Previous research indicated that the aggregation pheromone of the southern pine beetle, Dendroctonus frontalis, is produced only by females, the sex that initiates attacks. We provide evidence indicating that secondarily arriving males augment mass aggregation by releasing the attractive synergist (+)-endo-brevicomin. Healthy pines artificially infested with both sexes of D. frontalis were significantly more attractive to conspecifics than trees infested solely with females. Coupled gas chromatography-electroantennographic detection (GC-EAD) analyses of volatiles isolated from male beetles revealed substantially greater olfactory sensitivity by D, frontalis to endo-brevicomin than to any other component. The threshold of detection of both sexes for (+)-endo-brevicomin was four orders of magnitude lower than for its antipode and at least one order of magnitude lower than for either enantiomer of frontalin, the major female-produced aggregation pheromone component. Pairing with a female in a gallery stimulated individual male beetles to produce hundreds of nanograms of (+)-endo-brevicomin. (4-)-endo-Brevicomin was detected in a small percentage of female D. frontalis, whereas (-)-endo-brevicomin was never detected in either sex. In field trapping bioassays, we confirmed that (+)-endo-brevicomin is a potent synergist for attractive combinations of frontalin and pine turpentine. However, (+)-endobrevicomin failed to attract D. frontalis either when presented alone or in combination with turpentine. We postulate that mass colonization of host trees by D. frontalis is mediated by distinct semiochemicals from both sexes rather than females alone. Our discovery of a key aggregation pheromone component in such an apparently well-studied species implies that the pheromone models of other bark beetles could benefit from systematic reexamination using newer technologies. Additionally, baits fortified with (+)-endo-brevicomin may enhance pest management strategies that exploit attractants for D. frontalis.
Stem volatile extracts from ten trees that are sympatric with the western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) were assayed by gas chromatographic-electroantennographic detection analysis (GC-EAD). The extracts were from the primary host, ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinaceae); two nonhost angiosperms, California black oak, Quercus kelloggii Newb. (Fagaceae), and quaking aspen, Populus tremuloides Michx. (Salicaceae); and seven nonhost conifers, white fir, Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. (Pinaceae), incense cedar, Calocedrus decurrens (Torr.) Florin (Cupressaceae), Sierra lodgepole pine, P. contorta murrayana Grev. & Balf. (Pinaceae), Jeffrey pine, P. jeffreyi Grev. & Balf. (Pinaceae), sugar pine, P. lambertiana Dougl. (Pinaceae), Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco (Pinaceae), and mountain hemlock, Tsuga mertensiana (Bong.) Carr. (Pinaceae). Sixty-four compounds were identified from the ten trees, 42 of which elicited antennal responses in D. brevicomis, usually in both sexes. In addition, several synthetic compounds, including a number of the antennally-active compounds from the extracted trees and some bark beetle pheromone components, elicited antennal responses in a manner similar to that observed with the extracts. Of the antennally-active compounds known to be present in trees sympatric with D. brevicomis, only geraniol was unique to its host. Four antennally-active compounds were found in the host and in other conifers; five compounds were found only in nonhost conifers; eight compounds were found in either or both of the nonhost angiosperms; eight compounds were found in either or both of the angiosperms and in nonhost conifers, but not in the host; and 19 were found in both the host and in angiosperms and/or nonhost conifers. Several bark beetle pheromone components were found in the stem volatile extracts. Conophthorin was identified from both nonhost angiosperms; exo-brevicomin was identified in A. concolor; verbenone was identified from a number of nonhost conifers; and chalcogran was identified from P. tremuloides. The number of nonhost volatile chemicals that D. brevicomis encounters and is capable of detecting, and the diversity of sources from which they emanate, highlight the complexity of the olfactory environment in which D. brevicomis forages. This provides a basis for further work related to chemically-mediated aspects of foraging in this insect and perhaps other coniferophagous bark beetles, and highlights the need to consider foraging context in the design and implementation of semiochemical-based management tactics for tree protection.
A growing body of evidence suggests that bark beetles detect and avoid release points of volatile compounds associated with nonhost species, and thus such nonhost volatiles may have potential utility in the management of bark beetles. We used a coupled gas chromatograph-electroantennographic detector (GC-EAD) to assay the olfactory sensitivity of the southern pine beetle, Dendroctonus frontalis Zimmermann, to volatiles from leaves and bark of eight species of nonhost angiosperm trees that are common in the range of D. frontalis. Tree species sampled were red maple (Acer rubrum L.), mockernut hickory [Carya alba (L.) Nutt. ex Ell.], sweetgum (Liquidambar styraciflua L.), black tupelo (Nyssa sylvatica Marsh.), black cherry (Prunus serotina Ehrh.), southern red oak (Quercus falcata Michx.), blackjack oak [Quercus marilandica (L.) Muenchh.], and water oak (Quercus nigra L.). Beetle antennae responded to a total of 28 identifiable compounds in these samples. The relative olfactory responsiveness to 14 of these, as well as to nonanoic acid and four additional volatiles reported to be associated with nonhost angiosperms, was assessed in GC-EAD analyses of synthetic dilutions spanning six orders of magnitude. The largest response voltage amplitudes were obtained with trans-conophthorin, nonanoic acid, terpinen-4-ol, phenylethyl alcohol, and eucalyptol, whereas the lowest response thresholds were to nonanoic acid, nonanal, linalool, (E)-2-hexen-1-ol, and phenylethyl alcohol. Funnel traps baited with various combinations of eleven antennally-active angiosperm volatiles along with a standard attractant captured significantly fewer male and female D. frontalis than traps baited with the standard attractant alone. Our data suggest that a diversity of semiochemicals may be involved in host species discrimination by D. frontalis, and several may have utility in their management.
We conducted laboratory and field bioassays to characterize the pheromone system of an ash bark beetle, Hylesinus pruinosus Eichhoff (Coleoptera: Curculionidae: Scolytinae). Solitary females in newly initiated galleries in ash logs produced (+)-exo-brevicomin, whereas male beetles paired with females produced (+)-endo-brevicomin, lesser quantities of (+)-exo-brevicomin, and a third compound that could not be identified. Beetles produced these compounds also after exposure to juvenile hormone III, and they were the sole volatile chemicals isolated from beetles or aerations of infested logs that elicited electrophysiological responses from antennae of either sex. In the field, both sexes were strongly attracted to traps baited solely with either racemic or pure (+)-endo-brevicomin. Racemic exo-brevicomin was much less attractive to both sexes than racemic endo-brevicomin, and it did not increase attraction of endo-brevicomin when released in combination. Host odors (volatiles from mechanically damaged ash branches) failed to attract beetles or increase attractiveness of racemic exo-brevicomin. Our evidence suggests that male-produced (+)-endo-brevicomin is the major component of an aggregation pheromone for H. pruinosus, with (+)-exo-brevicomin and the unidentified male compound playing an indeterminate role in the chemical ecology of this species. Our data thus show an instance in which the major aggregation pheromone component of a bark beetle is produced by the secondarily arriving sex, a rare occurrence in bark beetles but one which has been reported previously for the Hylesini.
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