EFFECT OF MICROCLIMATIC CONDITIONS ON PRIMARY TRANSMISSION OF THE GYPSY MOTH FUNGAL PATHOGEN ENTOMOPHAGA MAIMAIGA (ZYGOMYCETES: ENTOMOPHTHORALES) IN MICHIGAN

摘要

The fungus Entomophaga maimaiga is capable of causing extensive epizootics in gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), populations. Development of an E. maimaiga epizootic begins when primary transmission of the pathogen from resting spores to early-instar gypsy moth occurs, and amplifies during secondary transmission of conidia from infected early-instar larvae to late-instar hosts. Gypsy moth density and other host-associated factors do not appear to influence primary transmission of E. maimaiga, suggesting that environmental conditions are integral in the germination of resting spores and, therefore, the initial development of epizootics. E. maimaiga infections of laboratory-reared gypsy moth larvae were evaluated under field conditions in 2001 and 2002 In an oak-dominated forest with low-density gypsy moth populations. Four-day field bioassays were continuously conducted for a 6-wk period each year, corresponding with natural gypsy moth larval development, and infection rates were related to microclimatic variables recorded hourly. Regression analyses of microclimatic variables on the percentage of gypsy moth larvae infected with E. maimaiga were significant (P < 0.05) in both years. In 2001, three variables, including the sum of hours with soil temperatures between 15 and 25 degrees C, average atmospheric water vapor pressure and total precipitation accounted for 33% of the variation in larval infection. In 2002, total precipitation was the only significant predictor of E. maimaiga infection and accounted for 28%, of the variation. Overall, results from this study provide further evidence that primary transmission (i.e., infection via resting spore germination) of the gypsy moth fungal pathogen E. maimaiga is strongly influenced by the occurrence of precipitation.