Early-summer pheromone biology of Galerucella calmariensis and relationship to dispersal and colonization

Galerucella calmariensis (Coleoptera: Chrysomelidae) has become an effective biological control agent for purple loosestrife (Lythrum salicaria). A male-produced aggregation pheromone was recently identified in this mostly univoltine beetle, and attractiveness to both sexes was demonstrated in the spring, when overwintered adults reproduce. Nothing was known about whether the pheromone functioned for the summer generation, when some of the beetles may undergo a brief reproductive period before entering diapause. Our results showed that both sexes of the beetles responded strongly to synthetic pheromone during summer, and males collected in the field during early summer and held under the prevailing photoperiod were able to emit pheromone. These males mated readily with field-collected females, and fertile eggs were laid, indicating a reproductive role for the pheromone during early summer. Males emitted pheromone well only when fed L. salicaria. Adult beetles were able to persist on marginal hosts such Salix interior and Rosa spp., but almost no pheromone was detected. Emission ceased and resumed as host access changed, even after a long interruption. Attractiveness of L. salicaria foliage was investigated. Six “green-leaf” volatiles were identified that were readily sensed by beetle antennae, but a preliminary synthetic blend of these was not attractive in the field. A model is presented for pheromone-mediated host colonization after dispersal, and relationships among pheromone biology, diapause, photoperiod, and host quality are discussed. Practical uses for the pheromone are noted, and new information is given about longevity of pheromone lures.