Keep a clean house

Plant protection of greenhouse-grown vegetable crops involves implementing a variety of strategies in order to alleviate problems with insect and mite pests, including sanitation. This is one of the easiest and least expensive practices to implement because sanitation can be performed during regular operating hours.

Inside and out

First, weeds located inside 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.

The weeds that serve as refuge for insects include: sowthistle, Sonchus spp. (aphids and whiteflies); oxalis, Oxalis spp. (thrips); and dandelion, Taraxacum officinale (whiteflies). Many weeds also serve as reservoirs for pathogens (fungi and viruses) that can be acquired by insects while feeding, and then subsequently transmitted, when feeding on vegetable crops including tomatoes and cucumbers.

Weeds that serve as reservoirs for viruses, specifically the tospoviruses, impatiens necrotic spot and tomato spotted wilt virus are: chickweed (Stellaria media), lambsquarters (Chenopodium spp.), nightshade (Solanum spp.), oxalis (Oxalis spp.), shepherd’s purse (Capsella bursa-pastoris), pigweed (Amaranthus spp.), and bindweed (Convolvulus spp.). An abundance of weeds underneath benches with soil flooring may retain enough moisture, thus providing a favorable habitat for fungus gnats and shore flies to breed.

Take action

A long-term strategy that should be implemented to reduce problems with weeds is installing landscape or fabric barriers, which are geotextile, non-biodegradable materials that prevent weeds from emerging from the soil and underneath benches. There are herbicides (weed-killers) registered for use in and around greenhouses, although caution must be exercised when applying herbicides inside greenhouses, especially those with vegetable crops. A pre-emergent herbicide may be applied prior to weed emergence, whereas a post-emergent herbicide is applied after weeds emerge.

However, be sure to avoid any inadvertent plant injury (phytotoxicity) when using herbicides, especially those that have systemic (when applied as a spray) and post-emergent activity. In general, apply herbicides when greenhouses are empty. Always read the label directions before mixing and loading to ensure that the herbicides can legally be used in greenhouses where vegetables are grown. Weeds that are mature or greater than 15.2 centimeters (6 inches) tall should be removed by hand, taking care to remove both the aboveground portion and roots. Areas or weed-free zones around the greenhouse outer perimeter [3.0 to 9.1 meters (10 to 30 feet)] will help to reduce the migration of insects, such as western flower thrips (Frankliniella occidentalis) and greenhouse whitefly (Trialeurodes vaporariorum) adults, and winged aphids through natural greenhouse openings (e.g. vents, sidewalls, and doors), which will decrease the incidence of disease transmission.

Out with the old

Plant debris or residues such as old leaves, fruit, and growing medium may also provide refuge for certain insect and/or mite pests. Insects and even mites can migrate to fresh plant material as plant debris desiccates. For example, a study (Hogendorp and Cloyd, 2006) showed that plant material and growing medium debris placed into uncovered refuse containers can be a source of insect pests. As plant material desiccates, adults can migrate onto the main crop. In addition, certain insect pests that were initially larvae, nymphs, or pupae when plant debris were placed into uncovered containers may develop into adults that can fly out of the container, and potentially infest the main crop.

Another study (Hausbeck and Moorman, 1996) indicated that plant debris deposited into uncovered containers may allow fungi such as gray mold (Botrytis cinerea) to sporulate. This results in the release and dispersal of spores, via air currents, that may infect the main crop. Always place debris or residues into refuse containers with tight-sealing lids. Also, growing medium debris may provide sites for fungus gnat adults to lay eggs and western flower thrips to pupate. A broom or shop vacuum can be used to remove plant or growing medium debris; however, avoid sweeping aggressively as this may distribute fungal spores in the plant debris throughout the greenhouse.

Old vegetable plants or those remaining at the end of the growing season should also be removed because they can be a potential source of insect and mite pest populations. They can serve as reservoirs for the viruses transmitted by insects such as the western flower thrips. Another important sanitation practice is to immediately remove any vegetable plants from the greenhouse that are heavily infested with insect or mite pests.

Raymond is a professor and extension specialist in horticultural entomology/plant protection at Kansas State University.

References

Hausbeck, M. K., and G. W. Moorman. 1996. Managing Botrytis in greenhouse-grown flower crops. Plant Disease 80: 1212-1219.

Hogendorp, B. K., and R. A. Cloyd. Insect management in floriculture: how important is sanitation in avoiding insect problems? HortTechnology 16: 633-636.