The behavior of adult Colorado potato beetles (Leptinotarsa decemlineata) in the presence of Beauveria bassiana sporulating on cadavers was studied to determine the likelihood of disease infection as beetles emerge from the soil and colonize host plants. In 2001, the movement of single adult Colorado potato beetles was monitored in arenas containing infective cadavers in differing spatial patterns between four potato plants. In 2002, a similar design was used, but was under the more natural conditions of a potato field. In both experiments, direction, time, and directness of beetle travel was not significantly affected by the presence or absence of infective cadavers, showing no avoidance of these cadavers by adult Colorado potato beetles. The likelihood that emerging adults would contact infective cadavers on the soil surface was quantified at different cadaver densities. A curvilinear relationship (y = 4.8313x0.4459) best describes the frequency of encounters of adults as a factor of increasing density of cadavers on the soil surface. Mortality of adults and production of conidia on cadavers after encounters with increasing densities of infective cadavers had similar relationships that show the decreased susceptibility of adults to B. bassiana compared with prepupae. These relationships were used to model horizontal infection to emerging adults in the summer months, subsequent to a conidial spray targeted at larvae. The model indicates that horizontal infection of adults may potentially be significant (30–70% of horizontal infection), and that simulated early sprays targeted at first-instar larvae resulted in the maximum levels of predicted horizontal infection.
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