Implications of climate change for biocontrol efficacy across the northern range of the invasive plant Linaria dalmatica

The effect of insect biological control agents on invasive plant populations can vary spatially, and spatial vari-ation in climate may drive regional variation in herbivory, and thus biocontrol efficacy. Within host plant populations, local plant abundance can also be affected by the spatial distribution of herbivores, but whether local patterns persist at larger scales is less well understood. We examined how infestation and damage of the stem-mining weevil Mecinus janthiniformis, a specialist biocontrol agent of the invasive plant Linaria dalmatica, varied within and among populations across the northern edge of the range in North America. We quantified weevil and invasive plant densities, as well as plant fecundity, stem diameter and height across sites spanning an area of similar to 39,000 km(2) in British Columbia, Canada. We found that specialist weevils did not respond to host plant density within sites across the study region. Instead, weevil attack and load were most sensitive to among-site variability in climate, with stems at warmer sites having four times as many weevils compared to stems at cooler sites. Weevils also reduced plant fecundity more at warmer sites, when controlled for plant size (stem diameter) with larger effects of weevils in thicker stems, indicating that L. dalmatica suppression is highest in warmer locations where weevils are more abundant and environmental conditions more favorable. Our results suggest that, with climate change, the efficacy of biocontrol for L. dalmatica will improve across the northern edge of the range. We recommend that at cooler sites, where biocontrols are less prevalent and effects on plant fecundity are weaker, alternative management strategies are necessary at this time. Across invasive plant species more generally, future studies that establish the role of climate in variability in biocontrol efficacy long after introduction can improve management of invasive plants under climate change.