Spray application is one of the most common ways to apply pesticides. This method of application requires liquid or dry pesticide formulations to be dissolved or suspended in water or oil-based mixtures prior to application. Spray applications are conducted using a range of equipment from handheld equipment, backpack sprayers, tractor-mounted sprayers, boom sprayers, aerial sprayers (airplanes, helicopter), drone sprayers, etc. Spraying is a versatile technique that can be used to treat crops, forests, lawns, and other vegetation. It generally provides uniform coverage of pesticides over the target surface therefore maximizing the effectiveness of pesticides. However, spraying affects where pesticides end up in the environment and their potential environmental impact. Therefore, understanding some basics principles of the characteristics of spray application and environmental conditions are vital for all spray applicators.
The spray application of pesticides involves the movement of pesticide from their spray equipment containers to the target areas through the medium of air. Although pesticides move in water and soil, the movement through air is an unavoidable feature for all spray applications. This movement of pesticides through the atmosphere poses a significant risk: the drifting of pesticide from the target area. This off-target movement is very important to consider as it can have serious consequences for non-target organisms and the environment. The environment can be simply defined as everything around us: air, soil, water, people, animals, plants, etc. The non-target area is everywhere in the environment that is not the target of the pesticide. Drift is the airborne movement of pesticides to non-target areas. Spray drift results from small spray droplets moving off-target.
The characteristics of the spray application and the environmental conditions will determine the probability for spray drift to occur. They are two main spray characteristics that determine a pesticide solution drift probability are the spray droplet size, and viscosity of the spray solution.
Spray Droplet Size:
The spray droplet size is determined by the type and size of the spray nozzle, the spray pressure, and the distance from the spray nozzle to target area.
- Spray nozzle types and sizes are classified according to a standard established by the American Society of Agricultural and Biological Engineers (ASABE) to assist applicators in selecting the right nozzles based on droplet sizes. The term called Volume Median Diameter (VMD) is used to estimate the average size droplets of spray volume. The units of VMD are measured in micrometer (microns). It is important to remember that larger spray droplets are generally less prone to drifting of target.
Characteristics of spray droplets (ASABE Standard)
- The spray pressure or force exerted by the spray tank equipment on the spray solution will affect the droplet size. Higher pressure generally results in smaller droplets due to the liquid experiencing greater force prior to exiting the nozzle. Conversely, lower pressure tends to form larger droplets. There are some nozzles that are designed to produce large droplets even at high pressures such as Air induction nozzles, Twinjet nozzles, Turbo flood nozzles, etc.
Viscosity of the spray solution:
The viscosity which is as the thickness of the pesticide mixture in the spray tank prior to spraying. This feature also influences the spray characteristics. The thicker the spray mixture inside the spray tank the larger droplets are typically produced resulting is less drifting.
So Far, we have considered the spray mixture characteristics that influence drifting. Now we will consider the environmental conditions, specifically the weather conditions.
Weather Conditions:
Weather conditions include wind speed, wind direction, air temperature, relative humidity, and temperature inversion.
- Wind Speed: High windy conditions will increase the probability of pesticide drifting off-target. It is generally recommended that when wind speeds are at or above 10 mph applicators do not spray pesticides.
- Wind direction: Spraying against the wind may result in uneven droplet coverage, as droplets are carried away before reaching the target, also the trajectory and pattern of the pesticide spray can be affected. Crosswinds can cause sprays to be carried laterally, potentially extending the reach of the spray beyond the target. Wind direction can have a direct impact on applicator safety. Spraying in the direction of the wind may result in exposure to pesticide drift and increase health risk to applicators. It is recommended that spraying pesticides should be conducted downwind or in the direction the wind is blowing.
- Air temperature and relative humidity: When temperatures are high and relative humidity is below 50% spray droplets are more likely to evaporate or turn to gas and drift off-target.
Overall, the versatility, effectiveness, accessibility, and cost-effectiveness of spray application make it the preferred method for many pesticide applications across various sectors, including agriculture, public health, forestry, and urban pest management. However, it’s important to consider environmental and safety factors when using spray applications to minimize risks to the environment, non-target organisms and human health. It is the responsibility of all applicators to be aware of the potential impact of spraying pesticides, especially the factors that influence the drifting of pesticides, and always seek to minimize the environmental impact while protecting human health and safety.
