|RAMOS, MARK - Former ARS Employee|
Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2014
Publication Date: 1/2/2015
Citation: Wraight, S.P., Ramos, M. 2015. 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. Biological Control. 83:51-67.
Interpretive Summary: The insect pathogenic fungus Beauveria bassiana (Bb) is an important natural enemy of Colorado potato beetle (CPB), and its CPB biocontrol potential has been researched worldwide for many decades. Results from small-scale tests of foliar spray-applications against small larvae have in some cases been positive, but large-scale pilot testing has generally resulted in poor efficacy. This failing has been attributed to low moisture/high UV radiation/high temperature conditions and rapid larval development. Making applications in evening (under favorable environmental conditions) and increasing the number and frequency of sprays have been recommended to overcome these problems, but results remain inconsistent. In a series of field tests, we investigated the effects of number and timing of applications on populations of larvae and next-generation adults. Mature larvae were collected from treatment plots and transferred to soil cages to elucidate the mode-of-action of Bb against CPB in the field environment. High rates of inoculation and infection were achieved under a broad range of weather conditions, but larvae persisted until they entered the soil to pupate. In the highly favorable soil environment, the fungus rapidly overwhelmed the larvae. Results showed that evening sprays were no more effective than morning sprays in controlling larvae and only slightly more effective in reducing adult populations. Multiple-spray programs reduced adult populations by 58–90 percent, but even a single spray, targeted against large larvae, reduced adult numbers by an average of 55 percent. These findings indicate that though Bb alone will not provide reliable control of larvae, it could play an important role in a pest management program aimed at suppressing CPB across large potato growing areas.
Technical Abstract: Spray programs comprising multiple or single foliar applications of the fungal pathogen Beauveria bassiana strain GHA (Bb) 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 x 1013 conidia in a spray volume of 468 gallons per acre (L/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 77 percent (range 58–90 percent) relative to control populations. Programs comprising a single application of Bb targeted against late-instar larvae reduced adult populations by an average of 55 percent (range 19–68 percent). Overall mean differences in efficacy of PM vs. AM applications were small and not significant in terms of larval control and defoliation damage. Average yields from the Bb multiple-spray programs were 18 percent greater than those from the controls. Observations indicated that moderate to high rates of inoculation and infection were achieved under a broad range of 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. These results 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.