We examined the effect of natural host‐plant quality on immune resistance in the autumnal moth, Epirrita autumnata (Borkhausen) (Lepidoptera: Geometridae). The division of mountain birches [Betula pubescens ssp. czerepanovii (Orlova) Hämet‐ahti (Betulaceae)] into two categories, high‐ and low‐quality food for larvae, was based on previous years’ results on the relative growth rate of the autumnal moth on the trees selected. The strength of the immune defence of autumnal moths was determined by measuring their encapsulation rate to exposure to a foreign antigen and the phenoloxidase (PO) activity of the pupal haemolymph. We found that individuals reared as larvae on naturally low‐quality food had a significantly higher encapsulation rate at the pupal stage than individuals reared on high‐quality food. Females also had a higher encapsulation rate than males. Food quality did not have statistically significant effect on PO activity, nor did this response variable show any differences between the sexes. Using half‐sib analyses, we found significant heritable variation in the encapsulation rate; the heritable variation in PO activity was near to significant, although equally strong. Heritability estimates (h2: 0.19–0.27) for immune defence traits were relatively low and only moderate when compared to other studies with insects. We also found a negative genetic correlation between pupal mass and PO activity, but not between PO activity and encapsulation rate. Our results suggest that the quality of food affects immune defence in the autumnal moth. Thus an intricate tritrophic relationship exists between the folivorous insect, the host tree, and the insect's natural enemies (e.g., pathogens, parasites, and parasitoids). This study demonstrates that natural variation within a food plant species has an effect on the innate immune system of an herbivorous insect.
During population outbreaks, top-down and bottom-up factors are unable to control defoliator numbers. To our knowledge, details of biotic interactions leading to increased population density have not been studied during real population outbreaks. We experimentally assessed the strength of plant defenses and of insect immunocompetence, assumed to contribute to active insect resistance against parasitoids and pathogens, in the geometrid Epirrita autumnata during a steep increase in population density. We demonstrated rapid (same-season) induced resistance in the foliage of its host, mountain birch. The response was systemic, spreading throughout the tree, and retarded larval growth rate by approximately 10%. On the other hand, no direct delayed carry-over effects were found in the next season in larval growth rate, mortality, or pupal mass. Larval damage to a tree during the previous year, however, significantly (by approximately 13%) accelerated the advance of the immune response (measured as melanization of an implant inserted into the pupal hemocoel). The encapsulation rate correlated positively with larval mortality in trees in which larvae had been introduced the previous year, but not in control trees. Both of these observations suggest that induced plant defense was associated with an increased insect immunocompetence during the population increase.
The quality of available food may affect insect herbivores directly (via growth and survivorship) and/or indirectly (by modifying insect vulnerability to parasitoids and pathogens). We examined the relationship between different phenolic compounds, belonging to various phenolic groups, in Betula pubescens spp. czerepanovii (mountain birch) foliage and the larval performance of the geometrid Epirrita autumnata (autumnal moth). Direct effects on insect performance were described by pupal weight, developmental rate, and survivorship; indirect effects were described by the encapsulation rate of an implant inserted into the insect hemocoel, a commonly used way to describe insect immune defense. We found profound differences in the effects of different phenolic categories: several individual hydrolyzable tannins were associated positively with larval performance but negatively with level of immune defense, whereas flavonoid glycosides were inversely related to larval survival but showed no association with the larvae immune defense.
Birch oxidases together with their substrates may form an important front line in defence against herbivores and pathogens.
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