Sexual communication in coccinellids: mechanisms, triggers, and evolutionary conservation

The first sex pheromone ever identified in a lady beetle was recently discovered in the Asian
lady beetle Harmonia axyridis (Coleoptera: Coccinellidae). Females display a stereotyped
calling behavior that results in the release of a volatile blend -mainly composed of five terpenoid
compounds- which attracts males and triggers copulation, but only in the presence of aphid
prey. Although similar behaviors have been observed in other coccinellid species, pheromonal
communication in closely related taxa remains poorly studied. Moreover, the precise olfactory
cues triggering pheromone production in H. axyridis, as well as the potential influence of larval
diet quantity and quality, remain unknown.
This study, which is an integral part of Anaïs Sion’s PhD thesis, aims to (1) identify the olfactory
cues that trigger sex pheromone production; (2) evaluate the impact of larval diet on adult
pheromone output; (3) explore the conservation of sex pheromones across phylogenetically
related species.
Using dynamic sampling of volatile organic compounds (VOCs) further analyzed by gas
chromatography (TD-GC-MS-FID), we first demonstrate that exposure to aphid-emitted VOCs
alone is sufficient to trigger sex pheromone production in H. axyridis. To identify specific cues
that trigger pheromone emission, we exposed ladybeetles to aphid alarm pheromone or aphid
honeydew, but no significant difference in pheromone production was found between the two,
due to large variations within each modality. Additionally, larval diet quality and quantity had
no significant effect on adult pheromone production, highlighting the pivotal role of some
aphid-derived cues during adulthood.
In H. quadripunctata, we identified three female-specific compounds—β-caryophyllene, β
elemene, and α-humulene—also present in H. axyridis, suggesting a conserved communication
mechanism. Further behavioral assays and analyses are needed to confirm their functional role.
This study contributes to our understanding of sexual communication in coccinellids,
specifically in H. axyridis and may inform future phylogenetic analyses within this family.