomis in relatively large amounts near the beginning of the aggregation phase of host colonization. All of these compounds had decreased at a similar rate 5 days later. This gradual decrease in exo-brevicomin and frontalin probably caused the observed reduction in attraction. The ecological significance of these compounds in relation to termination of the aggregation phase of D. brevicomis and reduction of interspecific competition is discussed.

Key Words-Dendroctonus brevicomis, Ips paraconfusus, Pinus ponderosa, bark beetle, exo-brevicomin, frontalin, myrcene, verbenone, trans-verbenol, attractants, inhibition, pheromones.

INTRODUCTION
The western pine beetle, Dendroctonus brevicomis LeConte, and the California 5-spined ips, Ips paraconfusus Lanier, colonize the phloem and cambium tissues of ponderosa pine, Pinus ponderosa Laws., during the same time periods in California. Once the beetles begin feeding, pheromones and other volatile compounds accumulate in their hindguts and are excreted with fecal material in the frass (Wood and Bushing, 1963; Pitman and Vité, 1963; Pitman et al., 1965; Wood et al., 1966; Renwick, 1967; Silverstein et al., 1966a, 1968). The pheromone components of D. brevicomis are exo-brevicomin (E) (female produced), frontalin (F) (male produced), and myrcene (M) (released by wounding of the host resin system) (Silverstein et al., 1968; Kinzer et al., 1969, Pitman et al., 1969). The attractive pheromone components of I. paraconfusus are ipsenol (male produced), cis-verbenol (produced by both sexes), and ipsdienol (male produced) (Silverstein and Rodin, 1965; Silverstein et al., 1966a, b; Wood et al., 1966; Renwick et al., 1976). The three pheromone components of each species must be released simultaneously for maximum activity in attracting both sexes in laboratory and field bioassays (Wood et al., 1976, 1967, 1968; Bedard et al., 1969, 1970, 1980a; Silverstein et al., 1968; Vité and Pitman, 1969a, b; Wood, 1970).

The female D. brevicomis initiates the entrance tunnel and is joined by a male, usually within a day or two (Miller and Keen, 1960). The males of emerged D. brevicomis are reported to contain verbenone in their hindguts (Renwick, 1967; Vité and Renwick, 1970; Renwick and Vité, 1970) which is released (Libbey et al., 1974; Browne et al., 1979) and inhibits the beetles' response to pheromones (Renwick and Vité, 1970; Hughes and Pitman, 1970; Wood, 1972; Bedard et al., 1980b). Verbenone has been proposed to cause beetles to terminate aggregation when the tree is fully colonized (Renwick and Vité, 1970; McNew, 1970). By contrast, in I. paraconfusus the male initiates the attack and is then joined usually by 3 females. Intraspecific compounds that inhibit the attractive response of I. paraconfusus have not been reported, and the cessation of the aggregation phase may simply result from a reduction in pheromone production (Pitman et al., 1966).

Both species frequently inhabit the same ponderosa pine. However, D. brevicomis usually begins colonization in the lower trunk and lower level of the crown while I. paraconfusus usually initiates attacks in upper areas of the crown (Struble and Hall, 1955; Miller and Keen, 1960). The proportion of the tree occupied by either species varies considerably, but the area of their overlap is usually quite limited. The mechanism of segregation may include the production and release of interspecific inhibitory compounds as shown by Birch and Wood (1975) for I. paraconfusus and I. pini (Say). The primary objective of this study was to determine if similar interspecific inhibition could be demonstrated for I. paraconfusus and D. brevicomis (i.e., the lowering of the catch at traps baited with pheromones of both species).

METHODS AND MATERIALS
I. paraconfusus beetles were collected from the Sierra National Forest near Bass Lake, Madera Co., California, at an approximate elevation of 1000 m. Logging slash containing late larval stages and callow adults was placed in emergence cages for the rearing and collecting of adult beetles (Browne, 1972).

