1 Factors in¯uencing horizontal transmission of the entomopathogen Beauveria bassiana in the Colorado potato beetle (CPB) were examined through a series of laboratory studies. 2 Cadaver density, cadaver life stage, ambient temperature and conidial density were the factors manipulated. 3 Mortality and sporulation of burrowing CPB prepupae both increased signi®cantly with increased sporulating second-instar cadaver density on the soil surface. 4 Mortality rates were signi®cantly higher when prepupae were released into laboratory arenas containing third-instar cadavers compared to second-instar cadavers. 5 Mortality and sporulation decreased signi®cantly as temperature increased from 15°C to 30°C, however, no temperature-dependent behavioural response by prepupae could be identi®ed as a potential cause. 6 An 86.1% decrease in conidial density per cadaver had no signi®cant effect on mortality or sporulation of prepupae, indicating that this level of environmental degradation of cadavers may not signi®cantly reduce the probability of horizontal transmission.

1 We modelled horizontal transmission of Beauveria bassiana in Colorado potato beetle (CPB) between larval cadavers and soil inhabiting prepupae. 2 The rate of disease transmission, based on the probability of a prepupa contacting sporulating cadavers on the soil surface, is a non-linear function of cadaver density and also dependent upon temperature. 3 The spatial pattern of cadavers is needed to estimate prepupal contacts with cadavers. Observational ®eld studies determined Johnson distributions to model the spatial pattern of cadavers and prepupae in the ®eld. The model also implicitly incorporates within-®eld larval spatial pattern into estimates of ®eld-level horizontal transmission. 4 Potential for horizontal transmission is higher in simulations using weather data from the warmer year of 1995, than in simulations of the cooler growing season of 1993. 5 Simulations of CPB populations under northern Maine climatic conditions recorded in 1993 and 1995 suggest that horizontal transmission can range from 3 to 24% depending upon the timing of primary infection of larvae in the ®eld. 6 Two simulated sequential applications of B. bassiana targeted at peak ®rst instars resulted in maximum horizontal infection in both years. 7 Sensitivity analysis suggests that horizontal transmission is most sensitive to changes in the proportion of cadavers that sporulate and least sensitive to changes in the time between larval death and the onset of cadaver sporulation. 8 Field validation of the model indicates good prediction of one measure of horizontal infection, the proportion of prepupae which eventually sporulated after being released in controlled ®eld experiments.

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