Climatic synchrony and increased outbreaks in allopatric populations of an invasive defoliator

Larch casebearer is an invasive defoliator in North America distributed within the regions of two allopatric hosts, eastern larch and western larch. Despite the establishment of a successful importation biological control program and ongoing parasitism by both native and introduced parasitoids, larch casebearer has recently undergone outbreaks on eastern larch and western larch. We analyzed defoliation data from aerial surveys to quantify spatiotemporal synchrony and found that defoliation by larch casebearer was synchronous both within and between eastern and western larch forests. We also analyzed monthly minimum temperatures across the study region and found that warming spring temperatures in March and cooling fall/winter temperatures in October through December were positively correlated at distances comparable to those between the allopatric outbreaks of larch casebearer. For allopatric populations with positively correlated population dynamics, climate is the most likely driver of synchrony. Thus, we suggest that a changing climate has facilitated recent outbreaks of larch casebearer.