THE CONSEQUENCES OF LARVAL AGGREGATION IN THE BUTTERFLY CHLOSYNE LACINIA

1. Females of Chlosyne lacinia (Geyer) (Lepidoptera: Nymphalidae, Meli taenae), the bordered patch butterfly, clump eggs in a few large clust ers on their host plant, Helianthus annuus. Resulting larvae form sibl ing aggregations to at least the third instar. 2. The effect of group size on survival and development of C. lacinia larvae was tested exper imentally in the field. Larvae developed faster and survived better in larger groups. 3. The effects of various predator guilds (ground-dwel ling arthropods, aerial arthropods and avian predators) on survival of larvae was then tested while controlling group size. Ground-dwelling arthropods, mainly fire ants Solenopsis xyloni, reduced larval surviva l greatly but other solitary invertebrate and avian predators did not alter survival. Group defences and aposematism of C. lacinia larvae ar e probably ineffective against predatory ants that attack en masse and recruit other colony members. 4. In laboratory experiments, two possi ble mechanisms underlying faster development of larvae in larger group s were tested: (i) overcoming the physical toughness of host plant lea ves, and (ii) social stimulus to feed. Results support the physical to ughness hypothesis but not the social stimulus hypothesis. 5. Feeding in large groups by C. lacinia larvae confers multiple advantages, incl uding protection from solitary predators and increased feeding efficie ncy because grouped, early-instar larvae can initiate feeding wounds o n tough sunflower leaves. These advantages of larval gregariousness, c oupled with reduced desiccation at the egg stage, apparently outweigh disadvantages of aggregation, such as interference and exploitative co mpetition among larvae.