The field experiments were conducted in the same areas where the beetles were collected at elevations from 900 to 1200 m. Logs with 50 male I. paraconfusus, logs from trees naturally infested with D. brevicomis, and synthetic pheromones of D. brevicomis were used in various tests. Logs (approximately 14 cm diam. by 28 cm long) were cut from ponderosa pine trees and stored less than one month at 4oC before male beetles were introduced head-first into drilled holes (Wood et al., 1968; Lanier et al., 1972). Each log then was wrapped with aluminum window screen to prevent attack by attracted beetles. Control logs were drilled and screened in the same way but without beetles. Beetles were introduced 48 hr before each test to allow maximum pheromone production (Wood et al., 1966). The log then was placed in a "small" sticky-trap (19 cm diam. X 30.5 cm high) supported on a pipe standard 1.2 m above the ground (Bedard and Browne, 1969).

Synthetic pheromones were used in some of the tests to provide a source of attraction or inhibition. Exo-brevicomin (> 95%), frontalin (> 95%), and myrcene (> 97%) (E,F,M) (Chem. Samples Co., Cleveland, Ohio) were released in the field from a releasing device (developed by Rodin for U.S. Forest Service). The device consisted of an inverted 7.6-cm salt shaker covered with aluminum foil which contained each compound in separate glass tubes sealed at one end. Exo-brevicomin and myrcene were in 5- X 55-mm tubes filled to a level 30 mm below the opening, while frontalin was in a 4- X 65-mm tube fllled to, a level 40 mm from the top. Weight losses for each of these compounds averaged 2 mg/day under fleld conditions (Browne, 1978). Verbenone (GLC purifled, > 99.8%, [alpha]D25 = +90o, Chem. Samples Co.) was released at about 4 mg/day from a glass container (1 cm diam. X 0.5 cm high) half fllled and placed inside the salt shaker (Bedard et al., 1980b).

This tree was baited with synthetic D. brevicomis pheromone (E,F,M) so it would be attacked by the beetles and then sawed into logs for use in tests described here.
Natural D. brevicomis pheromone substrates were obtained by placing several E, F, M release devices on a ponderosa pine tree so that beetles would initiate the mass-attack (Bedard et al., 1980b; Wood and Bedard, 1977). After about 2 days, the tree was felled and sectioned into 0.5- or 0.6-m logs and 2 logs were placed on pipe standards inside sticky-traps.
These large sticky-traps were built by criss-crossing two (76 X 203 cm) vinyl-coated flberglass screens (Browne, 1978) coated with Stikem Special® (Mickel and Pelton, Emeryville, California) over 4 (1.54 m long) stakes driven vertically into the ground in such a way as to form a box 76 cm wide X 67 cm high.

Inhibition of Response of D. brevicomis. The inhibition of the response of D. brevicomis to either E, F, M or its naturally produced pheromone by I. paraconfusus was tested by comparing the number of D. brevicomis caught daily at two sticky-traps. In the first test, two small sticky-traps placed 11-12 m apart each contained an E, F, M release device while one trap also contained a log infested with 50 male I. paraconfusus and the other trap contained an uninfested log. These 2 logs were interchanged each day of the test period while the E, F, M release devices and sticky-traps remained stationary (August-September, 1975). A test of the inhibition of D. brevicomis by synthetic pheromones of I. paraconfusus was not attempted because sufficient quantities of the compounds were not available.

In the second test, inhibition of the response of D. brevicomis to naturally produced pheromone by logs infested with I. paraconfusus was tested by comparing the catch of D. brevicomis at two large sticky-traps. The large sticky-traps, placed 11-12 m apart, each contained two logs (0.5 m long) from a tree naturally-infested with D. brevicomis. One trap also contained a log with 50 male I. paraconfusus and the other trap contained an uninfested log. The I. paraconfusus log and uninfested log were interchanged each day while the traps and D. brevicomis logs remained in place (September, 1975). The naturally infested D. brevicomis logs were obtained by sectioning a 32-cm-DBH (diameter of breast height) ponderosa pine tree 2 days after inducing an attack of D. brevicomis by placing E, F, M release devices at heights of 3, 6, and 12 m.

