Temporal and spatial variation of larval parasitism in non-outbreaking populations of a folivorous moth

In order to assess the role of parasitoids in the regulation of non-outbrea king populations of Epirrita autumnata, a geometrid lepidopteran with outbr eaking populations in northern Europe, we examined the temporal and spatial variation of larval parasitism in southwestern Finland during 6 successive years. The study was carried out on two spatial scales, among trees within sites of about 1 ha and among sites separated by distances of 2-10 km, usi ng experimental and observational approaches respectively. The overall perc ent parasitism was independent of host density on both spatial scales, whil e temporally it fluctuated only little. Of the two main parasitoids, the co mmoner one, Protapanteles immunis, showed a variable response to host densi ty on the larger spatial scale and negative density dependence on the small er scale. Temporally, parasitism caused by this species was independent of host density. Another parasitoid, Phobocampe bicingulata, showed positive d ensity dependence on the smaller spatial scale and had a variable response on the larger scale, but exhibited negative density dependence over time. T he results of this study caution against drawing conclusions concerning pop ulation regulation on the grounds of spatial density dependence alone. Larv al parasitoids apparently do not maintain low densities in the E. autumnata populations studied. However, they may suppress E. autumnata densities to a level low enough for density-dependent mortality factor(s) to become regu lating. Among other mortality factors of E. autumnata, pupal predation has been found to be temporally positively density-dependent.