Fig. 1. Using the beat method to detect the presence of twospotted spider mites on tomato
Photo: Raymond Cloyd
Fig. 2. Weeds under greenhouse bench
Photo: Raymond Cloyd

lant protection of greenhouse-grown vegetable crops such as tomatoes, peppers and cucumbers involves implementing a variety of strategies in order to alleviate problems with insect and mite pests, including the use of insecticides or miticides. However, the “first line of defense” in dealing with insect and mite pests (e.g., aphids, mites, thrips and whiteflies) in vegetable production systems is non-chemical plant protection strategies that include: scouting, sanitation, trapping, and cultural and biological methods.

Fig. 3. Weed fabric barrier underneath greenhouse bench
Photo: Raymond Cloyd

1. Scouting

Scouting is an important component of any plant protection program. The primary goals of scouting vegetable crops are: 1) correctly identify insect or mite pests feeding on crops; and 2) assess population dynamics and trends throughout the growing season. Pest identification is critical in determining the extent of the problem and what non-chemical means will help to alleviate future infestations. Determining population dynamics or the numbers of insect and/or mite pests will help track fluctuations (up and down) in pest populations during the growing season. This assists producers on when to take appropriate action. For vegetable crops (e.g., cucumbers, peppers and tomatoes) the “beat method” is effective in detecting the presence of aphids, mites and thrips. The beat method involves shaking plant leaves over a white sheet of paper (8.5 x 11.0 inches) attached to a clipboard (Fig. 1) and counting the number of insects or mites that fall onto the paper. You can either count the number of individual insect or mite pests or just determine presence or absence.

Fig. 4. Straw mulch placed in alleyways to reduce weed problems
Photo: Raymond Cloyd

2. Sanitation

Sanitation involves removing weeds and cleaning-up plant debris or residues, from within and outside the greenhouse facility. Weeds located inside (Fig. 2) and outside the greenhouse provide refuge for many insect and mite pests, including: aphids, leafminers, thrips, spider mites and whiteflies. Consequently, weeds allow insect and mite pests to survive, and potentially disperse onto vegetable crops. Many weeds may serve as refuge for insect pests. Furthermore, weeds serve as reservoirs for pathogens (fungi and viruses) that can be acquired by insects while feeding, and then transmitted when feeding on vegetable crops including tomatoes and cucumbers. A long-term strategy that will help to reduce problems with weeds is installing underneath benches, landscape or fabric barriers. Fabric barriers are geotextile, non-biodegradable materials that prevent weeds from emerging from the soil underneath benches (Fig. 3). For vegetable crops grown in the ground, mulches (e.g., straw) or fabric barriers may be used to reduce problems with weeds (Fig. 4). Weed-free areas or zones around the greenhouse outer perimeter — 10 to 30 feet — will help to reduce the migration of insects. This includes adult thrips and whiteflies, and winged aphids through natural greenhouse openings (e.g., vents, sidewalls, and doors), which can also reduce the incidence of disease transmission. Plant debris or residues, such as old leaves, fruit, and growing medium/soil may provide a refuge for certain insect and/or mite pests. Insects and even mites can migrate onto fresh plant material as plant debris desiccate. Therefore, be sure to place debris or residues into refuse containers with tight-sealing lids (Fig. 5) or dispose of debris/residues into compost piles. Old vegetable plants or those remaining at the end of the growing season should be removed because they can be a potential source of insect and mite pest populations. Moreover, old vegetable plants can serve as reservoirs for the viruses transmitted by insects such as western flower thrips (Frankliniella occidentalis). Another important sanitation practice is to immediately remove any vegetable plants from the greenhouse that are heavily-infested with insect or mite pests.

Fig. 5. Container with tight-fitting lid
Photo: Raymond Cloyd

3. Trapping

Yellow sticky tape can be placed among a vegetable crop in order to mass-trap or capture large numbers of insect pests including: adult aphids, thrips and whiteflies. Yellow sticky tape is positioned in rows hung vertically within the greenhouse (Fig. 6). In addition, yellow sticky tape can be placed near openings (e.g., side walls), which may capture adult insects as they enter the greenhouse from outdoors (Fig. 7).

Fig. 6. Yellow sticky tape placed vertically among a crop
Photo: Raymond Cloyd

4. Cultural methods

Vegetable crops that are overfertilized, especially with nitrogen-based fertilizers, tend to be more susceptible to aphids and spider mites. Over-fertilization may change plant quality thus making plants a better food source for insect and mite pests, which will enhance development, growth, and reproduction. Over-fertilizing vegetable crops results in higher levels of amino acids, thus leading to an increase in feeding by sucking insect and mite pests (those with piercing-sucking mouthparts). Therefore, only provide enough fertility for plant growth, based on information from laboratories associated with growing medium/soil samples.

Fig. 7. Yellow sticky tape placed outside of a greenhouse near openings
Photo: Raymond Cloyd

5. Biological methods

Biological control involves releasing natural enemies (parasitoids and predators) to regulate insect or mite pest populations. Natural enemies are commercially available for use against aphids, thrips, and mites (Fig. 8). Greenhouse producers must release natural enemies early in the cropping production cycle, and before insect and/or mite damage is noticeable (Fig. 9). When considering the use of biological control, it is important to correctly identify the pest in question so that the appropriate natural enemy can be purchased and released. Plant type may negatively affect the effectiveness of natural enemies. For example, the hairs (trichomes) on the leaves of certain plants including cucumber may inhibit the ability of predatory mites (e.g., Phytoseiulus persimilis) and parasitoids (e.g., Encarsia formosa) to regulate twospotted spider mite and whitefly populations.

Fig. 8. Package containing an assortment of natural enemies
Photo: Raymond Cloyd

In conclusion, the implementation of non-chemical plant protection strategies such as scouting, sanitation, trapping, and cultural and biological methods is important in the production of greenhouse-grown vegetable crops in order to reduce or alleviate problems associated with insect and mite pests.

Fig. 9. Releasing predatory mites early in tomato production
Photo: Raymond Cloyd

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