Each trap held the treatment and check an equal number of days to minimize variation due to trap position and to E, F, M or natural pheromone release rates. D. brevicomis were picked from the traps each day and placed in mineral spirit solvent to remove the stikem. The sex of the beetles was determined, and the sex ratios with binomial confidence limits (95%) were determined by the following formulas adapted for use with the quadratic formulas of Spiegel (1961) for determining binomial confidence limits for proportions:

Sex ratio = number of males / number of females = P / (1- P)
Upper confidence limit = Pu / (1 - Pu )
Lower confidence limit = Pl / (1 - Pl )

The nonparametric Wilcoxon test for two paired samples (McCall, 1970) was employed for the comparison of the number and sex caught on the treatment and check traps the same day. Also, the Wilcoxon test was used to compare the treatment and check catches on successive days in the same trap position, i.e., compare position a, day 1, containing the I. paraconfusus log to position a, day 2 without the I. paraconfusus log. However, only day 1 with 2, day 3 with 4, and day 5 with 6 were compared. Finally, the Wilcoxon test was used to determine if one sex was inhibited more than the other. The ratio of the number of male D. brevicomis caught on the treatment traps to the number caught on the check traps was compared to a similar ratio of catch for females.

Inhibition of Response of I. paraconfusus. The possibility that the attraction of I. paraconfusus to male pheromone substrates could be inhibited by the presence of natural D. brevicomis pheromone substrates was investigated. The naturally infested D. brevicomis substrates were obtained as described above by placing two E, F, M elution devices on a 39-cm-DBH ponderosa pine tree at heights of 1.5 and 6.8 m at 8 AM, on August 24, 1976. At 8 AM, August 27, the tree was sectioned into 0.6-m logs, numbered 1-9, with the first log beginning at the 1.2-m height. Each of three replicates consisted of two large sticky-traps 11-12 m apart both containing a log with 50 male I. paraconfusus and one trap also contained two of the D. brevicomis logs (# 1 + 6, 2 + 5, or 3 + 8). On each day of the test (August 27-30, 1976) the D. brevicomis logs were alternated between paired traps while the I. paraconfusus logs and traps remained stationary. The bark beetle predator, Enoclerus lecontei Wolc. (Coleoptera: Cleridae) also was collected each day from the traps.

Finally, the possibility that E, F, M could inhibit the attraction of I. paraconfusus to naturally produced pheromone was tested by placing two small sticky-traps 11-12 m apart, each containing a log infested with 50 male I. paraconfusus. One of the traps also contained an E, F, M release device. The E, F, M release device was alternately placed on one trap of the pair each day during the test (August 30-September 4, 1976). The possibility that verbenone could inhibit the response of I. paraconfusus to naturally produced pheromones was tested in a similar way (June 25-28, 1976).

Identification and Quantification of Pheromones in Hindgut of D. brevicomis Removed from a Naturally Infested Tree. The sex ratio within the galleries and the production of pheromones compounds by D. brevicomis were monitored near the start and at the end of the inhibition of I. paraconfusus test period. Log 4 was dissected at 4 PM on August 27 by carefully peeling the bark off with a draw knife, collecting the beetles from the galleries, and recording the number of excavations. Log 5 (used in the test) was dissected similarly at 10 PM on August 31. The identiflcation and quantities of pheromones found in the hindguts of D. brevicomis feeding in ponderosa pine for approximately 3 days and 8 days (tree parts used in the test of inhibition of I. paraconfusus) were determined by extraction and gas-liquid chromatography (GLC). The last abdominal segment was grasped with fine forceps and pulled away from the beetle to remove the hindgut and a portion of the midgut from 50 males and 50 females collected on the two dates from the dissected logs. The guts were placed in 1-ml Mini-Vials (Applied Science Lab., Inc.) containing 0.1 ml ground glass and 0.35 ml diethyl ether, and crushed with a small glass rod. The extracts were injected onto two GLC (1.8 m X 2 mm ID) glass columns (10% FFAP on 80/100 Gas Chrom Q with the N2 carrier gas at 30 ml/min at 100oC and 3% Apiezon L on 100/120 Gas Chrom Q with the N2 flow at 12 ml/ min at 100oC. Standards of exo-brevicomin, frontalin, trans-verbenol (GLC purified, > 99%) (Glidden Organics, Jacksonville, Florida), and verbenone were compared quantitatively to the peaks from the hindgut extracts. The pheromones and some of the associated compounds in the hindgut extracts also were identified by coinjecting on both columns a portion of each hindgut extract with comparable quantities of exo-brevicomin (2.5 X 10-8 g/µl), frontalin (2.5 X 10-8 g/µl), trans-verbenol (10-8 g/µl), and verbenone (6 X 10-9 g/µl).

