Spray deposits in a mature oak canopy from aerial applications of nuclear polyhedrosis virus and Bacillus thuringiensis to control gypsy moth, Lymantria dispar (L.)

Aerial spray applications were carried out to evaluate a naturally occurring nuclear polyhedrosis virus (NPV) for gypsy moth control. Mature oak forest plots were sprayed with a repeated application of 5 X 10(10) occlusion bodies (OB) ha-1 per application, and applications at 5 X 10(10) and 10(11) OB ha-1, all with volume application rates of 5 l ha-1 per application. A Bacillus thuringiensis (Bt) treatment comprising a double application at 30 BIU in 2.4 l ha-1 was evaluated as a positive control. Spray applications were made with a fixed-wing aircraft equipped with rotary cage atomizers, in light winds with weak turbulence, moderate air temperatures and high relative humidity. Spray deposits in the treatment areas were assessed by microscopy and fluorometry, by means of a tracer dye. Deposit densities, drop-size spectra and volumetric deposits were quantified on oak leaves and artificial foliage in the tree canopy and on ground samplers. The NPV applications resulted in average deposit densities (number of stains cm-2) on oak leaves, artificial foliage and ground cards of 8, 15 and 11, respectively, and the Bt applications resulted in densities of 13, 10 and 12 on these substrates; coefficients of variation were high at 73-97%. Drop-size spectra on artificial foliage from the NPV applications had number and volume median diameters that averaged 81 and 279 micrometers. By contrast the foliar drop-size spectra for Bt applications had number and volume median diameters that averaged 31 and 95 micrometers, due to lower flow rates and faster cage rotation. Volumetric spray deposits on artificial foliage from the NPV applications averaged 11% of the volume application rate compared to 25% on the ground. The Bt applications resulted in average volumetric foliar and ground deposits of 17 and 21% of the application rate. Comparisons with published data are made, and a suggestion for improving NPV applications for gypsy moth control with the use of finer sprays and more turbulent conditions.