The general goal in spraying for pest control is to get good coverage of the target pest. Thereafter, retention and spread of the pesticide on pest surface, penetration of pest surface, and a high level of pest control.
Ideally, good coverage of the target pest is required for good control when using pesticides. However, good coverage of a pest depends on factors related to the properties of the pesticide spray formulation and the carrier used for delivery.
In most instances, the carrier used to deliver the pesticide spray is water and applicators often use a surfactant to mitigate some of the issues related to water that affects spray droplet size.
What is a surfactant?
A surfactant, or surface acting agent, is a class of adjuvant. An adjuvant is a chemical substance that is added to a sprayer tank that helps modify the activity of the pesticide or modify the application characteristics.
A surfactant is composed of chemical compounds that improve the emulsifying, dispersing, spreading, wetting, or other surface-modifying properties of liquids.
When you use a surfactant, some of the benefits include:
- increased pesticide droplet coverage on target pests,
- better spray retention and droplet drying on target pests,
- increased pesticide penetration of leaf cuticles, insect exoskeletons, etc., and
- increased cellular accumulation of pesticide thereby reducing environmental losses.
The size of the spray droplet, however, is the most critical aspect of pesticide spraying and must be addressed to ensure good pest control.
Why is droplet size important?
The size of droplets is often the key to success when pesticides are applied using ground or aerial equipment. The major concern is usually with spray drifting off from the intended target. Spray drift is directly related to the size of droplets.
Droplet size is usually expressed in microns (micrometers); where one micron is equal to one thousandth of a millimeter, and the volume median diameter (VMD) is used to standardize droplet size.
VMD is a measure used to describe droplet size within the industry. VMD is the midpoint droplet size where half of the volume of spray is smaller, and half of the volume is in droplets larger than the mean size of droplets. The accompanying illustration provides a visual description of VMD.
VMD data were used by the American Society of Agricultural and Biological Engineers to generate a droplet size classification system to be used widely among applicators as follows:
To aid understanding of the VMD-based droplet size classification system, ASABE also developed the following chart to provide a comparison of droplet size to a common object as follows:
How is VMD spray droplet size information used?
Based on the ASABE spray droplet size classification system, most agrochemical applications recommend a fine, medium, or coarse spray.
- Fine spray provides enhanced retention for directed spraying on the target including postemergence weed control and contact fungicide and insecticide applications.
- Medium sprays are the most widely used spray type, e.g. for systemic-acting fungicides, insecticides, and herbicides.
- Coarse sprays are used with systemic, residual, and soil-applied herbicides.
What are some major factors that affect spray droplet size?
Regardless of application, the challenge for most applicators is ensuring that the pesticide is applied on the target at the correct rate. However, a number of factors must be mitigated when pesticides are applied and include:
- The higher the pressure, the smaller the droplet size. With smaller droplets, better coverage of target pests is gained, resulting in higher chemical performance.
- Small droplets increase the potential for spray drift.
- Nozzles size, height, angle, and spray pattern
- Selection of nozzles is perhaps more important than making pressure adjustments to minimize drift.
- Position and orientation of nozzles on the spray boom are also important factors.
- Droplet size distribution can be greatly affected by nozzle orientation.
- Applicators must determine the desired spray swath width by striking a balance among nozzle size, spray angle, pressure, and height above the target.
- Specific gravity of the fluid
- Small, lightweight particles fall much slower than large particles.
- Small droplets of low specific gravity are the ones that are more likely to drift from the target area.
- Viscosity and surface tension
- As the viscosity of a fluid increases, it will tend to form large droplets when sprayed.
- Fluids with high surface tension will form large droplets which reduces wetness on pest surfaces. Ultimately, pest control is reduced.
- Overall coverage of targets is much even (and better) when spray application is primarily with small droplets.
Applicators usually must address one or more factors that affect spray droplet size whenever they apply pesticide. One important way of doing so, and to ensure good pesticide performance, is with the addition of a surfactant to the spray mixture.
Influence of surfactant on spray droplet size
Surfactants are commonly used when the potential for pesticide spray drift is high. Pesticide spray drift is an important issue today because:
- The Pesticides are expensive and allowing them to drift off-target is wasteful.
- Pesticides can have undesirable consequences when they drift off-target, e.g., harm to human health and the environment.
- Pesticides that drift off-target can damage sensitive crops in surrounding areas.
There are three types of surfactants that can be used to address spray droplet size and drift. These are:
- Wetting surfactants (commonly referred to as wetting agents)
- Drift control surfactant
- Sinking surfactants
Wetting surfactant (or wetting agent)
- Designed to improve the coverage of a pest’s surface when you spray pesticide, commonly referred to as ‘wetting’.
- Reduces the surface tension of spray droplets and prevents droplets from beading up.
- By lowering spray droplet surface tension, the pesticide spray increases the area covered by each droplet.
Drift Control Surfactant
- Modifies the properties of the spray solution to increase the viscosity of the spray and reduce drift from the target.
- Can also function as a penetrant and sticker with rain resistance properties.
- Further reading on drift control surfactants:
- Creates thick mayonnaise-like droplets that do not drift off-target when applied in aquatic systems.
- Further reading on sinking surfactants:
- Carefully read label instructions for the pesticide and surfactant to ensure you create the best spray mixture that addresses droplet size, lowers drift potential, and maximizes pest control.
- Visit Brewer International for more information about surfactants that enhance wetting and reduce the potential for spray drift.
- American Society of Agricultural and Biological Engineers [ASABE]. ASABE S572.1 Droplet Size Classification. http://info.sprayerdepot.com/hs-fs/hub/95784/file-32015844-pdf/docs/asabe_s572.1_droplet_size_classification.pdf. Accessed, Jan. 5, 2022.
- North Carolina State University Center for Integrated Pest Management [NC-CIPM]. Pesticide Environmental Stewardship – Understanding Droplet Size. https://pesticidestewardship.org/pesticide-drift/understanding-droplet-size/. Accessed, Jan. 5, 2022.
- Sijs, R., S. Kooji, D. Bonn. 2019. How Surfactants Influence the Drop Size in Sprays. J. Fluid Mechanics. 11 pg. https://arxiv.org/pdf/1907.09723.pdf. Accessed, Jan. 5, 2022.
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