Delayed efficacy of Beauveria bassiana foliar spray applications against Colorado potato beetle: Impacts of number and timing of applications on larval and next-generation adult populations

• Foliar sprays of B. bassiana GHA were applied vs. Colorado potato beetle larvae.
• Inoculation/infection was achieved, but larvae survived until the pre-pupal stage.
• Multiple applications vs. larvae reduced next-generation adult populations by 58–93%.
• A single late-season application reduced adult populations by 36–75%.
• Evening applications were only marginally more effective than morning applications.

Spray programs comprising multiple or single foliar applications of the fungal pathogen Beauveria bassiana strain GHA (Bb-GHA) made during morning (AM) vs. evening (PM) hours were tested against Colorado potato beetle Leptinotarsa decemlineata (CPB) in small research plots of potatoes over multiple field seasons, and efficacy against larval and first-generation adult populations was evaluated. The pathogen (formulated as BotaniGard WP®) was applied at the rate of 2.5 × 1013 conidia in a spray volume of 468–480 L per ha. Modes of efficacy were investigated by collecting pre-pupal larvae and holding them in soil cages in the field and laboratory. Results revealed that larval control achievable with multiple-spray programs was poor regardless of environmental moisture conditions, but that emerging adult populations were reduced by an average of 80% (range 58–93%) relative to control populations. Programs comprising a single application of Bb targeted against late-instar larvae reduced adult populations by an average of 60% (range 36–75%). Overall mean differences in efficacy of PM vs. AM applications were not significant in terms of larval control and defoliation damage. Emergence of next-generation adults was significantly lower in the PM vs. AM treatments, but differences were small in terms of percent control. Average yields from the Bb-GHA multiple-spray programs were 18% greater than those from the controls; the mean 5% increase resulting from the Bb-GHA single-spray programs was not significant. Observations indicated that moderate to high rates of inoculation and infection were achieved under a broad range of environmental conditions in the crop canopy, but larvae persisted until they entered the soil to pupate. Most deaths due to mycosis occurred among pre-pupal larvae in their pupation cells in the soil. These results, in combination with similar findings of delayed efficacy reported by other researchers, indicate that foliar applications of Bb could be a significant component of an IPM program aimed at suppressing CPB populations on large farms or over large areas.

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