USE OF HOST PLANTS BY OPEROPTHERA-BRUMATA L (LEP, GEOMETRIDAE) DURINGTHE FIRST RECORDED OUTBREAK IN THE SUBCONTINENTAL BOREAL ZONE OF FENNOSCANDIA

The first known outbreak of Operopthera brumata in the subcontinental boreal zone was found in 1994. In the summer of 1995 use of the host p lants by O. brumata was studied during the outbreak. Eight transects w ere studied from outbreak epicentres to the undefoliated perimeter. Al ong the transects the defoliation level of each tree and shrub species was estimated. The epicentres were in deciduous tree stands in early secondary succession on abandoned pasture land. The defoliation of dif ferent species of deciduous trees growing the area differed significan tly. Of the dominant canopy species of the epicentres, Prunus padus an d Sorbus aucuparia were the most defoliated, while Alnus incana and Be tula pubescens had a medium level of defoliation. The average level of defoliation was 90% for P. padus and S. aucupara, and about 25% for A . incana and B. pubescens. Shrub species such as Rhamnus frangula, Rib es spp. and Sambucus racemosa had very low levels of feeding. In the s tudy area, Quercus robur, normally the main hostplant of O. brumata, w as rare; but Prunus padus, which has a similar growing strategy, was v ery abundant in the outbreak epicentres but absent around the perimete rs. In general, P. padus was the most defoliated tree species in the s tudy area. We suggest that P. padus is the main host plant of O. bruma ta in SE Fennoscandia, and only in those areas where it is abundant ar e O. brumata populations able to increase to the outbreak level.