Pesticides such as insecticides, miticides and fungicides are still primarily used to deal with insect and mite pests, and diseases in gardens and greenhouses. Some pesticide labels recommend adding an adjuvant into pesticide spray solutions to increase the performance of an application. However, some pesticides already have an adjuvant or adjuvants incorporated into the formulation. So, what is an adjuvant? An adjuvant is a material added to a pesticide solution in order to improve or alter deposition, toxicity, mixing ability, persistence, and/or other attributes that will enhance pesticide performance. Benefits of using adjuvants are 1) improve wettability of spray solution, 2) minimize evaporation of spray droplets, 3) improve weatherability of pesticides, 4) increase penetration through plant tissues or insect skin, 5) adjust pH of spray solution, and 6) enhance spray droplet deposition. There are many types of adjuvants including surfactants, stickers, spreader-stickers, extenders, activators, compatibility agents, buffers and acidifiers, deposition aids, defoaming agents, thickeners, attractants, and spray colorants.
Adjuvants are readily available from nurseries, garden centers, and chemical supply companies. One of the most widely-used adjuvants is surfactants. These are surface active agents used to improve the ability of a pesticide to stick or adhere to and be absorbed by the target surface. This enhances spray coverage by reducing the surface tension of the spray droplets, which allows spray droplets to spread-out; providing a larger surface area in which a plant can absorb the pesticide or increase contact with insect and/or mite pests, and foliar fungal spores. In addition, a surfactant, when added to a pesticide spray mixture, improves cohesion and dispersal of sprays, and increases wetting (or coverage) on plant leaves, stems, and fruits. A surfactant can also reduce the surface tension of spray droplets thus allowing for better coverage on waxy or hairy leaf surfaces of certain plants or outer covering of insects and mites. Oils such as vegetable, soybean (soy), cottonseed, and even soaps may act as surfactants by reducing the surface tension of water.
The three primary surfactants are: nonionic, anionic, and cationic. Nonionic surfactants are the most used surfactants. They increase penetration of pesticides through the plant tissues. This may increase the effectiveness of systemic and translaminar (which means the material penetrates the leaf tissue and forms a reservoir of active ingredient within the leaf) insecticides, miticides, and fungicides by improving absorption into plant tissues. Anionic surfactants are used to prevent pesticides from washing-off plants due to rainfall or watering. Also, they prohibit pesticides from being rapidly absorbed through the plant tissues (plant surfaces have a negative charge), which keeps pesticides on the outer surface of leaves when exposed to environmental conditions such as sunlight and temperature. In addition, anionic surfactants may increase the effectiveness of contact pesticides and stomach poison insecticides because they retain residues on the leaf surface instead of being absorbed by plant leaves. This results in better suppression of fungal leaf diseases and leaf-feeding insects such as caterpillars. Cationic surfactants are strongly attracted to plant surfaces. As such, caution is warranted due to potential for plant injury (=phytotoxicity). Therefore, these surfactants are not used as much as nonionic and anionic surfactants.
There are a number of factors that may influence the selection of a surfactant including 1) physical properties of target surface, which is associated with leaf waxiness and the presence of hairs or trichomes on leaves; 2) physical and chemical properties of a pesticide that are affiliated with solubility properties (ability to be dissolved in water or not); 3) application method (foliar, granule, or drench); 4) environmental conditions and cultural practices [rainfall, sunlight, and irrigation (watering)]; and biology and pest habitats regarding location on the plant and protective covering associated with certain insect pests such as mealybugs and scales. It is important to understand, even when using a surfactant that plant stress may reduce effectiveness of foliar-applied systemic and translaminar pesticides due to rapid drying of spray droplets.
Adjuvants, such as surfactants, may enhance the performance of a pesticide application and as such should be considered when dealing with insect and mite pests, and diseases in gardens and greenhouses. However, always read the label of a pesticide to determine if it is legal to add a surfactant to the pesticide spray solution as some pesticides already contain adjuvants in the formulation.
Raymond A. Cloyd is a Professor and Extension Specialist in Horticultural Entomology/Plant Protection.