Distribution of resting spores of the Lymantria dispar pathogen Entomophaga maimaiga in soil and on bark

Cadavers of late instar Lymantria dispar (gypsy moth) larvae killed by the fungal pathogen Entomophaga maimaiga predominantly contain resting spores ( azygospores). These cadavers frequently remain attached to tree trunks for several weeks before they detach and fall to the ground. Density gradient c entrifugation was used to quantify resting spores in the soil and on tree b ark. Titers of resting spores were extremely high at 0-10 cm from the base of the tree and the number decreased with distance from the trunk of the tr ee. Titers were also highest in the organic layer of the soil with numbers decreasing precipitously with increasing depth in the soil. While resting s pores were obtained from tree bark, densities per unit area were much lower than those found in the organic soil layer at the base of the tree. Field bioassays were conducted with caged L. dispar larvae to compare infection l evels with distance from the tree trunk as well as on the trunk. Highest in fection levels were found at 50 cm from the tree base with lowest infection on the tree trunk at 0.5 m height, although we expected the highest infect ion levels among larvae caged at the bases of trees, where highest spore ti ters occurred. Laboratory experiments demonstrated that L. dispar larvae ex posed to resting spore-bearing soil at the soil surface became infected whi le larvae exposed to soil with resting spores buried at least 1 cm below th e surface did not become infected.