Induced resistance of host tree foliage during and after a natural insect outbreak

Abstract

1. Plant resistance against insect herbivores often increases after experimental damage to foliage, but few studies have obtained field estimates of the effect of induced resistance on insect populations during and after a natural insect outbreak. 2. This study measured the effect of quality of the host tree, mountain birch (Betula pubescens ssp. czerepanovii), on the periodically fluctuating folivore Epirrita autumnata (Lepidoptera, Geometridae) during peak and postpeak years of an outbreak in Finnish Lapland. Comparisons were made both within and between study sites to assess host plant quality, and thereby the effect of delayed induced resistance (DIR). 3. In within‐site comparisons, a set of experimental trees was defoliated by wild larvae in the peak year of the outbreak, whereas control trees were protected from defoliation by spraying with an insecticide. The effect of host plant quality was quantified in the following year by measuring the pupal mass of E. autumnata larvae reared in enclosures on these trees. 4. In between‐site comparisons, the sizes of pheromone‐trapped males were measured at both outbreak and low density sites during the progress of the outbreak. The size of trapped males was subsequently used to estimate the corresponding fecundity of females at the same sites. 5. Pupal mass of E. autumnata reared on trees defoliated in the previous year was 0–10% lower than on those trees protected from defoliation by the insecticide. Field‐collected adults indicated a similar pattern: they were smaller at outbreak sites than at low‐density sites, and the size reached its minimum in the post‐peak year. However, the estimated loss of reproductive capacity of females resulting from DIR was too small to be the sole explanation for the termination of the outbreak. 6. Whether the weak DIR response in this system was a characteristic of the 1990s outbreak alone remains unclear, because different terminating agents may be important for different individual outbreak peaks. During this outbreak, larval parasitism and developmental asynchrony between larvae and birch were probably more important reasons for population collapse than DIR.