Yponomeuta evonymellus outbreaks in southern Finland: spatial synchrony but different local magnitudes

Defoliations of Prunus padus by Yponomeuta evonymellus were monitored in tw o areas in southern Finland. During a long-term study (1980-1995) in the ar ea with high peak defoliations, P. padus trees recorded two defoliation pea ks, leading to complete defoliation of many trees. The negative relationshi p between pupal mass and current year tree defoliation during the peak year s suggested shortage of food affected reproductive potential of Y. evonymel lus, Furthermore, no delayed induced resistance was observed in these popul ations. Interannual correlations ill degrees of defoliation experienced by individual trees were low, i.e. high defoliation in one year did not predic t the degree of defoliation the following year. We also detected a sharp in crease in parasitism rates with defoliation between years. In the other stu dy area located about 150 km NW, only 4% of more than 200 study trees exper ienced defoliations higher than 50%. In this population, high defoliation o f tree individuals in one year predicted low defoliation in the following y ear. In addition, rates of parasitism were less variable and intermediate c ompared to those in the high peak area. Altogether, the relatively rapidly operating negative feedback in the low peak - compared with the high peak a rea - was consistent with the difference between both areas in the levels o f peak densities.