Reduction of oviposition time and enhanced larval feeding: two potential benefits of aggregative oviposition for the viburnum leaf beetle

Abstract

Although aggregation in non‐social arthropods is common, its adaptive value is not always clear. Oviposition behaviour of the viburnum leaf beetle (Pyrrhalta viburni) is aggregative, with females preferring to lay eggs on twigs already infested by conspecifics. We previously showed that aggregative oviposition aided in overcoming host plant defences. Here we explore two additional benefits of this behaviour: reduction of time investment in oviposition; and improvement of host use by larval group feeding. In video‐recorded trials, the presence of conspecific egg masses did not have an impact on the time associated with oviposition, but it did affect females' pre‐oviposition behaviour: the length of foraging movements was 21% shorter on infested than on non‐infested twigs, and was concentrated near the infested area. In two separate studies of insect performance, larvae were reared to maturity in groups of different densities (five or 10, and five or 20, respectively) on several hosts (four and 17 Viburnum spp., respectively). Independent of host species, adults were > 13% heavier (groups of 10 vs. five), and pupation success was 27% higher (groups of 20 vs. five) at higher larval density. Two additional effects were dependent on host species: larval survivorship (groups of 10 vs. five), and adult mass (groups of 20 vs. five). These effects were not associated with plant defensive traits (i.e. trichomes or leaf toughness), but adult mass was associated with host nutritional quality (i.e. foliar nitrogen content). These results show that several traits of P. viburni larval performance are positively density‐dependent, and suggest that benefits of group feeding could have played a role in the evolution of oviposition behaviour in P. viburni. Our results also demonstrate that plant quality can mediate benefits of aggregation, underscoring the importance of ecological context in understanding the feeding strategies of insects.