A simple empirical model to predict forest insecticide ground-level deposition from a compendium of field data

Abstract

Deposit data from 205 aerial forest insecticide applications conducted in field trials by the Canadian Forest Service, Great Lakes Forestry Centre over a 15-year period are summarized. Deposit measurements were taken under "worst case" scenarios in the sense that direct applications were made over water bodies, and ground samplers were intentionally placed in open or cleared areas of forest. The median % deposit on shoreline collectors (32 separate applications) was 5.7%, on mid-stream collectors (44 separate applications) was 6.2%, and on forest floor collectors (129 separate applications) was 4.9%. Forest floor deposit was most closely associated with application rate and droplet size (r = 0.624, p < 0.001 and r = 0.662, p = 0.011, respectively) but these variables combined only explained 44% of the variation in deposit. Data from all three collector types were grouped by 10% deposit increments and combined to provide a data set from all deposition scenarios. A negative exponential model was fitted to the proportion of these combined sites regressed on % deposit in 10% increments and plotted as a deposit probability distribution curve (p < 0.001, r2 = 0.992). The probability distribution curve indicated that 5-10% deposit would be expected about 57-91% of the time, whereas 50% deposit or greater would be expected about 2% of the time or less. In a probabilistic risk assessment for aerially applied insecticides in a conifer-dominated forest environment, the probability distribution curve based on empirical data presented here can be used to refine the characterization of exposure scenarios from which effects estimates can be derived.

Positive Results of an Early Intervention Strategy to Suppress a Spruce Budworm Outbreak after Five Years of Trials

“…Nozzles on the aircraft break the liquid mixture into small droplets (atomize) so that when the drops land on foliage they are small enough to be eaten by SBW. Because the forest canopy acts as a filter, 90%-95% of the spray is deposited in the forest canopy [48]. The portion that reaches the ground stays in the upper 5 cm of the soil and leafy debris, does not leach away, and is broken down over time by soil microbes, sunlight, and moisture [49,50].…”

“…Tebufenozide deposited in the canopy is relatively rainfast and is not easily washed off by rainfall [51] and tebufenozide that reaches the ground is not harmful to soil invertebrates [52], Water bodies are identified on maps and are excluded from all treatment areas during the planning phase, so there is no targeting of visible water bodies. Tebufenozide that lands on water has no noticeable environmental impact; research showed that there were no significant harmful effects on most organisms at concentrations expected after aerial spraying, even if a water body were to be unintentionally sprayed [48,53,54]. The most recent review of tebufenozide states that 'No adverse effects on birds, mammals or aquatic species are likely to occur from exposure to tebufenozide' [55].…”