Associate Professor Queen's University Kingston, Ontario, Canada
Abstract: A common method to manage invasive species is the introduction of natural enemies (i.e., herbivores, pathogens, and parasites) as biological control agents. However, the success of these efforts can be highly variable owing to a variety of ecological and genetic factors. For example, Galerucella pusilla and G. calmariensis are two specialist herbivore beetles that were introduced to control invasive populations of the perennial herb purple loosestrife (Lythrum salicaria) in North American wetlands. The response to this biological control program has been highly variable with some populations exhibiting drastic population shrinkage over time while others are seem unaffected indicating possible genetic variation in defence or tolerance to herbivory. Recent studies in L. salicaria, have demonstrated that growth and timing of reproduction are highly variable across the introduced range and that this variation is heritable, resulting in flowering time clines that are locally adapted to variation in season length. We investigated how genetic constraints imposed by climate adaptation may limit the ability of L. salicaria to evolve in response to herbivore pressure. To this end, we conducted a multi-year common garden experiment at the Queen’s University Biological Station (QUBS) in Ontario, Canada. The garden includes 12 experimental blocks, each with 200 individuals representing 20 genetic populations and 10 seed families per population. Half of the blocks were exposed to Galerucella beetles and other herbivores while the other half were protected by insecticide application. These seed families were sampled along a 10° latitudinal gradient in northeastern North America where northern plants have evolved early flowering to reproduce in shorter growing seasons relative to southern plants. We estimated the strength of phenotypic selection on flowering time and computed the relationship between strength of selection in the presence and absence of herbivory. Over seven years of data, early-flowering families experience a greater decline in fitness in the presence of herbivory relative to late-flowering families (p < 0.001), indicating that northern populations are less tolerant to herbivory relative to southern populations of L.salicaria. This variation in tolerance is tied to genetic constraints on flowering time. This research demonstrates an important interaction between biotic and abiotic selection. Specifically, adaptation to the abiotic stressor of climate limits the ability of populations to respond to the biotic stressor of insect herbivory. These interactions may be important for understanding the spread of invasive species under climate change and has implications for management strategies that utilize biological control.