Language and restrictions regarding pesticide drift vary among products, but all put the responsibility clearly with the applicator.
One insecticide label states: Only apply this product if the wind direction favors on-target deposition. Do not apply when the wind velocity exceeds 15 mph.
Another label does not give wind speed instructions but addresses the complexity of the problem facing applicators: Avoiding spray drift at the application site is the responsibility of the applicator. The interaction of many equipment and weather-related factors determine the potential for spray drift. The applicator is responsible for considering all of these factors when making the decision to apply this product.
Several herbicide labels listed 10 mph as the maximum wind speed during application, while several other insecticide labels indicated a maximum wind speed of 15 mph. Both state or assume that wind direction should be away from sensitive areas.
Some labels specify the use of nozzles and pressures that deliver medium or coarse spray droplets. The definitions are based on ASABE S572.1 droplet size classification (American Society of Agricultural and Biological Engineers).
If a nozzle output has a volume mean diameter (VMD) of 300 microns (droplets of very fine rain), 50% of the spray volume is in droplets that are less than 300 microns and 50% are above 300 microns. Temperature and humidity affect evaporation of spray droplets. High temperature and low humidity results in faster evaporation, and droplets become smaller. Smaller droplets are moved farther by winds.Figure 1. The average droplet size from a specific nozzle is its volume mean diameter (VMD) (Image: UK)
Drift additives may be used to reduce the number of small droplets that are produced by a spray system. Check product labels for statements or prohibitions. Use of a drift additive can affect spray coverage and product performance. These products are unlikely to prevent drift if used improperly or in unfavorable environmental conditions.
By Lee Townsend, Extension Entomologist