Effects of Metarhizium anisopliae on the pollen beetle Meligethes aeneus and its parasitoids Phradis morionellus and Diospilus capito

The efficacy of two Finnish strains of Metarhizium anisopliae against the rape blossom beetle Meligethes aeneus (Coleoptera: Nitidulidae) and the effect on its parasitoids Phradis morionellus (Hymenoptera: Ichneumonidae) and Diospilus capito (Hymenoptera: Braconidae) were studied in laboratory and semi-field experiments. Several bio-assays were performed using either direct exposure of the host beetle to the pathogens, or pots containing soil inoculated with M. anisopliae conidia at a rate of 2 x 10(8) per ml soil. Parasitised and unparasitised M. aeneus larvae were collected in the field and were placed for pupation in the test soil. The effect of treatment on M. aeneus and D. capito was estimated after adult emergence, and the effect on P. morionellus was estimated by dissecting hibernating pupae. While both of the M. anisopliae strains were highly pathogenic to M. aeneus adults and larvae upon direct exposure, soil treatment resulted in no significant difference between the number of insects emerging (or found) from the treated and untreated pots. However, the treated insects appeared to be latently infected and the actual infection rate had to be estimated after their death and incubation in a moist chamber. The mortality of the beetles caused by direct exposure to the fungus was 85% (range 70-88%). The rate of latent infection following indirect exposure via treated soil in deep pots in laboratory was 80% (range 49-100%), while that in P. morionellus was only 17% (range 0-85%), and in D. capito significantly higher, 76% (range 72-100%). The results indicate that M. anisopliae is a potentially useful candidate to be used as a bio-insecticide to control the pollen beetle, and that at least the most abundant parasitoid, Phradis morionellus, is less affected than the target pest itself. The possible ecological role of latent infections in the host as well as in some of the parasitoids needs to be clarified.