RESULTS AND DISCUSSION
Inhibition of Response of D. brevicomis. The attraction of D. brevicomis to E, F, M or to naturally infested D. brevicomis logs was inhibited by logs infested with I. paraconfusus males (Figures 1 and 2; Table 1).

FIG. 1. Inhibition of the response of Dendroctonus brevicomis to E, F, M by volatiles released by Ips paraconfusus males boring in ponderosa pine. Treatment and check were alternated each day between positions a and b.
In both cases, D. brevicomis males and females were inhibited to about the same degree (Table 1). The pheromone-producing substrates used in the tests had only one species present in each log, so the observed inhibition probably was due to responses of flying beetles to the pheromones and/or other compounds released by the two species. If both species had been placed in the same log (Birch and Wood, 1975), there could have been interactions between feeding beetles which might have inhibited production and release of compounds in one or both species. Furthermore, by separating the two species in different logs and by interchanging the I. paraconfusus infested log and the uninfested log each day, the variation due to day, position, trap, and unequal pheromone release could be equally distributed between treatment and check. In the tests for inhibition of D. brevicomis response to E, F, M or to natural pheromone, an average of 17 and 142 I. paraconfusus were caught per day, respectively, showing that pheromone was released during each test period.

The I. paraconfusus infested logs had an immediate effect on the response of D. brevicomis because traps containing the same D. brevicomis logs caught significantly different numbers of beetles on successive days (Figure 1; Table 1).
TABLE 1. Inhibition of response of D. brevicomis and I. paraconfusus to natural and synthetic pheromone by verbenone or volatiles from infested logs
SexInhibition P valueb (total)
Treatment
vs.
control		No caught
(male:female)		Sex ratio
(male:female)		Male
Female		Same day
Successive day		Female > Malec
Inhibition of D. brevicomis
D. brevicomis,4630.84
I. paraconfusus212:251(0.70-1.00)a
0.0070.002
vs.0.31
0.0100.001
D. brevicomis8960.93
432:464(0.82-1.06)
-
EFM,3291.02
I. paraconfusus166:163(0.82-1.26)
< 0.001< 0.001
vs.0.13
< 0.001< 0.001
EFM9280.87
433:495(0.77-0.96)
-
Inhibition of I. paraconfusus
I. paraconfusus,8990.51
D. brevicomis305:595(0.45-0.59)
0.0100.004
vs.0.008
0.0030.015
I. paraconfusus2,6220.33
651:1,971(0.30-0.36)
-
I. paraconfusus,1, 1560.20
EFM194:962(0.17-0.24)
0.9600.390
vs.-
0.2900.750
I. paraconfusus1,2830.20
214:1,069(0.17-0.23)
-
I. paraconfusus,670.34
verbenone17:50(0.20-0.59)
0.0360.012
vs.0.036
0.0120.012
I. paraconfusus7980.13
91:707(0.10-0.16)
aBinomial confidence limits (95%) in parentheses.
bP values obtained with the Wilcoxon signed-rank test from comparisons of catch on the same day except as indicated.
cA small P value (< 0.05) indicates that female beetles were inhibited more strongly than males.
This effect was terminated as soon as the I. paraconfusus log was replaced with a check log. The inhibition of D. brevicomis response to E, F, M by an I. paraconfusus log proved that the reduced catch was caused by compounds released by I. paraconfusus rather than possible effects of species interaction on release of E, F, M by D. brevicomis in logs. Although E, F, M might effect I. paraconfusus pheromone production, nevertheless, I. paraconfusus inhibited the D. brevicomis response to E, F, M. The number of D. brevicomis caught on traps with E, F, M was relatively constant each day (Figure 1) while the number caught on traps with natural pheromone decreased each day (r = -0.39; Syx = 53.1) (Figure 2).

FIG. 2. Inhibition of the response of Dendroctonus brevicomis to natural pheromones by volatiles release by Ips paraconfusus males boring in ponderosa pine. Treatment and check were alternated each day between positions a and b.
Inhibition of Response of I. paraconfusus. The response of I. paraconfusus to pheromones produced by males was inhibited by volatiles released from naturally infested D. brevicomis logs (Figure 4; Table 1). However, the response of I. paraconfusus was not inhibited by the synthetic pheromones, E, F, M, of D. brevicomis (Table I). Instead verbenone, which is produced by male D. brevicomis, inhibited the attraction of I. paraconfusus to logs infested with males (Figure 3; Table 1).

