Coexisting but temporally separated cohorts of insects with a multiannual l
ife cycle may have dissimilar average abundance, resulting in periodically
fluctuating population density. In the case of the boreal moth genus Xestia
with a 2-year life cycle and a distinct abundance difference between the t
wo coexisting cohorts, empirical results and a simple model suggest that th
e oscillatory dynamics are maintained by interaction with a parasitoid wasp
. Here we report theoretical results on a spatially extended version of the
basic model and relate the modeling results to empirical observations. A s
patially extended model may have domains oscillating in different phases as
is the case between western and eastern Finnish Lapland. Spatial heterogen
eity tends to fix the location of phase boundaries. In contrast, spatially
homogeneous temporal fluctuations tend to synchronize populations in large
regions. (C) 2001 Academic Press.