Does Larval Dispersal Cause the Expansion of Gypsy Moth Outbreaks?

Pest managers often suggest that gypsy moth outbreaks spread from one area to another. An example of this concept is the so-called "focal area" theory. Under this theory, outbreaks start in small, specific areas (foci) and expand out into larger, concentric zones about this area in successive years. The hypothesized mechanism for this expansion is dispersal of first instar larvae. A simple simulation model was developed to describe the role of dispersal and biomodal density-dependent mortality in the spatial dynamics of gypsy moth densities. Using this model, it was impossible to explain large-scale expansion of outbreaks from foci. Thus, dispersal is an unlikely cause of the commonly observed expansion of defoliated areas. Instead, we hypothesize that these patterns are caused by the spatial distribution of resistant tree stands and that, on a regional scale, spatially separate populations operate largely independently of one another. This conclusion suggests that suppressing populations in one area is unlikely to have an affect on the development of outbreaks in adjacent areas. North. J. Appl. For. 8(3):95-98.