Host-parasitoid dynamics in periodic boreal moths

Numerous insect species with fixed multiannual life cycle emerge synchronously in large numbers, mostly with a period of two or three years. Moths in the genus Xestia with a two-year life cycle are strikingly abundant in boreal forests around the Northern Hemisphere every second year but scarce in alternate years. Many hypotheses have been put forward to explain their periodic occurrence, including interaction with natural enemies. Here we present biological, numerical and modelling evidence strongly suggesting that the scarcity of the even-year cohorts of eight coexisting Xestia species in eastern Finnish Lapland is due to regulation by the parasitoid wasp Ophion luteus, possibly reinforced by predation by shrews.