Flea beetles (Coleoptera, Chrysomelidae, Halticinae) on rapeseed and sugarbeet in Finland

Surveys of the incidence of flea beetles on sugarbeet and rapeseed were carried out in eight localities in southern and central Finland in 1972 and 1980—83. The first flea beetles emerged from overwintering in late April to early May, depending on the temperature. The majority, however, appeared during the second half of May, when daily temperatures occasionally reached +20°C. Flea beetles found their host plants by olfactory orientation. Two population peaks occurred during the growing season. The overwintered flea beetles formed the first peak in late May to early June and the adults of the new generation the second peak starting in late July. The incidence of flea beetles fluctuated greatly on the cultivation, as the flea beetles moved only short distances and showed thus very local occurrence. The availability of host plants greatly affected their reproduction rate, and thus the annual and regional differences in the incidence of the flea beetles were great and depended solely on the availability of host plants. Therefore it was difficult to establish any countrywide differences in the incidence of the beetles. Only Phyllotreta undulata (Kutsch.) and P. striolata (F.) were of any importance aspests of rapeseed. P. undulata made up some 80—90 % and P. striolata some 10 % of the total number of flea beetles on rapeseed. P. striolata was more abundant on radish than on rapeseed. Sugarbeet was damaged only by Chaetocnema concinna (Marsh). Other species of flea beetles were also observed in small numbers on rapeseed, radish and sugarbeet. They did not, however, cause any damage, but spread from adjacent cultivated plant species or weeds. The suction trap collected only a few flea beetles, but clearly revealed their activity periods. Damage caused by flea beetles is most harmful during the short seedling stage. At that time one flea beetle per plant was considered the threshold level for control measures. Later, even several flea beetles did not significantly hamper the growth. Seed coating efficiently prevented damage by flea beetles. The general incidence of flea beetles observed during this study was so low that coating of the seed was not justified. Chemical control of blossom beetle efficiently reduced flea beetles as well. The abandoning of the cultivation of winter rape also reduced the total number of flea beetles.