Long-term inducible resistance in birch foliage: triggering cues and efficacy on a defoliator.

Heavy damage of the mountain birch foliage, as well as application of small amounts of insect frass to the soil beneath the trees, reduced growth of Epirrita autumnata larvae reared in these trees in the following year. Foliage damage in the previous year decreased larval survival, too. Both foliage damage and insect frass in the soil decreased a fecundity index which combined the effects of size and survival. Because application of small amounts of fertilizers had an effect indistinguishable from that of insect frass, the effect of the frass may base on responses of trees to an increase in soil nutrient concentration in mid-summer. In previously untreated control trees, all performance indices (growth, survival, and egg production) of Epirrita correlated positively with the distance of the birch from the closest birch defoliated in the previous year, indicating "communication" between adjacent trees. Epirrita egg production in trees that had been both defoliated and treated with frass in the previous summer was at least 70% lower than in previously unmanipulated control trees.