Application Parameters Affecting Field Efficacy of Beauveria bassiana Foliar Treatments against Colorado Potato Beetle Leptinotarsa decemlineata

Abstract

The effects of various spray-application parameters on the efficacy of Beauveria bassiana foliar treatments against Leptinotarsa decemlineata larvae were evaluated during three field seasons. Treatments were applied to small plots (6 rows × 7–12 m) using a CO2-powered backpack hydraulic sprayer with nozzles affixed to lateral drop tubes and directed upward at a 45° angle to target ventral leaf surfaces. The sprayer delivered 280 liters/ha at 3.45 bar. Three rates (1.25, 2.5, and 5 × 1013 conidia/ha) of an emulsifiable oil formulation of B. bassiana and two spray intervals (3–4 vs 6–8 days) were tested. Three applications at the medium and high rates made at 3- to 4-day intervals produced low to moderate levels of control (43–65%). The low rate and applications at weekly intervals were less effective. A wettable powder (WP) formulation was also compared to the oil formulation. Three applications at the medium rate were made at 3- to 4-day intervals. During one field season, rain commenced soon after two of the three initial applications. Under these conditions, the oil formulation was significantly more effective than the WP formulation (65% vs 8% control), suggesting greater rainfastness of the oil formulation. Tests with the oil formulation also compared applications from a sprayer configured with drop tubes as described above to those from a sprayer configured with nozzles directed to spray downward from 30 cm above the canopy. The sprayer with nozzles attached solely on drop tubes deposited an average of 752 conidia/mm2 on the upper surfaces of the leaves and 482 conidia/mm (39%) on the lower surfaces, whereas the alternative sprayer deposited 1062 conidia/mm2 onto the upper surfaces and only 50 conidia/mm2 (<5%) onto the leaf undersides. Applications from below canopy provided greater control of larvae than applications from above canopy when initiated against early instars. Applications initiated against late-instar larvae failed to provide useful control of larvae; however, nearly all treatments applied against late instars, including sprays at weekly intervals and from above canopy, produced significant reductions (53–84%) in populations of first-generation adult beetles.