Agricultural soils throughout Europe have been shown to be almost deserts with respect to the occurrence of insect pathogens capable of infecting the most important oilseed rape (OSR) pests (Zec-Vojinovic et al., 2006). In contrast, soils in the natural ecosystems (such as forests) typically harbour these pathogens in quantities several orders of magnitude greater than those in the agricultural fields (Vanninen et al. 1989). It appears that many of the current agricultural practices are not compatible with insect pathogens. Soil-dwelling entomopathogens such as fungi (EPF) and nematodes (EPNs) probably suffer from multiple stresses, such as frequent exposure to strong UV-light when fields are without vegetation cover, dilution in soil via inversiontillage, lack of suitable host insects over long periods during the rotation, and the use of suppressive agrochemicals (e.g. fungicides, herbicides and mineral fertilizers). A specific case is Nosema (Anncaliia) meligethi, the intracellular obligate parasite of pollen beetle (Meligethes aeneus), which occurs at endemic levels outside the OSR-growing areas, but is virtually absent from areas where the crop is grown (Lipa & Hokkanen, 1992; Hokkanen & Lipa, 1995). This incompatibility pattern is a likely result of frequent insecticide treatments, which probably kill diseased (weakened) individuals more effectively than healthy ones, thus practically curing the pollen beetle populations from the Nosema disease.
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