Southern pine beetle, Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae): quantitative analysis of chiral semiochemicals
Semiochemicals released from logs infested by southern pine beetle (SPB), Dendroctonus frontalis, from a total of eight infestations located in Texas, South Carolina, and Virginia were collected four to eight days after initial attack. The quantities and chiralities of most semiochemicals, as analyzed by gas-liquid chromatography, showed geographic and temporal variations. Changes in the quantities of α-pinene (aP), frontalin (F), and endo-brevicomin (eB), are believed to result from responses of the host and the beetle to each other's activity at a given time and differences in their respective health. The chiralities of aP, F, and eB at all locations generally remained stable over time, yet variation across the insect's geographic range, particularly for aP and F, is believed to be due to genetic variation of individuals. Geographic and temporal variations in the quantities and chiralities of cis-verbenol (cV), trans-verbenol (tV), and verbenone (V) are presumed to be due to the multiple pathways of origin (SPB, autoxidation, and microorganisms).
Analysis of the same semiochemicals isolated from hindguts of individual beetles from Texas, South Carolina, and North Carolina showed quantities of cV and tV to be substantially greater in females than in males; whereas, males contained much greater amounts of V. Geographic differences were found in quantities of tV and V in both sexes and in aP and F in males only. The chiralities of most semiochemicals present in SPB hindguts differed markedly from those released from infested logs. Males produced predominantly (+)-F and (-)-eB, (-)-cV, and (-)-V; however, the chirality of tV varied considerably among areas. In contrast, females produced predominantly (+)-cV and (+)-V and (-)-F, (-)-eB, and (-)-tV. The (+) enantiomer of aP predominated in both sexes, but the proportion of (+ )-aP was generally lower than that released from SPB-infested logs from the same areas. Geographic differences in chirality of tV and V were significant in males and for eB in females.
In laboratory trials, aP autoxidized under ambient temperatures to form tV, myrtenol (M), V and to a lesser extent, cv. Both the quantities and chiralities of these compounds were dependent on the chirality of the aP precursor. Significantly greater amounts and proportions of the (+) enantiomer of each compound were produced when (+)-aP was predominant than when the antipode of the precursor was predominant. The extent to which autoxidation products playa role in bark beetle behavior is expected to be dependent on the proportion of aP in pine resin and its chirality present in a pine species.
The information gained from this research was used to elaborate on previously proposed behavioral sequences occurring during the mass attack of host trees by SPB and to suggest new avenues to improve the use of semiochemicals in pest management efforts.