Host plant quality and defence against parasitoids: no relationship between levels of parasitism and a geometrid defoliator immunoassay

Abstract

Host plant quality has a major influence on the performance, and ultimately on the fitness of an herbivorous insect, but may also have indirect effects on the third trophic level by affecting an herbivore's defensive ability against natural enemies. In a three-year field study, we examined the effects of natural food quality on the ability of autumnal moths, Epirrita autumnata (Lepidoptera, Geometridae), to defend themselves against parasitoids. In each year, we confirmed the variation in quality of host trees (mountain birch, Betula pubescens ssp. czerepanovii) by determining the mass of pupae reared in mesh bags attached to the trees and the water content of leaves. Individuals grown on high quality trees possessed significantly higher encapsulation rate of a foreign antigen as pupae compared to those on low quality trees during the first and third study years; a parallel trend was also found in the second study year, although this difference was not statistically significant. However, in spite of observed differences in encapsulation rates, individuals reared on high and low quality trees did not differ in their levels of parasitisation when exposed to hymenopteran parasioids in the wild and thus were equally vulnerable. Accordingly, the encapsulation response seems not to play a major role on the population ecology scale in the studied system. Our findings also stress the importance of direct resistance tests, which should be conducted along with tests of insect immune function.

Reversed Impacts by Specialist Parasitoids and Generalist Predators May Explain a Phase Lag in Moth Cycles: A Novel Hypothesis and Preliminary Field Tests

“…3). The larval and pupal parasitoid species of this study correspond to the parasitoid fauna found during the latest increase and peak phases of the cycle (Klemola et al 2007, Klemola, N. et al 2008 and are listed in Table 2.…”

“…We tested our hypothesis on reversed effects of specialist and generalist natural enemies with one-year field experiments on the larval and pupal stages, which seem most prone to mortality from natural enemies (Tanhuanpää et al 1999, 2001, Klemola, N. et al 2008, Hansen et al 2009). Specifically, we estimated the probability of larval and pupal parasitism, as well as pupal predation of both species by vertebrates and invertebrates, under field conditions in 2007.…”

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

“…In post-peak and decline phases of the autumnal moth cycle, the overall larval parasitism rates are noticeably high, occasionally approaching 100%, of which a major proportion (>90%) can be caused by solitary parasitoids (Bylund, 1995;Tanhuanpää et al, 2002;Klemola et al, 2007Klemola et al, , 2008aK. Ruohomäki and T. Klemola, unpublished data).…”

“…Klemola et al, unpublished data). The parasitism rate ranges from 0 to 100 (Bylund, 1995;Klemola et al, 2008a). For 5th instar larvae of the autumnal moth, the value a is the total amount of leaves consumed by a non-parasitised larva, while b is the total amount of leaves eaten by a parasitised individual.…”

“…During the density increase of the autumnal moth, predators are unable to effectively track the rapid changes in herbivore densities (Enemar et al, 1984;Ruohomäki et al, 2000). However, in both the peak and decline phases of the moth cycle, there are at least 15 hymenopteran parasitoid species present in high numbers, which attack either the egg, larval or pupal stages of E. autumnata (Tenow, 1972;Ruohomäki, 1994;Bylund, Oecologia (2009) 159(3): 539-547 1995Tanhuanpää et al, 2002;Klemola et al, 2007Klemola et al, , 2008a. Of all parasitoids present, the proportion of solitary species is high (Klemola et al, 2007.…”