We reared larvae of Junonia coenia Hubner (Nymphalidae) on artificial
diets with trace concentrations of iridoid glycosides and on leaf diet
s with higher concentrations of iridoid glycosides. We offered these c
aterpillars to predacious ants and observed the effects of the followi
ng on predation: diet (artificial vs. leaf), site (ant colonies in dry
vs. wet areas), instar (early vs. late), and time (changes in predati
on over five days). Diet and site were consistently significant predic
tors of the ants' propensities to reject prey and the caterpillars' ab
ilities to escape predation. Leaf-diet caterpillars escaped more frequ
ently than artificial-diet caterpillars, and ants from dry sites were
more likely to reject prey than ants from wet sites. The percentage of
iridoid glycosides found in individual caterpillars was also a good p
redictor of the probability of rejection by predators and prey escape.
Caterpillars with higher levels of iridoids were more likely to be re
jected and to escape, suggesting that sequestered iridoid glycosides a
re a defense against predaceous ants.