Copyright

J. R. Aldrich (home page)& A. Zhang

USDA-ARS Insect Chemical Ecology Laboratory, Agricultural Research Center-West, B-007, Beltsville

Many, if not most, of the known heteropteran pheromones also attract certain tachinids which exploit the bug pheromones as host-finding kairomones. The system we will describe involves the sympatric predaceous stink bugs, Podisus maculiventris and P. neglectus (Pentatomidae: Asopinae). Male-produced pheromones and the nymphal defensive secretions of these Podisus spp. are usurped as kairomones principally by two tachinid species: Euclytia flava (a generalist pentatomid parasitoid) and Hemyda aurata (a specialist asopine parasitoid). The pheromone of P. maculiventris consists mainly of (E)-2- hexenal and ?-terpineol, while that of P. neglectus is predominantly (E)-2-hexenal with linalool. Curiously, the nymphal secretion of P. maculiventris contains linalool, along with (E)-4-oxo-2-hexenal and tridecane; whereas nymphs of P. neglectus secrete linalool and ?-terpineol, plus (E)-4-oxo-2-hexenal and tridecane. GC-EAD experiments with P. maculiventris, E. flava and H. aurata revealed that the tachinids are much more electrophysiologically sensitive to the pheromone of the bug than are the bugs themselves. Furthermore, the antennae of E. flava are more responsive to ?-terpineol than linalool, but H. aurata antennae respond to ?-terpineol and linalool more equally. In addition, the antennae of both tachinids respond strongly to minute amounts of (E)-2-octenal and (E)-2-decenal, alkenals that are characteristic defensive compounds of stink bug adults. Captures of E. flava and H. aurata individuals in traps baited with blends containing either ?-terpineol or linalool were positively correlated with the respective antennal sensitivities of the tachinids toward these monoterpenoids. Field tests in which some pheromone-baited traps also contained a high dose of (E)-2-octenal showed that the addition of (E)-2-octenal tended to repel the bugs. However, (E)-2-octenal significantly increased attraction of E. flava females to the pheromone of P. neglectus, whereas attraction of E. flava individuals to P. maculiventris pheromone was either unaffected (female flies) or sometimes reduced (male flies) by the presence of (E)-2-octenal. Our working hypothesis to explain these results is that the intensity of a tachinid’s antennal response toward particular host volatiles is directly proportional to host suitability. This interpretation is substantiated by GC-EAD showing that methyl (E,E)-2,4-decadienoate—the main pheromone component of Euschistus spp., acceptable but not preferred stink bug hosts of E. flava—elicited significantly weaker responses from the antennae of E. flava females than seen for ?-terpineol and linalool.