Pioneer-Follower dilemma game in Acanthoscelides obtectus (Coleoptera: Bruchidae)

Abstract

Larvae of Acanthoscelides obtectus show two contrasting behaviors when entering a bean. One is pioneer behavior in which a larva enters the bean through an entrance hole made by itself; the other is follower behavior in which a larva enters the bean through an entrance hole made by a pioneer. Previous studies have shown that the number of followers is much greater than that of pioneers. If there is a cost to being a pioneer, and if larvae can choose either of the two strategies, there is a dilemma: to be a pioneer or not. This dilemma is similar to the chicken game because if all larvae avoid choosing the risky pioneer strategy in favor of the follower strategy, none of the larvae can enter a bean, and they will die. In the present study, in order to investigate whether the larvae of A. obtectus are facing a dilemma of entry order, we experimentally measured the cost to pioneers and tested the flexibility of larval entering strategies. Pioneers’ costs were measured by entrance success rate, and the flexibility of larval strategies was tested by gauging the proportion of pioneers at various larval densities. The entrance success of pioneers was lower than that of followers, and the proportion of pioneers decreased as the number of competing larvae increased. These results suggest that the Pioneer-Follower interaction in A. obtectus satisfies the conditions for a dilemma game: the larvae of A. obtectus are in a dilemma of entry order.