MODIFICATION OF THE DEVELOPMENT OF BEAUVERIA TENELLA MYCOSIS IN MELOLONTHA MELOLONTHA LARVAE, BY MEANS OF REDUCED DOSES OF ORGANOPHOSPHORUS INSECTICIDES

Infection experiments have been carried out in the laboratory by mixing increasing quantities of spores of Beauveria tenella and reduced doses of parathion or trichloronate (organophosphorus insecticides) with the rearing medium of white grubs (Melolontha melolontha) to verify whether it is possible to favour the development of the muscardine disease. The results are similar to those which were obtained previously with reduced doses of BHC: with low dosage of spores (5 × 104 per gram of peat) which does not produce the mycosis after 4 months at 20°C, the addition of small quantities of parathion or trichloronate gives a real increase of muscardine; with an inoculum of 1 × 106 spores per gram, the mortality by the fungus is approximately the same with or without addition of insecticide, but the development of the disease is more rapid in the case of a simultaneous contamination by the spores and one of the two insecticides. These results give the possibility of carrying out field experiments with low quantities of a non long lasting insecticide so as to favour the development of the muscardine disease in a natural population of white grubs.