Spray deposits from aerial insecticide spray simulant applications to a coniferous plantation in low and high wind speeds

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Aerial insecticide applications were made on a jack pine plantation in low and high wind speeds, using a fixed-wing aircraft equipped with rotary atomizers. A 16 ha area with an average canopy height of 6.5 m was sprayed with a suspension of fluorescent particles in tripropylene glycol monomethyl ether at 0.42 l ha −1, using a 65 m swath width. The spray was released from 18 m above ground and had a volume median diameter of 41 μm. Wind speeds at release height were 1.2 and 4.5 m s −1 in trials 1 and 2, and the air layers between canopy top and spray release height were stable and unstable respectively. Average drop counts per conifer needle from trials 1 and 2 were significantly different at 0.91 and 1.92, and foliar deposits at 4 and 6 m were significantly greater in the high wind trial. Drop counts per needle increased with height and the count ratios at 2, 4, and 6 m were 1:2:4 and 1:3:10 in trials 1 and 2. Drop count distributions on needles from different heights showed significant differences and a larger proportion of needles received drops in the high wind trial, implying the potential for improved insecticide efficacy. Drop densities (cm −2) on Kromekote ground cards from trials 1 and 2 were significantly different at 3.26 and 1.85, implying substantially reduced contamination of the understorey in high winds. Drop sizes were similar, with average volume and number median diameters of 71 and 45 μm. Drop sizes on Kromekote combs placed in the canopy were also similar in both trials, with average volume and number median diameters of 49 and 21 μm. The results indicate an excellent opportunity for improving on-target spray deposits and efficacy while reducing understorey contamination from silvicultural insecticide applications, as well as their cost and temporal constraints.