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

Females of predaceous ladybirds use sensing chemicals in larval tracks as an 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 regimes (C1, C4, C8, P1, P4, P8). We measured the developmental time of the fourth larval instar, pupae, and fresh adult body mass. Developmental time increased at the highest density in the combination C8 but remained unchanged across densities in the dishes with pheromone (P1–P8). Body mass was significantly lower at the highest density in both regimes (C8, P8) and was slightly higher in the presence of pheromone (P). Ladybird larvae respond independently of their density and of the presence of pheromones. The compounds present in the tracks, previously known as oviposition-deterring pheromone, may be associated with a buffering effect on density-related developmental delays.