FIG. 3. Inhibition of the response of Ips paraconfusus to natural pheromone by verbenone.
Although both sexes were inhibited by verbenone, females appeared to be more strongly affected than males (Table 1). Female I. paraconfusus are reported to be more responsive to synthetic pheromones and to naturally produced frass than male beetles (Wood and Bushing, 1963; Pitman et al., 1965; Borden, 1967) so that verbenone might disproportionately inhibit females. The structural similarity of verbenone to cis-verbenol indicates that verbenone may competitively inhibit the acceptor site for cis-verbenol in I. paraconfusus. The other pheromones of I. paraconfusus, ipsenol and ipsdienol, have been shown to be essentially unattractive in the field without the addition of cis-verbenol (Wood et al., 1967; 1968). The enantiomeric composition of verbenone was approximately 68% (+) and 32% (-). The possible role of specific enantiomers, as demonstrated already for D. brevicomis (Wood et al., 1976) and Gnathotrichus sulcatus (Borden et al., 1976), remain to be examined.

Young et al. (1973) isolated ipsenol, cis-verbenol, ipsdienol, and verbenone from both the male frass and headspace volatiles of I. confusus LeC., a sibling species of I. paraconfusus, that feeds in pinyon pine, Pinus monophylla Torr. and Frem. Lanier and Wood (1975) found that I. paraconfusus and I. confusus infested logs were interspecifically attractive in areas where only one of the species occurred. However, I. paraconfusus preferred its own frass when samples of each species were presented simultaneously in the laboratory. In another experiment, significantly more I. paraconfusus females entered male nuptial chambers of their own species than those of I. confusus when logs containing each species alone were placed together in the field. Finally, in that experiment there were 107 volunteer attacks by I. paraconfusus on the logs containing I. paraconfusus while only three occurred on the I. confusus logs. Therefore, the preference of I. paraconfusus for its own species may be due, in part, to the release of verbenone from male I. confusus infested logs.

The response of I. paraconfusus to its naturally produced pheromone was inhibited by volatiles released from D. brevicomis logs in the field (Figure 4; Table 1), and as with verbenone, both sexes were inhibited but females to a greater extent (Table 1).

FIG. 4. Inhibition of the response of Ips paraconfusus to natural pheromone by volatiles released by Dendroctonus brevicomis boring in ponderosa pine logs removed from trees under attack.
The ponderosa pine logs infested with D. brevicomis obtained by baiting a tree with E, F, M appeared to be comparable to naturally infested trees. The attack density (166/m2) and sex ratio (1 m : 1.16 f; no more than 1 m/f) were within the limits observed for a tree under colonization for less than 3.5 days (Miller and Keen, 1960; Bedard et al., 1980b). The logs infested with D. brevicomis appeared to be releasing E, F, M since D. brevicomis were attracted each day of the 4-day inhibition test (Figure 5).

FIG. 5. Attraction of Enoclerus lerontei and Dendroctonus brevicomis to traps containing logs infested with Ips paraconfusus and to traps containing logs with both Ips paraconfusus and Dendroctonus brevicomis.
However, the number of D. brevicomis caught declined each day during the trapping period which was from 3 to 7 days after baiting (r = -0.44; Syx = 15.6). This reduction in attraction to naturally infested logs was probably due to a reduction in pheromone production and release (Figure 6). Similarly, a decline in number caught also was observed during the study of D. brevicomis inhibition (Figure 2) as well as in other studies observing infested logs or trees (Miller and Keen, 1960; Vité and Pitman, 1968; Stephen and Dahlsten, 1976). However, our fmdings do not support the statement by Vité and Pitman (1968) that "production of pheromone continues only as long as the host resists extensive feeding and gallery construction." Browne et al. (1979) have quantified these pheromonal components released into the air around infested logs in the laboratory and trees under attack in the field but the relationship of these estimates to production of these compounds in the beetle is not known.

Interspecific inhibition of the response to pheromones may function to segregate the two bark beetle species in distinct areas of the tree and to reduce areas of competition for food and space. Verbenone and compounds isolated from I. paraconfusus that inhibit response of D. brevicomis to its pheromones may prove useful as behavior modifying chemicals that would inhibit the aggregation phase of host colonization and perhaps reduce tree mortality.

