Technical Abstract: 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. But it was unstudied how (or if) the pheromone functioned for the subsequent generation in summer, when reproduction is limited but when dispersal is frequent. Previous research in summer found aggregation patterns during host colonization that suggested a pheromone was operating. Our results supported this concept: Beetles responded strongly to the pheromone in summer. Some summer-collected males emitted pheromone, and mating and oviposition of fertile eggs followed. (These findings were indicative of a partial second generation, which sometimes occurs, and also reinforced the reproductive role of the pheromone). Males emitted pheromone only when on L. salicaria, and emission could cease quickly and resume again, even after a long interruption. Marginal hosts such as Salix interior or Rosa spp. could sustain life but did not facilitate pheromone emission. To dispersing beetles, the pheromone would signal existence of a site with both conspecifics and suitable host. Attractiveness of L. salicaria foliage was investigated. Six "green leaf" volatiles were identified that were readily sensed by beetle antennae, but a synthetic blend of these was not active in the field. The pheromone is a powerful, practical monitoring tool and is especially effective for dispersing populations that are difficult to detect by other means. New information is presented about longevity of pheromone lures.