INFLUENCE OF DESTRUXINS ON DISEASE DEVELOPMENT OF METARHIZIUM-ANISOPLIAE IN SCARABEID LARVAE

The presence of mycotoxins like destruxins in insects infected by the entomopathogenic hyphomycete Metarhizium anisopliae (Metsch.) Sor. is well established. In the case of the scarabeid larvae of Cetonia aurata L. and of Oryctes rhinoceros L. contaminated by this fungus, a cytotoxic effect on hemocytes was demonstrated in previous studies. Thus, the role of purified destruxins A, B and E in modifying the pathogenicity of 2 isolates of M. anisopliae in these insects was studied. Single and split inoculations of destruxins, either alone or in combination with fungal spore injection were experimented. The difference in action of destruxins applied in split doses, in these 2 host species indicated a rapid detoxification in larvae of C. aurata as against a cumulative effect in O. rhinoceros. The results indicated that the development of the disease in C. aurata larvae, inoculated with the specific strain M. a. 32 was not modified by combined injection of a single dose (2 ; 6.6 and 20 .mu.g/g of larval weight) or by split dose applications (4 injections of 2 .mu.g/g at 2 days interval). With the non specific strain M. a. 51, a high sensitivity of larvae was seen at a later period after the treatment. In O. rhinoceros larvae, 3 split applications of destruxins at a dose of 1 .mu.g/g stimulated the activity of the specific strain M. a. 51, since LT 50, which was 11.2 days at a dose of 10 conidia/larva, has been found to be reduced to 6.7 days by addition of the toxins. Thus, the role of destruxins in influencing the pathogenicity of M. anisopliae depends on: (1) the host species and their ability to detoxify destruxins, and (2) the pathotypes and their capacity to produce toxins in their specific or non specific host, as indicated in the case of C. aurata larvae.