Abstract: Geographical variation in the periodicity of gypsy moth outbreaks
The existence of periodic oscillations in populations of forest lepidoptera is well known. While information exists on how the periods of oscillations vary among different species, there is little prior evidence of variation in periodicity within the range of a single lepidopteran species. The exotic gypsy moth is an introduced foliage-feeding insect in North America. Its populations are characterized by high amplitude oscillations between innocuously low densities and outbreak levels during which large regions of forest are defoliated. These outbreaks are recognized to arise periodically with considerable synchrony across much of the gypsy moth’s North American range. Our analysis indicates that gypsy moth outbreaks in North America are periodic but they exhibit two dominant periodicities: a primary period of 8-10 yr (as previously reported) and a secondary period of 4-5 yr (a new finding in this study). The outbreak periodicity varied geographically and this variation was associated with forest type. We found that in the most susceptible forest types, those with a significant oak composition, outbreak periodicity had a more dominant 5-year period while in the sub-optimal forest types cycles were clearly operating on a 10-year period.