E. lecontei, a predator of both bark beetle species, has been reported to be attracted to D. brevicomis infested trees (Berryman, 1966; Vité and Gara, 1962) as well as to I. paraconfusus synthetic pheromones (Wood et al., 1968) and to several ponderosa pine monoterpenes such as M (Pitman and Vité; 1970). E. lecontei was attracted in higher numbers to traps containing logs infested with both bark beetle species than to logs with I. paraconfusus only (P = 0.006). Because logs infested with D. brevicomis were not tested alone, we could not determine if this increased response was due to synergism among compounds produced by both species or simply due to the attraction produced by D. brevicomis alone.

Actual logs from which D. brevicomis beetles were dissected and pheromone components quatified as shown in Figure 6 below.
Identification and Quantification of Pheromones in Hindguts of D. brevicomis Removed from a Naturally Infested Tree. The quantities of F and E found in male and female D. brevicomis, respectively (Figure 6), following dissection from the logs used in the test demonstrating inhibition of the I. paraconfusus response, indicates that these pheromones were present in the greatest amounts at the beginning of the trapping period when the catch of D. brevicomis also was highest (Figure 5).

FIG. 6. Quantities of pheromonal compounds (E,F), verbenone, and trans-verbenol found in hindguts of male and female Dendroctonus brevicomis feeding in ponderosa pine logs cut from a tree under colonization.
Previous studies have shown that hindguts of emerged D. brevicomis males that have not fed on the new host had verbenone and trace amounts of trans-verbenol while the females contained trans-verbenol and either undetectable or trace amounts of verbenone (Renwick, 1967; Renwick and Vité, 1968, 1970; Vite and Renwick, 1970; Pitman et al., 1969). In those studies, the amounts of trans-verbenol and verbenone present in these beetles were not quantified. The presence of verbenone in feeding males has been reported (Hughes, 1973), and this compound was recently found in air surrounding either female or female/male infested logs (Browne et al., 1979). In our studies, verbenone occurred in the largest amounts in male D. brevicomis at the same time that the greatest inhibition of the response of I. paraconfusus occurred (Figure 4). For example, on the first day of the test, the traps containing both D. brevicomis infested logs and a log infested with I. paraconfusus caught only 23% as many I. paraconfusus as did traps containing logs infested with I. paraconfusus alone. On succeeding days, traps with both species caught 27%, 60% and 39% as many. The quantities of verbenone found in male D. brevicomis were similar to the amounts of F and E in these beetles. Furthermore, I. paraconfusus response was inhibited by quantities of verbenone which were similar to release rates of E and F necessary to attract D. brevicomis. Therefore, verbenone was probably released in sufficient quantities to account, at least in part, for the observed inhibition of the response of I. paraconfusus to its pheromone.

E and F as well as verbenone occurred in the largest concentrations in the hindguts of D. brevicomis near the beginning of the aggregation period (3 days after baiting), and these compounds had decreased at a similar rate 5 days later (Figure 6). It has been postulated that termination of the aggregation phase of host colonization may be caused by the release of verbenone in large enough quantities to inhibit the attraction to pheromones (Renwick and Vité, 1970; McNew, 1970; Bedard et al., 1980b). Our results, on the other hand, suggest that the observed reduction in attraction (Figures 5 and 6) was caused by a reduction in the quantities of E and F produced and not by an increase in verbenone production. Nevertheless, verbenone may play a role in regulation of the attack density and thus would influence the termination of the aggregation phase. However, the eventual decrease in attraction is probably caused by a reduction in E, F release.

Acknowledgements- We would like to thank L.E. Browne, W.J. Loher, P.A. Rauch, and E. Zavarin of the University of California at Berkeley; M.C. Birch and D. Light of the University of California at Davis; and W.D. Bedard of the Forest Service, U.S.D.A., for their suggestions and review of the manuscript. We would also like to thank K.Q. Lindahl, Jr., for statistical review, and W.D. Bedard and P.E. Tilden for their helpful suggestions and unpublished data regarding the field behavior of D. brevicomis and for the use of the Forest Service research facilities at Oakhurst, California.

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