Does group feeding by toxic prey confer a defensive benefit? Aristolochic acid content, larvae group size and survival of pipevine swallowtail (Battus philenor) larvae.

Aggregative feeding is widespread in Lepidopteran larvae suggesting that this

behavior serves on adaptive function. Many studies of the potential benefits of

aggregative feeding in Lepidopteran larvae have been conducted. However, no studies

have directly examined the benefits of cryptic larvae being both chemically defended and

gregarious. Group feeding occurs disproportionately more in chemically defended

larvae than in larvae that have no chemical defense. Most of these larvae are cryptic

when they are most highly aggregated and most vulnerable to predation. In this study,

the benefits of group feeding in terms of decreased predation were explored in first instar

larvae of pipevine swallowtail larvae, Battus philenor, a species that exhibits chemical

sequestration. Contrary to our expectation, we found that groups of larvae fed a diet

with high levels of the toxin aristolochic acid, which they sequester naturally and use as

a defense against natural enemies, had significantly lower survivorship due to predation

in both the field and in the laboratory experiments compared to groups of larvae fed a

diet with low aristolochic acid content. We also found that aristolochic acid does not

deter the generalist predator Hippodamia convergens, the ladybird beetle, suggesting

that this compound is not a universal predator deterrent as previously assumed. Thus,

instead of finding a benefit to group feeding and chemical defense in cryptic larvae, we

have found a negative impact of group feeding in this population of B. philenor. Based

on this evidence, we speculate that other benefits of group feeding might be outweighing

the negative consequences of increased predation during the first instar. Future

research on chemical defense, aposematism, and aggregative feeding should take into

consideration that chemical defenses might not be universally effective against all

natural enemies.