Green muscardine disease of Hylobius pales (Herbst) (Coleoptera: Curculionidae)

Abstract

The mode of infection of larval and adult Hylobius pales with Metarrhizium anisopliae, an entomopathogenic fungus, was investigated by means of histology, scanning electron microscopy and the controlled application of conidia. The objective of the study was to explain the large amounts of inoculum necessary to achieve high beetle mortality under field conditions. Spore germination was minimal on beetles and intermediate on larvae, apparently as a result of antibiosis from bacterial and fungal contaminants. High germination percentages resulted after the antibiotic principle was eliminated by surface-sterilizing insect parts. Germ tubes grew extensively over the host surface before they produced appressoria. Penetration of the integument was observed to only take place through procuticular portions, never through sclerites. While no proof for intestinal infection was obtained, invasion from within the buccal cavity was confirmed.Despite limitations imposed by antibiosis, the growth of germ tubes over the host surface and the impenetrability of sclerites, the following field trials with Metarrhizium against Hylobius are encouraged. Plants to be protected should be dipped in spore suspensions at the time of planting. Spores could be applied around the base of plants to be protected. Any means of infecting larvae would seem superior to efforts aimed at infecting the beetles.Present address: College of Natural Resources,