Spray deposit patterns and persistence of diflubenzuron in some terrestrial components of a forest ecosystem after application at three volume rates under field and laboratory conditions

Abstract

AbstractSpray deposit patterns and persistence of diflubenzuron [1‐(4‐chlorophenyl)‐ 3‐(2,6‐difluorobenzoyl)urea] in white pine (Pinus strobus L.) and sugar maple (Acer saccharum Marsh.) canopies, forest litter and soil were studied after aerial application of a 250 g kg−1 wettable powder formulation, ‘Dimilin® WP‐25’, at 70 g active ingredient (a.i.) ha−1, using three volume rates (10, 5 and 2.5 liters ha−1) over three blocks in a mixed forest near Kaladar, Ontario, Canada, during 1986. Spray droplets were sampled at ground level using ‘Kromekote®’ cards, and diflubenzuron deposits were collected on glass plates. Droplets were the largest (with a volume median diameter of 250 μm) at the 10 liters ha−1 rate, resulting in the highest number of droplets per cm2 on the Kromekote cards and deposits of diflubenzuron on glass plates. Deposits on foliage, litter and soil were also correspondingly the highest. At the 5.0 and 2.5 liters ha−1 rates, volume median diameter values were smaller (195 and 150 μm, respectively) and deposits on the substrates were markedly lower. In the spray block that received 10 liters ha−1, diflubenzuron persisted in foliage as long as 120 days after treatment, but it lasted for only about a week in forest litter and soil samples. At 5 and 2.5 liters ha−1, diflubenzuron failed to persist in foliage as long, and residues in litter and soil, which were barely above the quantification limit, persisted only for a few days.Laboratory studies, conducted under constant meteorological conditions using different droplet‐size spectra, showed that deposit levels were not affected when the volume median diameter of the spray cloud decreased from 253 μm to 145 μm, but were markedly reduced as this progressively decreased from 92 to 37μm. The dissimilarities between the field and laboratory findings were attributed to meteorological and other factors influencing droplet deposition on tree canopy in aerial applications of pesticides over forests.