Reduction in size and fecundity of the autumnal moth, Epirrita autumnata, in the increase phase of a population cycle

Abstract

Increasing fecundity with increasing density has been observed for many cyclic herbivore populations, including some forest Lepidoptera. We monitored population density, body size and reproductive capacity of the cyclic lepidopteran, the autumnal moth (Epirrita autumnata, Geometridae), from the early increase phase to the devastating outbreak density in northernmost Norway. Larval density of the species increased exponentially from 1998 to 2002 and remained at the outbreak level also in 2003. Within the same period, the body size and fecundity of individuals reduced as analysed from several parallel datasets on larvae, pupae and adults. In another study area in northernmost Finland, the density increase of the autumnal moth was moderate only, and true outbreak density was not attained during the study. Despite that, a reduction was again detected in the size and fecundity of individuals. Possible factors responsible for the reduced size and fecundity of individuals in the Norwegian population were quantitative shortage of foliage, rapid and delayed inducible resistances of the host, mountain birch (Betula pubescens ssp. czerepanovii), as well as crowding-induced responses of larvae. These factors likely acted in concert, although non-delayed responses to the density were emphasized. Our findings did not support the hypotheses of climatic release, inducible susceptibility of the host tree and mast depression (i.e. lowered chemical defence of the host tree after its mast seeding) as promoters of the fecundity-based density increase of the autumnal moth, since the reduced fecundity in relation to increased density was strongly against the predictions of these hypotheses. Therefore, we suggest that the density increase of autumnal moth populations is promoted by high survival rather than exceptionally high fecundity.

Geographically partitioned spatial synchrony among cyclic moth populations

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Foliar Phenolics are Differently Associated with Epirrita autumnata Growth and Immunocompetence

“…Interestingly, the index of immune defense was higher in trees with artificially introduced larvae in the previous year than in control trees, many of which still were devoid of larvae. Because the increase phase of the E. autumnata cycle is characterized by a low level of parasitism (Klemola et al 2004), this result suggests an unanticipated connection between induced changes in plant defense and the resistance of defoliators toward their enemies (parasitoids and diseases). It also implies that defoliator outbreaks and cyclic peaks may be facilitated, perhaps even caused, if defoliators benefit from the compounds produced by the plants against them (Haukioja 2005b).…”

Larval parasitism of the autumnal moth reduces feeding intensity on the mountain birch

“…According to our model, the reduction in food consumption owing to parasitoids may be enough to reduce the defoliation during the decline phase when the parasitism rate is high, but seems to be ineffective at the peak phase when larvae can cause total defoliation. This has also been observed in nature, where mountain birch forests have been totally defoliated and the effect of parasitism can be seen only after a delay (Tenow, 1972;Bylund, 1995;Klemola et al, 2004). However, only some autumnal moth populations reach outbreak levels in northern Fennoscandia (e.g.…”