Photoperiod response influences both voltinism and impact in a biological control agent: comparing six sources of the loosestrife leaf beetle ( <i>Galerucella calmariensis</i> ) in a common environment

Multivoltine insects, those having more than one generation per year, often use daylength as a cue for terminating reproduction and entering diapause prior to the onset of winter. In a common garden experiment in Corvallis, Oregon, USA (44.6°), we compared voltinism and impacts of the loosestrife leaf beetle Galerucella calmariensis, a classical biological control agent for the wetland weed purple loosestrife (Lythrum salicaria), sourced from 6 locations in their introduced range (39.4 to 48.8° N). The populations were known to have diverged in their critical daylength for diapause since introduction in 1992. After rearing the populations in similar conditions for a generation, and overwintering the adult beetles outdoors, we followed cohorts of eggs on size-matched potted plants as they developed with natural timing through 2 generations in a randomized complete block design. Adult beetles from both the first and second generations were sampled to determine their reproductive status and voltinism. We harvested, dried, and weighed inflorescences and roots of the plants to measure agent impact. The 6 beetle populations were found to differ significantly in both voltinism and impacts, with a higher voltinism corresponding with greater impact. We also showed that voltinism in the introduced location was, to some extent, predictable based on laboratory determined photoperiod response curves, but other factors including plant quality also appeared to be influential. These results show that the efficacy of a biological control agent varies by geographic source and that this can be at least partially predicted by the population's diapause response to photoperiod.