Spatial Displacement of Release Point can Enhance Activity of an Attractant Pheromone Synergist of a Bark Beetle

Abstract

Flight responses of the southern pine beetle, Dendroctonus frontalis Zimmermann, to widely-spaced (>130 m) traps baited with pine volatiles (in turpentine) and the female-produced pheromone component frontalin were enhanced when a bait containing the male pheromone component (+)-endo-brevicomin was attached directly to the trap. However, displacing this bait 4-16 m horizontally from the trap significantly increased its synergistic effect. (+)-endo-Brevicomin enhanced catch to the same degree when the bait was positioned either on the trap or 32 m away. In another experiment, pairs of frontalin/turpentinebaited traps were established with 4 m spacing between traps and >100 m spacing between pairs. Attachment of either a racemic or (+)-endo-brevicomin bait to one trap of a pair caused a significant increase in catch by both traps, but catch in the trap lacking endo-brevicomin was increased more than in its endo-brevicomin-baited twin. In a third experiment, widely-spaced groups of three traps (in a line with 1 and 4 m spacing between the middle and outer traps) were baited uniformly with frontalin and turpentine, and the release rate of (+)-endo-brevicomin from the middle trap was varied across three orders of magnitude. Release rates sufficient to enhance total D. frontalis catch by the trio also caused relatively higher catches to occur in the outer traps than in the middle one. These experiments indicated that both male and female D. frontalis fly to and land preferentially at sources of frontalin and host odors when these are located some distance away from a source of endo-brevicomin. This behavior may have evolved in D. frontalis to allow host-seeking beetles to locate growing, multi-tree infestations while avoiding fully-colonized trees within these infestations. Our data demonstrate that trap spacing alone can qualitatively change the outcome of bait evaluation trials and may explain why many earlier experiments with endo-brevicomin failed to identify it as an aggregation pheromone synergist for D. frontalis. We believe that important aggregative functions of semiochemicals of other bark beetle species may have been similarly overlooked due to choice of experimental procedures.

Semiochemicals in the Natural History of Southern Pine Beetle Dendroctonus frontalis Zimmermann and Their Role in Pest Management

“…One implication of these findings more generally for bark beetle semiochemical research is that field experiments with unrecognized multifunctional semiochemicals could be interpreted as indicating either inhibitory/repellent or synergistic/attractive properties for the test compound depending on trap spacing (Sullivan and Mori, 2009). A similar problem might arise as a result of natural sources of pheromone in the environment where tests are performed.…”

“…Female-produced trans-verbenol (trans-4,6,6-trimethylbicyclo[3.1.1]hept-3-en-2-ol) may also participate but the insufficient studies to date suggest that its presence may not be essential (see below). The ternary blend is the most attractive lure yet identified for this species [at least when released in the absence of natural or artificial competing sources of attractant (Moreno et al, 2008;Sullivan and Mori, 2009;Sullivan et al, 2007bSullivan et al, , 2011Vit e et al, 1985)], and the species possesses exceptional olfactory sensitivity to the two insect-produced components (Payne, 1975;Sullivan, 2005;Sullivan et al, 2007b). The pheromone component combination parallels that of closely related, aggressive species, D. brevicomis LeConte and D. adjunctus Blandford in which each sex contributes a single, different bicyclic ketal to the aggregation pheromone (ie, brevicomin or frontalin) that-at least in the case of D. brevicomis-acts synergistically (Bedard et al, 1980b;Browne et al, 1979;Byers et al, 1984;Hughes et al, 1976).…”

“…I believe that this remarkable phenomenon is at least partially explained by the fact that (+)-endo-brevicomin released from a point source at a rate approximately equal to a single mass-attacked tree can have synergistic effects on frontalin/host odour-baited traps located within a radius of at least 32 m (Fig. 3), a zone that would encompass many currently attacked and adjacent unattacked trees near the head of a beetle infestation (Sullivan and Mori, 2009). Furthermore, the synergistic effect in this study was significantly greater (ie, catches were higher) when the endo-brevicomin device was 4-16 m distant rather than on the trap.…”

“…However, a property that distinguishes endo-brevicomin and its behaviour from the above listed aggregation attractants which likewise cause displaced landing effects at high concentrations (besides it being a synergist and not an attractant) is that at no release rate does endo-brevicomin appear to direct landing to its point of release more than to the immediately surrounding area (Sullivan and Mori, 2009); rather, the compound apparently produces a zone of synergistic activity, with the release rate determining the size of the zone and the radius of maximum synergism (Sullivan and Mori, 2009, author's unpublished data). This behavioural effect also appears to occur about equally for both sexes of D. frontalis rather than predominantly the first arriving sex.…”

“…A similar situation occurs with the aggregation pheromone component lanierone in Ips pini (Say) which is a potent attractive synergist for conspecifics but has no effect on attraction of predators including T. dubius . Lack of response by D. frontalis-seeking T. dubius to endo-brevicomin may be due to the fact that endo-brevicomin does not direct landings by D. frontalis to its point of release [whereas frontalin and host odours apparently have this effect; (Sullivan and Mori, 2009)], and furthermore since it is a "multifunctional" pheromone component it can have inhibitory effects on D. frontalis at high concentrations (Sullivan et al, 2011). As such, its presence might not increase the odds of a T. dubius encountering a prey insect at least at close range.…”