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

## TL;DR

Harlequin ladybird larvae adjust their development based on population density and pheromone exposure, affecting adult body mass and survival.

## Contribution

The study reveals how larval development and adult body mass are influenced by density and pheromone presence in ladybirds.

## Key findings

- Larvae in clean dishes developed slowly at high density, while those in dirty dishes developed faster.
- Adults from high-density environments were lighter, likely due to frequent larval encounters.
- Pheromones in larval tracks may buffer developmental delays caused by high density.

## Abstract

Larvae of ladybird beetles living in a high number together can compete for food and cannibalize other larvae. They can feel their density as a number of encounters and smell their footprints. We reared larvae of Harlequin ladybird Harmonia axyridis in three densities (1, 4, 8 in a dish) and either in dishes cleaned daily or with accumulated smell. We measured the time of development of larvae, and the body mass of the adults subsequently emerged. Larvae in dirty dishes kept developing fast even at high density, while larvae in clean dishes developed slowly at the high density. They thus avoided becoming defenseless pupae in the presence of other feeding larvae. Adult beetles that resulted from larvae living at high density were much lighter than at low density. This effect was probably caused by the frequency of meeting other larvae in a dish, not by the amount of the chemicals in footprints, because beetles from dirty dishes were slightly heavier. High-density populations may begin to self-regulate due to negative effects on growth and survival, reducing the risk of overpopulation.

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.