The aggregation pheromones of the aphid predator, Hippodamia convergens

The convergent ladybird beetle (Hippodamia convergens) forms dense aggregations during winter diapause, the mechanisms of which are not well understood. We evaluated the role of three defensive allomones and cuticular hydrocarbon extracts in the formation of H. convergens aggregations. A 2-choice laboratory bioassay demonstrated that dipausing ladybirds preferred to walk up the branch of the Y-tube that had previously been exposed to conspecifics. Analysis of the surface of this pre-exposed branch revealed that walking ladybirds leave behind a blend of non-volatile hydrocarbon compounds. Applying an extract of these hydrocarbons to one branch of the Y-tube also yielded significant preference for the treated branch.  As opposed to diapausing females, mated female ladybirds avoided the treated arm. This suggests that depending on the physiological context, these same hydrocarbon footprint pheromones could also function as an oviposition deterrent in much that same way as those laid by ladybird larvae. In a separate series of bioassays, diapausing H. convergens aggregated within the section of an arena exposed to either of two pyrazines, 2-isobutyl-3-methoxypyrazine and 2-sec-butyl-methoxypyrazine. Laydbirds also aggregated to a blend of three pyrazines at their natural ratio. During fall migration to overwintering sites, significantly more ladybirds aggregated in artificial hibernacula baited with pyrazines. These three pyrazines also function as warning odors that in conjunction with other aposematic displays contribute to the multi-modal, anti-predatory defense of coccinellid beetles and some other arthropods. Confirmation of the role of some pyrazines in H. convergens aggregations suggests that these defensive allomones have been co-opted for intraspecific communication.