Patterns of abundance along a limiting gradient are important for understanding variable impacts by introduced weed biological control agents. In areas at extremes of the gradient (i.e. at range margins), interannual variability in limiting factors (e.g. climate) may cause shifts in the location of range margins and alter species interactions and biological control dynamics. In the US, the biological control agent Agasicles hygrophila (flea beetle) has a range thought to be limited by cold winters to areas where mean winter temperatures remain above 11.1° C, but its host (alligatorweed) is more broadly distributed. More than 50 years have passed since the initial introduction of A. hygrophila, but no study has yet evaluated the dynamics of control by A. hygrophila at the range margin. Over two years (2015, 2016), we studied abundance of agent and host at sites along a climatic gradient within and at the northern range margin of the herbivore to test whether 1) flea beetle abundance declines with increasing latitude, 2) variability in flea beetle abundance increases with latitude, 3) alligatorweed abundance increases with latitude (due to decreased herbivore impacts), 4) variability in winter temperatures between the two years explained patterns in insect abundance and impacts to alligatorweed.
2014-2015 winter temperatures were colder than 2015-2016 which was reflected in an apparent northward range margin shift from the first to second study year. Based on published winter temperature thresholds, we estimated flea beetle overwintering to occur south of N 30.4° (our southernmost sites) during winter 2014-2015 but all sites (up to N 33°) in the following year. This corresponded to a negative relationship between insect abundance and latitude in 2015 but not 2016. Variability in insect abundance was positively related to latitude only in 2016. Plant abundance increased with latitude both years but was lower overall during the second year, likely due to extended feeding by A. hygrophila during the warm winter. We suggest that relatively subtle variation in winter temperatures from year to year can cause disproportionately large differences in biological control due to range margin shifts.