Fig.1. Airborne drift from a field sprayer.
Drift is often associated with the physical movement of spray droplets away from the target site at the time of application. This type of drift, usually called airborne drift (Fig. 1), results from factors associated with application methods and equipment. Airborne drift may be simply a spray swath displacement that results in targeted deposition of the spray droplet at a distance not more than 30 feet from the point of spray release. Small spray droplets, however, may travel several thousand feet before depositing into a non-target area. Smaller airborne droplets may evaporate in the atmosphere and travel for great distances. The resulting vaporized droplets usually do not create significant problems because they are dispersed over large areas. Airborne drift can be reduced to a minimum with good judgment and proper selection and operation of application equipment.
Drift sometimes occurs even days after the application is made. This type of drift, usually called vapor drift (Fig. 2), is most commonly associated with volatilization (changing from a liquid to a gas) of pesticides with subsequent movement from the target area. The vapor drift becomes a significant concern only if the pesticide applied is highly volatile and the atmospheric conditions become suitable for rapid vaporization of the pesticide.
When pesticide drift injury occurs on a sensitive crop, it is difficult to determine the relative contribution of airborne and vapor drift to overall off target movement. Airborne drift is responsible for most drift damages, but, fortunately, it can be controlled.