Larval aggregation in a Neotropical butterfly: risky behaviors, per capita risk, and larval responses in Ascia monuste orseis

Abstract

Gregariousness in insects has been shown to improve performance, feeding, and defense against predation. Nevertheless, an increased risk of antagonistic interactions such as competition and predation occurs in group living. We examined the effects of both group size and larval age of aggregations of the butterfly Ascia monuste orseis (Godart, 1819) on larval permanency and attack by natural enemies. Group behavior was also investigated with respect to predation risk, as well as larval responses after enemy attack. One larva and groups of 10 and 50 larvae were assigned on host plants and monitored daily to evaluate larval permanency. At each instar, a subset of groups was observed for a 4-h period to identify natural enemies and to record both group behaviors performed at the moment of attack and responses to the attacks. Permanency on the host was higher in aggregations than in solitary larvae. Larger aggregations conferred greater protection to A. monuste orseis larvae against natural enemies than did smaller aggregations, through lower per capita predation. Parasitoidism was more intense in small-sized larvae, whereas late instars were more susceptible to predators. Similarly to solitary species, larvae were most susceptible to attacks when performing searching and feeding compared to resting and moving. Moving away and headrearing were observed for all instars and groups after enemy attacks and rarely protected larvae against natural enemies. Our results show that behaviors that involve head movements posed more attack risks than those that involve moving the whole body or absence of movements, and that larval aggregation in A. monuste orseis rarely exhibits defensive behaviors against natural enemies. Larval defense is provided through dilution effect. Natural enemies can use several cues to find their prey, as for example, prey morphology, chemistry, and behavior. Nevertheless, group behavior has been poorly investigated with respect to mortality risks in phytophagous insects. This study investigated whether certain group sizes and behaviors are more risky than others in the gregarious Ascia monuste larvae. Larvae were susceptible to both parasitoids and predators, and larger groups conferred more protection against natural enemies than smaller ones. Similarly to species with a solitary habit, behaviors that involved head movements as searching and feeding posed more attack risks than moving and resting. Defense behaviors were not very effective against natural enemies. Our study reveals that both solitary and gregarious species may share more similarities in terms of susceptibility to natural enemies than previously thought.