Entomol. exp. appl. 67: 241-246, 1993. 9 1993 Kluwer Academic Publishers. Printed in Belgium. 241 Overwintering survival and spring emergence in Meligethes aeneus: effects of body weight, crowding, and soil treatment with Beauveria bassiana H. M. T. Hokkanen Department of Applied Zoology, University of Helsinki, SF-O0710 Helsinki, Finland Accepted: November 19, 1992 Key words: integrated control, biological control, Brassica campestris, Nitidulidae, Finland, density dependence, mortality, fat reserves Abstract High overwintering mortality was recorded for Meligethes aeneus in the field in Finland, ranging from 85 to 98~o depending on the source of the overwintering population. The main factor explaining vari- ation in survival was the body weight of the beetles in the autumn: only the heaviest beetles survived. Body weight, on the other hand, was influenced by treatment of the rape field soil with Beauveria bassiana, resulting in a 50 ~o decrease in winter survival compared with the reference. The soil treatment did not appear to have direct mortality effects (pathogenesis) on the beetle neither in the summer nor in the winter, and there was no apparent effect on survival of crowding of the insects at the overwintering site. Density dependence in overwintering survival ofM. aeneus may function through the availability of pollen food in the autumn, and intraspecific competition for it. Treatment of the rape field soil with B. bassiana may be a feasible way to increase the winter mortality and to lower the pollen beetle populations, but requires further research. Introduction Particularly at the northernmost areas of their range, insects are likely to suffer from heavy nat- ural mortality during overwintering (Danks, 1978; Leather, 1992a). Mortality may be due to inade- quate cold hardiness, but often also to the deple- tion of fat reserves during the long winter (Oney- ka & Boreham, 1987; Leather, 1992b). Meligethes aeneus F. (Col., Nitidulidae) is rel- atively well adapted to the climate of Northern Europe, and it occurs up to the Arctic Circle. With the large increases in the growing area of oilseed crucifers during the last 10-15 years ev- erywhere in Europe, it has become a very serious pest (Bromand, 1990). Although the basic biology of the species is rather well studied (e.g. BOrner & Blunck, 1920; Kaufmann, 1925; Friedrichs, 1954; Nolte, 1954; Fritsche, 1957; Jourdheuil, 1960; Nilsson, 1988a- c), little is known about its overwintering survival. MOiler (1941) characterized the typical overwin- tering sites and their requirements, but did not give data on survival. The beetle is strictly univoltine throughout its distribution area. It overwinters in the adult stage, just beneath the litter layer in the soil, typically in forest areas surrounding the fields (M~tller, 1941; Blazejewska, 1958). A few individuals may also overwinter in the field. Light soils on a hillside are preferred, possibly because there the danger of flooding is minimal (MOiler, 1941).