Stinky pyrazines repel predators while inviting conspecifics: Defensive allomones as aggregation pheromones in the aphid predator Hippodamia convergens (Coccinellidae)

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 cues. We evaluated the role of conspecific odors in the formation of H. convergens aggregations in a lab bioassay. Beetles were shown to aggregate within a particular section of an open, still-air 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.