Chemical defense acquired via pharmacophagy can lead to protection from predation for conspecifics in a sawfly

Predation is an important selection pressure acting on organisms, with
organisms evolving diverse anti-predator strategies to combat it. One such
strategy is chemical defense in which organisms either synthesize or
extrinsically acquire defensive chemicals. Little is known about the
intraspecific transfer of such chemicals and if such chemicals acquired
from conspecifics can also serve as defense against predation. Here, we
used adults of the turnip sawfly, Athalia rosae, which can acquire the
plant chemicals ‘clerodanoids’ via pharmacophagy after exposure to the
plant, Ajuga reptans. We show that clerodanoid access mediates protection
against predation by mantids for the sawflies. Moreover, even indirect
access to clerodanoids, via nibbling on conspecifics that had access to
the plant, resulted in protection against predation albeit to a lower
degree than direct access. Furthermore, sawflies that had no direct access
to clerodanoids were less consumed by mantids when they were grouped with
conspecifics that had direct access. Most of such initially undefended
sawflies could acquire clerodanoids from conspecifics that had direct
access to the plant, although in low quantities. Together our results
demonstrate that clerodanoids serve as chemical defense that can be
intraspecifically transferred. Moreover, the presence of chemically
defended individuals in a group can confer protection onto conspecifics
that had no direct access to clerodanoids, suggesting a ‘herd-protection’
effect.