Effect of Application Rate on Droplet Size Spectra and Deposit Characteristics of Dimilin^® Spray Mixtures in an Aerial Spray Trial

The wettable powder formulation of diflubenzuron, Dimilin^® WP-25, was aerially sprayed as an aqueous suspension over four 15 ha plots in conifer forests in Newfoundland using different volume and emission rates of application. A Piper Pawnee aircraft equipped with six Micronair^® AU5000 atomizers delivered the spray mixtures. Plot P9-35/4.7 was treated with 35 g active ingredient (AI) in 4.7 L/ha, plots P10-70/4.7 and P16-70/4.7 with 70 g AI in 4.7 L/ha, and plot P11-30/2.0 with 30 g AI in 2.0 L/ha. The first three plots were each sprayed at an emission rate of 48.13 L/min, the fourth plot at 20.48 L/min. Spray droplets were sampled with Kromekote^® cards to determine the size spectra of the spray cloud reaching the ground level. The use of a tracer dye, Erio Acid Red, facilitated droplet measurements on the sampling cards. Glass plates were used to collect deposits of the tracer dye for colorimetric determination of the percent recovery of the spray volume emitted.

The higher emission rate of 48.13 L/min used in plots P9-35/4.7, P10-70/4.7, and P16-70/4.7 resulted in large droplets, higher droplets/cm², and greater recovery of the applied spray volume at ground level. The lower emission rate of 20.48 L/min used in plot P11-30/20 to accommodate the lower application rate resulted in smaller droplets, lower droplets/cm², and a smaller percent recovery of the spray volume on glass plates. The smaller droplets have a tendency to linger longer near the forest tree canopy, increasing the chance of impaction on the conifer needles. The lower application rate might provide a better target coverage at canopy level, with minimum ground contamination, and warrants further field testing. The relationship between rate of application and deposit characteristics of Dimilin WP-25 is discussed.