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

Abstract

In order to assess the role of parasitoids in the regulation of non-outbreaking populations of Epirrita autumnata, a geometrid lepidopteran with outbreaking 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, using experimental and observational approaches respectively. The overall percent parasitism was independent of host density on both spatial scales, while temporally it fluctuated only little. Of the two main parasitoids, the commoner one, Protapanteles immunis, showed a variable response to host density on the larger spatial scale and negative density dependence on the smaller scale. Temporally, parasitism caused by this species was independent of host density. Another parasitoid, Phobocampe bicingulata, showed positive density dependence on the smaller spatial scale and had a variable response on the larger scale, but exhibited negative density dependence over time. The results of this study caution against drawing conclusions concerning population regulation on the grounds of spatial density dependence alone. Larval 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 regulating. Among other mortality factors of E. autumnata, pupal predation has been found to be temporally positively density-dependent.

Large larvae of a flush-feeding moth (Epirrita autumnata, Lepidoptera: Geometridae) are not at a higher risk of parasitism: implications for the moth's life-history

“…Jaenike, 1990;Mayhew, 1997, for exam ples with herbivorous insects). Moreover, in E. autum nata, high levels of larval parasitism (Teder et al, 2000, and this study) suggest strong intra-and interspecific exploitative competition among parasitoids that further contributes to the ephemeral character of the resource for parasitoids. This, in turn, should select for high oviposition rates, and reduced selectivity as a correlated response.…”

“…In terms of daily survival, the larval stage is definitely the most vulnerable one in the life cycle of E. autumnata (Tanhuanpaa et al, 1999(Tanhuanpaa et al, , 2001. Mortality of larvae attributable to parasitoids may reach levels (Ruohomaki, 1994;Teder et al, 2000, and this study) that exceed mortalities in other stages of the life cycle com bined. This should favour short developmental periods.…”

“…Larvae of E. autumnata are attacked by several hymenopteran parasitoids (Ruohomaki, 1994;Teder et al, 2000). The most common encountered in the pre sent study were the three solitary koinobiont species: Protapanteles immunis (Haliday) and Cotesia jucunda (Marsh.)…”

Causes of cyclicity of Epirrita autumnata (Lepidoptera, Geometridae): grandiose theory and tedious practice

“…However, southern forest communities are generally more productive and diverse; the concomitant increase in generalist predators offers a potential explanation for regional differences in population dynamics. In our studies, the highest mortality rates (close to 90%) occurred in the larval stage, mainly caused by passerine birds and parasitoids (see Tanhuanpää et al 2000). However, patterns in larval predation (Tanhuanpää et al 2000) and parasitism (Teder et al 2000) suggest that they are of limited importance in the regulation of southern populations.…”

“…In our studies, the highest mortality rates (close to 90%) occurred in the larval stage, mainly caused by passerine birds and parasitoids (see Tanhuanpää et al 2000). However, patterns in larval predation (Tanhuanpää et al 2000) and parasitism (Teder et al 2000) suggest that they are of limited importance in the regulation of southern populations. Six successive years of data from 12 to 15 populations of southern E. autumnata reveal surprisingly constant annual parasitism rates of 28%-35% (Teder et al 2000).…”