Effect of a Marking Pheromone and Population Density on Ladybird Larval Development

Females of predaceous ladybirds use sensing chemicals in larval tracks as oviposition deterring pheromone to avoid cannibalism of eggs. We hypothesized that larvae would also respond to the presence of conspecific tracks by slowing their developmental rate and delaying pupation, thereby reducing the time spent as a defenseless pupa in the presence of feeding conspecifics. We reared larvae of the harlequin ladybird Harmonia axyridis in dishes that were replaced daily by a clean one (C) or continuously in a dish with larval tracks accumulated (P). We used three larval densities (1, 4, 8 larvae per dish) for both treatments (C1, C4, C8, P1, P4, P8). We measured the developmental time of the fourth larval instar and fresh adult body mass. Developmental time increased at the highest density in the clean treatment C8 but remained unchanged across densities in the pheromone treatment (P1-P8). Body mass was significantly lower at the highest density in both treatments (C8, P8) and was slightly higher at the presence of pheromone (P). Ladybird larvae respond independently to their density and to the presence of pheromones. The compounds present in the tracks, previously known as oviposition deterring pheromone, may therefore also function as development enhancing pheromones.