Stinky pyrazines repel predators while inviting conspecifics: Defensive allomones as aggregation phermones in the migratory Hippodamia convergens

Background/Question/Methods

The aposematic Hippodamia convergens is an important biological control agent for aphid and mite pests. They are highly mobile predators that migrate from agricultural fields to overwinter in mountain-top aggregations. These aggregations recur in hibernacula traditional in location, despite turnover in migrants. The formation of aggregations and the permanence of these hibernacula may potentially be due to the ability of H. convergens to orient towards pheromone signals. We evaluated the role of conspecific odors in the formation of H. convergens aggregations in a lab bioassay using a still-air olfactometer. The olfactometer consisted of a circular arena with a porous floor under which four compartments of equal dimension delivered a localized source of volatile treatment odors or blank controls. Groups of diapausing beetles were allowed time to move through this heterogeneous odor-environment before eventually arresting in aggregations or otherwise. The distribution of individuals in relation to the treatment odor was characterized by analyzing their radial degree positions using circular statistics.

Results/Conclusions

Beetles were shown to aggregate within a particular section of the open arena when exposed to odor from a group of live conspecifics. This aggregative preference is maintained in odor treatments with particular alkylmethoxypyrazine components of coccinellid headspace odor. 2-Isobutyl-3-methoxypyrazine exhibited the strongest aggregative effect. Beetles also aggregated to some doses of 2-sec butyl-methoxypyrazine, but appeared to be repelled at higher doses. A third constituent, 2-isopropyl-3-methoxypyrazine, generally had little effect on the distribution of beetles. These three pyrazines commonly function as warning odors that in conjunction with other aposematic displays (red-black coloration, gregarious behavior, reflex bleeding) contribute to the multi-modal anti-predatory defense of many coccinellids, as well as other arthropods. These laboratory bioassays provide strong evidence that the defensive allomones of H. convergens have been co-opted for intraspecific communication. The evolutionary implication of this semiochemical parsimony and its potential role in biological control are discussed.