Browsing by Subject "loisinta-aste"

Pollen beetle is one of the most harmful pests of rapeseed and turnip rape in Europe. It can cause considerably high yield losses. In addition, due to routine use of pyrethroids, pyrethroid resistant strains of pollen beetle have developed in several European countries, including Finland. One alternative for pesticides is biological control, which can be enhanced by improving the survival of natural enemies of pests. In Finland two species of natural enemies of pollen beetle occur: Phradis morionellus and Diospilus capito. The aim of this study was to investigate long term changes in percent parasitism and test if certain factors have influence on it. The main data on percent parasitism was collected during 1985-1995 from different locations of the turnip rape cultivation area in Finland. To get some current data, percent parasitism was observed in 2015, though from smaller area than in the main data. In addition to data on total percent parasitism, specific data on both parasitoid species was available for most of the years. Also the density of pollen beetle larvae was observed during 1990-1995. Supplementary data was collected on soil types, growing zones, precipitation and average temperature and annual severity of aphid and pollen beetle attack. To test differences between geographical areas, the observed locations was divided into 18 minor areas, and further into six grand areas. Several statistical analyses were ran on such a diverse data. The results show that percent parasitism differed significantly between years, geographical areas and growing zones. Percent parasitism was significantly relative to temperature in May and rain in July, but no differences were found between related soil types. Percent parasitism was not relative to density of host larvae, but correlated negatively with host density in the following year. Strong negative correlation between both severity of aphid attack and annual sale of dimethoate indicates that the parasitoids are exposed to pesticides in those years with high annual abundance of aphids. From the two parasitoid species, P. morionellus occurred much more abundant than D. capito. As observed, percent parasitism can be very high, but in some years and geographically it can be diminished due to several factors. By decreasing harmful activities, parasitism could be enhanced and biological control of pollen beetle could be improved.