Spruce Budworm Mortality as a Function of Aerial Spray Droplet Size

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Abstract
The size and number of aerial spray drops impinging on spruce budworm in its conifer forest habitat were determined by means of a new tracer method that uses fluorescent particles in a liquid spray. Examination of 1113 larvae affected by an experimėntal insecticide that had been applied to a 5000-acre (2024-ha) test area in Montana showed that 93 percent had not been contacted by any droplets larger than 50 µ in diameter. Small numbers of droplets 50 to 100 µ in diameter were found on 7 percent of the larvae, along with lethal numbers of smaller drops. No evidence was found that significant numbers of drops larger than 100 µ reached the target insects. Because about 95 percent of the spray applied to forests by current methods consist of droplets larger than 50 µ, the biologically effective portion of the drop spectrum is only a few percent. The data foreshadow a major potential reduction in insecticide requirements for the successful control of spruce budworm.

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