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

1. Plant resistance against insect herbivores often increases after experim ental damage to foliage, but few studies have obtained field estimates of t he effect of induced resistance on insect populations during and after a na tural insect outbreak. 2. This study measured the effect of quality of the host tree, mountain bir ch (Betula pubescens ssp. czerepanovii), on the periodically fluctuating fo livore Epirrita autumnata (Lepidoptera, Geometridae) during peak and postpe ak years of an outbreak in Finnish Lapland. Comparisons were made both with in and between study sites to assess host plant quality, anti thereby the e ffect of delayed induced resistance (DIR). 3. In within-site comparisons, a set of experimental trees was defoliated b y wild larvae in the peak year of the outbreak, whereas control trees were protected from defoliation by spraying with an insecticide. The effect of h ost plant quality was quantified in :ht following year bq measuring the pup al mass of E. autumnata larvae reared in enciosures on these trees. 4. In between-site comparisons, the sizes of pheromone-trapped males were m easured at both outbreak and low density sites during the progress of the o utbreak. The size of trapped males was subsequently used to estimate the co rresponding fecundity of females at the same sites. 5. Pupal mass of E. autumnata reared on trees defoliated in the previous ye ar was 0- 10% lower than on those trees protected from defoliation by the i nsecticide. Field-collected adults indicated 3 similar pattern: they were s maller at outbreak sites than at low-density sites, and the size reached it s 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 expla nation for the termination of the outbreak. 6. Whether the weak DIR response in this system was a characteristic of the 1990s outbreak alone ri mains unclear, because different terminating agent s may be important for different individual outbreak peaks. During this out break, larval parasitism and developmental asynchrony between larvae and bi rch were probably more important reasons for population collapse than DIR.