The shield defense of the sumac flea beetle, Blepharida rhois (Chrysomelidae: Alticinae)

Summary.

Both adults and larvae of the sumac flea beetle, Blepharida rhois (Forster), are dietary specialists that feed on smooth sumac, Rhus glabra, on Long Island, NY. Instead of discarding their feces, B. rhois larvae retain it on their backs to form fecal 'shields'. We observed that ants attacking shielded larvae retreated and groomed vigorously, indicating the possible presence of chemical deterrents. To examine whether shields were chemical rather physical barriers against predation, we employed a generalist ant predator as a bioassay. Shields of larvae reared on R. glabra thwarted ants while larvae that had their shields removed were readily taken. Moreover, larvae reared on a substitute diet of lettuce were defenseless. However, protection was restored after their lettuce-derived shields were replaced with shields obtained from larvae reared on R. glabra. We then extracted and fractionated shields in order to locate active deterrents. To determine whether larvae synthesized defensive compounds or obtained them from the host, leaves were also analyzed and compared to the chemicals found in shields. The shield defense was a mixture of three fatty acids, a suite of tannins, their metabolites and phytol. All shield compounds or their precursors were obtained entirely from the host plant. Pure standards of shield compounds were found to be deterrent when assayed. This is one of the first instances of an insect using a mixture of primary and secondary substances for defense against predators.