Pesticides, in this case, insecticides and miticides, are widely used in greenhouse production systems to suppress insect and mite pest populations, preventing or minimizing direct and indirect damage to greenhouse-grown horticultural crops. However, pesticides will only be effective in suppressing insect and mite pest populations when used appropriately. The correct use of pesticides will ensure that you obtain sufficient mortality of insect and mite pests, which could consequently reduce future inputs from pesticides, thus diminishing the potential for insect and mite pest populations to develop pesticide resistance. This article discusses seven points that will help you make pesticides work better.

1. Correctly identify insect and mite pests.

The proper identification of insect and mite pests is important in selecting the appropriate pesticide because many selective pesticides have a narrow range of activity on specific insect or mite pests. For instance, there are a number of pesticides that are only labeled for use on one pest group (e.g. mites), whereas other pesticides have activity on two different insect types (e.g. aphids and thrips) or insects and mites (e.g. thrips and twospotted spider mites Tetranychus urticae). You should always have several reference publications available that have clear images (pictures of the actual insect and mite pest, and damage to plants) in order to help you correctly identify a given pest or pests.

Fig. 1. Spraying plants with pesticides
Photo: Raymond Cloyd

2. Obtain thorough coverage of all plant parts.

In order to obtain high mortality levels (>80 percent) of insect and mite pests, especially when using contact pesticides, it is important to thoroughly cover all plant parts, including leaves, flowers, stems and fruits (Fig. 1). Since the life stages (egg, larva, nymph, pupa or adult) of nearly all insect and mite pests reside on the leaf underside (Fig. 2) to avoid ultraviolet light desiccation, coverage of leaf undersides is essential in killing insect and mite pests. When plants are “small” (Fig. 3), the ability to cover all plant parts is easier, which results in higher mortality of insect and mite pests. However, when the plant architecture (number of leaves and branches) is complex (Fig. 4), insects such as mealybugs and thrips can escape exposure from insecticide residues because there are more hiding places and it is more difficult to cover the surface area of the entire plant.

3. Time applications accordingly.

Pesticides, in general, should be applied in the early morning or late afternoon when most insect and mite pests are active. Furthermore, information associated with scouting will help to time applications as scouting will determine when the most susceptible life stage (larva, nymph or adult) is predominantly present. This information will ensure higher levels of mortality of insect and mite pest populations after pesticide applications.

Fig. 2. Aphids on a leaf underside
Photo: Raymond Cloyd

4. Monitor water quality.

Although greenhouse producers are well aware of the importance of water quality in regards to plants, water quality (especially pH and hardness) is just as important in regards to pesticide effectiveness. For example, if the water pH is >7.0, then certain pesticides may experience alkaline hydrolysis, where pesticides undergo chemical degradation. Therefore, be sure to maintain the spray solution pH between 5 and 7. In addition, always read the pesticide label to obtain information affiliated with the appropriate spray solution and water pH.

5. Rotate pesticides with different modes of action.

The rotation of pesticides with distinct modes of activity will reduce the prospect for resistance developing in insect and mite pest populations. Mode of action refers to how a pesticide affects the metabolic or physiological processes of insect or mite pests. Be sure to use the same mode of action within a pest generation before switching to another pesticide with a different mode of action in the next generation. The rotation of pesticides will vary depending on the season with more frequent rotations occurring during mid-spring through late fall (in most locations) compared to winter through early spring due to the effects of temperature and presence of host plants. Refer to pesticide label information for the mode of action associated with the specific active ingredient.

Fig. 3. Small plants in greenhouse that are easy to spray and get thorough coverage of all plant parts
Photo: Raymond Cloyd

6. Apply pesticides frequently enough.

Most commercially available pesticides used in greenhouse production systems only kill the young (larva and nymph) and adult life stages of insect and mite pests, with minimal direct activity on the egg and pupal life stages. Therefore, multiple pesticide applications will be required to kill life stages that were initially “missed” by previous pesticide applications, such as larva and nymph that were in the egg stage, and adults that were in the pupal stage. In addition, more frequent applications will be needed when simultaneously dealing with multiple age structures or overlapping generations.

Application frequency will depend on temperature as development (life cycle: egg to adult) of insect and mite pests will increase under higher temperatures, consequently requiring more frequent applications. However, more frequent applications may place undue “selection pressure” on insect and mite pest populations, increasing the potential for insect and mite pest populations to develop pesticide resistance.

7. Use the proper label rate.

Fig. 4. Complex plant architecture that is difficult to obtain thorough coverage of all plant parts
Photo: Raymond Cloyd

Always read the pesticide label and follow specific rates accordingly to make pesticides work better. Avoid consistently exposing pest populations to the highest label rate to mitigate the potential for insect and mite pest populations developing pesticide resistance. The lowest label rate, especially early in the crop production cycle, may be just as effective as the highest label rate. The reason for this is that insect or mite pests present early during crop production, in general, have not been exposed to pesticides, which may result in higher initial mortality levels. In conclusion, pesticides will work better for you in suppressing insect and mite pest populations if you abide by the seven points discussed in this article: 1) correctly identify insect and mite pests; 2) obtain thorough coverage of all plant parts; 3) time applications accordingly; 4) monitor water quality; 5) rotate pesticides with different modes of action; 6) apply pesticides frequently enough; and 7) use the proper label rate.

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. or 785-532-4750