Influence of rain and conidial formulation on persistence of Beauveria bassiana on potato leaves and Colorado potato beetle larvae

The influence of rain on removal of Beauveria bassiana conidia from potato (Solanum tuberosum) leaves and Colorado potato beetle (Leptinotarsa decemlineata) larvae was investigated. In the first experiment, substrates were exposed to simulated rain of varying intensities (25, 77, and 89 mm/h) and durations (0, 15, 30, 45, and 60 min). Substantial numbers of colony-forming units (CFU) were removed from leaves (89 to 95%) and from larvae (34 to 70%) by rain exposure, and the majority of CFU were removed within the first 15 min of exposure. Rain intensity had a minimal effect on conidial persistence on both leaves and larvae, but more CFU were removed from larvae exposed to the highest rain intensity treatment. Conspicuous clumping of conidia was observed on the cuticles of both leaves and larvae following application. After rain exposure, conidial clumping was less conspicuous. In a second experiment, the influence of sis formulations on the persistence of conidia exposed to rain was investigated. Treatments consisted of a water control, sunflower oil, a wettable powder, and four oil emulsions applied at two rates. Formulation significantly affected removal of CFU from potato leaves but not Colorado potato beetle larvae. Conidia applied in sunflower oil were not removed from potato leaves by exposure to 39 mm of rain in 30 min. In contrast, conidial persistence during this period was substantially reduced for conidia applied in water, a wettable powder containing clay, and an oil emulsion formulation applied at a low volume rate (2.3 liters of oil in 281 liters of water/ha). Although simulated rain removed substantial quantities of B. bassiana CFU from leaves and to a lesser extent from Colorado potato beetle larvae, a substantial number of the conidia applied in water remained after exposure to relatively high intensity rains. Evidence suggested that conidia in direct contact with the cuticle of leaves and larvae were less prone to be removed by rain than conidia in aggregates and thus not in contact with the cuticle, and emulsifiable oil formulations may be used to enhance conidial retention on foilage. (C) 2000 Academic Press.