Defoliations of Prunus padus by Yponomeuta evonymellus were monitored in two areas in southern Finland. During a long-term study (1980–1995) in the area with high peak defoliations, P. padus trees recorded two defoliation peaks, leading to complete defoliation of many trees. The negative relationship between pupal mass and current year tree defoliation during the peak years suggested shortage of food affected reproductive potential of Y. evonymellus. Furthermore, no delayed induced resistance was observed in these populations. Interannual correlations in degrees of defoliation experienced by individual trees were low, i.e. high defoliation in one year did not predict the degree of defoliation the following year. We also detected a sharp increase in parasitism rates with defoliation between years. In the other study area located about 150 km NW, only 4% of more than 200 study trees experienced defoliations higher than 50%. In this population, high defoliation of tree individuals in one year predicted low defoliation in the following year. In addition, rates of parasitism were less variable and intermediate compared to those in the high peak area. Altogether, the relatively rapidly operating negative feedback in the low peak — compared with the high peak area — was consistent with the difference between both areas in the levels of peak densities.