Attraction to conspecific eggs may guide oviposition site selection in a solitary insect

Conspecific attraction is a form of social information use whereby individuals are attracted to the presence of conspecifics because they may indicate high-quality sites or resources. Conspecific attraction results in aggregation of individuals with similar needs and may therefore intensify competition, in particular, at high densities. Thus, the occurrence and strength of conspecific attraction may be dependent on density, but the effects of predicted intensity of future competition for resources on individual decisions have rarely been quantified. We studied realized early fecundity and oviposition site selection in the butterfly Pieris napi in relation to a density gradient of conspecific eggs on available host plants in an explicit laboratory experiment. Relying on conspecific assessment of host quality is expected to select for conspecific attraction, whereas competition avoidance is expected to select for avoidance of high conspecific densities. Presence of conspecific cues did not substantially affect realized fecundity as females exposed to an environment containing conspecific cues laid approximately equal number of eggs as females exposed to an environment lacking such cues. Instead, when females were able to choose among host plants with or without previously laid conspecific eggs, they preferred plants that already carried eggs in relation to egg-free host plants, independently of the initial egg density. Indeed, the maintenance of conspecific attraction, rather than avoidance, in P. napi implies that the possible benefits of conspecific attraction in oviposition site selection may outweigh the costs of competition in the wild.