Three ingredients for pesticide success

Pesticides are commonly applied by greenhouse producers to suppress insect and mite pest populations, and minimize problems with diseases. In fact, horticultural crops grown in greenhouses require extensive inputs from pesticides in order to maintain the aesthetic quality of both the foliage and flowers. However, it is important to use pesticides properly in order to maximize performance. The three key factors associated with maximizing pesticide performance are timing, coverage and frequency.

Timing refers to applying pesticides (in this case insecticides and miticides) when the most susceptible life stage of a given insect or mite pest is present. In general, for insect and mite pests, the most susceptible life stages to most contact pesticides are the larva/nymph and adult. The egg and pupa tend to be more resilient, and thus less susceptible to pesticide applications. Therefore, greenhouse producers need be sure they understand the biology of a given insect and/or mite pest to increase the effectiveness of a pesticide application.

When using systemic insecticides applied to the growing medium, always make applications prior to noticing phloem-feeding insects such as aphids and whiteflies because the active ingredient of the systemic insecticide may take time to move or translocate throughout plants following application, although this depends on water solubility (the higher the water solubility, the faster the active ingredient will translocate through the plant vascular system). Examples of systemic insecticides that may be used on greenhouse-grown horticultural crops include acephate (Orthene), imidacloprid (Marathon), dinotefuran (Safari), thiamethoxam (Flagship), cyantraniloprole (Mainspring), and spirotetramat (Kontos). Be sure to take note that temperature influences the life cycle and thus, the presence of susceptible life stages. The higher the temperature, the faster that insects and mites complete development. The effect of temperature on insect and mite development needs to be taken into consideration when timing applications of pesticides.

Coverage is especially important when using contact pesticides. Always try to obtain thorough coverage of all plant parts including the leaves, stems and flowers. The use of an adjuvant may be required to enhance coverage, which increases the spreadability of the spray solution and enhances coverage, especially on waxy leaves (a number of horticultural crops have waxy leaf surfaces). An adjuvant is a material added to a pesticide mixture or solution in order to improve or alter deposition, toxicity, mixing ability, persistence, and/or other attributes that will enhance performance. One type of adjuvant is a surfactant, which reduces the surface tension of spray droplets allowing for better coverage on waxy or hairy leaf surfaces of certain plants or the outer covering (cuticle) of insects and mites.

Translaminar pesticides are those in which the material (spray solution) penetrates leaf tissues and forms a reservoir of active ingredient within the leaf. This provides residual activity against a number of plant-feeding pests such as the twospotted spider mite (Tetranychus urticae). Regardless, always thoroughly cover all plant parts; especially the upper and lower sides of leaves as many insecticides and miticides with translaminar properties have contact activity as well. Examples of insecticides/miticides that have translaminar activity include abamectin (Avid), chlorfenapyr (Pylon), etoxazole (TetraSan), pyriproxyfen (Distance/Fulcrum), spinosad (Conserve) and spiromesifen (Judo).

Frequency will depend on the residual activity of a given pesticide. As always, read the product label to obtain information on the frequency of application. In general, recommendations are usually for making pesticide applications once every seven days. However, frequency of application will be contingent on the residual activity of the pesticide (short-term vs. long-term). Nonetheless, too many applications may also result in phytotoxicity (plant injury) to certain greenhouse-grown horticultural crops. For example, phytoxicity may occur if you apply insecticidal soaps (potassium salts of fatty acids) or horticultural oils (mineral, petroleum or neem-based) too frequently (three times per week). Furthermore, applying the same pesticide continuously may promote resistance development in insect and/or mite pest populations, which is why you always rotate pesticides with different modes of action after completion of a generation. In addition, the time of year or season (spring vs. summer) may affect the frequency of application due to the influence of temperature on the life cycle of the insect or mite pest. As the ambient air temperature increases, the time required for the life cycle to be completed is reduced; thus resulting in the need for more frequent applications.

In conclusion, timing, coverage and frequency are important factors to consider in order to ensure success when using pesticides to suppress insect and/or mite populations, and even diseases of greenhouse-grown horticultural crops. However, proper pesticide stewardship is critical to avoid problems such as phytotoxicity and resistance. Finally — and most importantly — read the label of all pesticides prior to use, and follow label directions explicitly.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.