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Sequestration of dietary terpenes by the goldenrod beetle; A proposed mechanism for protection.
Johnson, Robert*,1, Young, Brenda 2, Grebenok, Robert3, 1 Medaille College, Buffalo, NY, USA2 Daemen College, Amherst, NY, USA3 Canisius College, Buffalo, NY, USA
ABSTRACT- Solidago altissima is known to contain a number of terpene allelochemicals and hosts the specialist beetle Trirhabda sp. In areas of persistent Solidago occurrence, Trirhabda populations may reach high densities causing defoliation of initial growth by larvae and regrowth by adults; however, little is known about Trirhabda population regulation, or if their allelochemical-rich diet plays a role in protection against predators. To test the hypothesis that Trirhabda larvae sequester dietary allelochemicals, GC/MS analysis was conducted on leaf samples from eight distinct patches of Solidago clones and 5-6 actively feeding larvae from each patch. To test the hypothesis that larvae are able to deter insect attacks, two-choice feeding bioassays were conducted in June 2003 using both ants (Formica pennsylvanica) and spined soldier bugs (Podisus maculiventris) with small mealworms as controls. Plant analysis identified five sesquiterpenes and two diterpenes. The diterpene solidagone and the sesquiterpene hydrocarbon germacrane-D were respectively the components of greatest mean concentration. Larval terpenoid profiles were less complex and had approximately 25% of the concentrations found in plants; however, germacrane-D and solidagone were likewise the two major terpene components. A positive relationship was seen between solidagone host and whole larvae concentrations (r2=0.562, F=10.26 df=1,8, P=0.013). Prolonged ant attacks on larvae were deterred relative to controls (X2 = 32.6, df = 1, P<0.0001). Deterrence was scored as a single ant bite followed by retreat and mandible cleaning. In contrast, the soldier bug exhibited no feeding preference or differential mortality. This study suggests that Trirhabda larvae selectively retain dietary germacrane-D and solidagone and offers a chemical mechanism for the observed deterrence of ant attacks. The lack of deterrence against a sucking insect also suggests that potential chemical deterrents may not be retained in larval hemolymph but in surface glands or secreted.
Key words: Solidago altissima, Trirabda sp., Goldenrod beetle, Terpenes