CANCELLED - Caterpillars optimize defense via host plant mixing

Background/Question/Methods

Insect herbivores can sequester plant secondary metabolites (PSMs) for their own defense. Those that mix plants in their diets (i.e., grazers) present the interesting possibility that herbivores tailor their PSM intake to optimize defense against natural enemies. If PSMs act additively toward defense, host-plant mixing may reduce detoxification costs associated with over-ingestion of particular PSMs, and if they act synergistically, mixing may offer herbivores greater defense than would be possible by eating more of a single plant. Alternatively, PSMs may differ in their efficacy against different enemies. The woolly bear caterpillar, Grammia incorrupta, is a dietary generalist that grazes upon dozens of herbaceous plants in the grasslands of the southwestern USA. In this study, we test the hypothesis that dietary mixing of plants containing iridoid glycosides (IGs) and those containing pyrrolizidine alkaloids (PAs) enhances G. incorrupta defense against generalist predators. We offered caterpillars that had been conditioned on four treatment diets (rearing diet, PA plant, IG plant, PA plant + IG plant) to Aphaenogaster cockerelli, an ant that is ubiquitous in their natural habitat. We measured caterpillar deterrency by counting the number of ants that rejected each caterpillar before acceptance, and by the duration of ant-caterpillar interactions.

Results/Conclusions

Caterpillars conditioned on rearing diet, devoid of PSMs, were the least deterrent to ants; those fed single-plant diets showed an intermediate level of deterrency; and those fed a combination of PA and IG plants were the most deterrent. Treatments did not differ in the time ants spent interacting with caterpillars before making a decision. The size of the deterrency effect in the two-plant treatment compared to single-plant treatments implies that caterpillars optimize defense via chemical synergies. Combined with the finding that G. incorrupta uses PAs to self-medicate against parasitoids, these results suggest that PSM optima in the diets of grazing herbivores vary over time depending on which enemies pose a threat. Grazing herbivores may be uniquely capable in responding to these threats using combinatorial chemical defense.