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 occ urring nuclear polyhedrosis virus (NPV) for gypsy moth control. Mature oak forest plots were sprayed with a repeated application of 5 X 10(1 0) occlusion bodies (OB) hd(-1) per application, and applications at 5 X 10(10) and 10(11) OB ha(-1), all with volume application rates of 5 1 ha(-1) per application. A Bacillus thuringiensis (Bt) treatment com prising a double application at 30 BTU in 2.4 1 ha(-1) was evaluated a s a positive control. Spray applications were made with a fixed-wing a ircraft equipped with rotary cage atomizers, in light winds with weak turbulence, moderate air temperatures and high relative humidity. Spra y deposits in the treatment areas were assessed by microscopy and fluo rometry, by means of a tracer dye. Deposit densities, drop-size spectr a and volumetric deposits were quantified on oak leaves and artificial foliage in the tree canopy and on ground samplers. The NPV applicatio ns resulted in average deposit densities (number of stains cm(-2)) on oak leaves, artificial foliage and ground cards of 8, 15 and 11, respe ctively, and the Bt applications resulted in densities of 13, 10 and 1 2 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 mu m. By c ontrast the foliar drop-size spectra for Bt applications had number an d volume median diameters that averaged 31 and 95 mu m, due to lower f low rates and faster cage rotation. Volumetric spray deposits on artif icial foliage from the NPV applications averaged 11% of the volume app lication rate compared to 25% on the ground. The Bt applications resul ted 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. Copyright (C) 1 996 Elsevier Science Ltd.