Back To Basics: Insect Control

Greenhouse growers offer their tomatoes, lettuce, and cucumber plants a favorable environment to produce fruit. At the same time, they create a favorable habitat for insect pests. Unlike crops that grow in the field, greenhouse vegetable pests are not plagued by the weather, natural predators, or parasites in the greenhouse. As a result, these pests can reproduce rapidly and cause quite a bit of damage if left unchecked.

The pests that are the most common to these three crops include aphids, whiteflies, leafminers, tomato pinworms, cucumber beetles, and spider mites. In this article, we will discuss these insects, the damage they can cause, and some helpful tips on control measures.

Aphid. The pear-shaped and soft-bodied aphid may be winged or wingless. In either case, this pest feeds in colonies, causing discoloration or mottling of foliage. It often transmits virus disease and excretes honeydew on which sooty mold grows.

Cabbage loopers. This green caterpillar with white stripes is about 30 millimeters in length. It has three pairs of legs near the head. Young larva typically can be found on the underside of leaves, consuming the tender leaf tissue and leaving most of the veins intact.

Cucumber beetle. The cucumber beetle is oval-oblong and is 5 to 6 millimeters in length with a bright yellowish-green body with 12 black spots on wing covers or a pale yellow body with three black stripes on wing covers. This pest not only leaves holes in foliage and stems, it feeds in blossoms and scars fruit.

Leafminer. This pest is a colorless to bright-yellow maggot with a pointed head. When it enters fruit, the leafminer makes a serpentine mine that is slightly enlarged at one end. As an adult, it is 1.0 to 1.8 millimeters long.

Tomato pinworm. These young, yellowish-gray larva are only a few millimeters long but they are able to leave blotch-like mines in leaves. Older yellow, green or gray purple-spotted larva are up to 8 millimeters in length. They bore into stems, buds, and fruit.

Two-spotted spider mite. The spider mite is very tiny, almost microscopic. It is pale to dark green in color with two or four darkly colored spots. Adults and nymphs have eight legs; larvae have six legs. This mite feeds on undersides of leaves. Infested foliage will have silvery or pale yellow stipples.

Whitefly. There are two types of whiteflies: greenhouse and silverleaf. The greenhouse whitefly is a white, moth-like insect about 1.5 millimeters long that is usually found with tiny yellow crawlers and/or green oval flattened immobile nymphs and pupae. These pests cause leaves to yellow and drop from plants.  Some plants are stunted and unreproductive. A black, sooty mold often is present on leaves.

The silverleaf whitefly is slightly smaller and a darker yellow color than the greenhouse whitefly (0.96 millimeters for the female and 0.82 millimeters for the male). It causes similar damage to plants as the greenhouse whitefly.

Management And Control

A well-planned, properly administered management program can frequently prevent damaging populations of insects and keep levels under control. Prevention is key.

Sanitation, screened openings, insect-free transplants, daily inspection of plants, and isolation are important considerations in greenhouse pest control.

The use of biological controls also is encouraged. Yellow sticky traps help keep adult whiteflies and aphids in check and monitor their movement and development in concert with the use of the parsitic wasp Encarsia formosa against the whitefly; the predatory mite Phytoseilus persimilis against the spider mite; and the parasitic wasp Diglyphus isaea against leafminers. The introduction of lady beetles, lacewings, or midges for aphids, and the use of the natural insecticide Bacillus thuringiensis for worms offers additional tactics in the battle against bugs in the greenhouse. Regular and frequent inspections of plants for pests and beneficials and good record keeping are essential to pest management.

Environmental management can also influence pest development in the greenhouse. Temperatures above 85°F and below 60°F slow the development of pests and give parasites an advantage. High and low relative humidity also slows the development of certain pests. Likewise, the misting of foliage during the day will dislodge aphids, leafminer adults, and adult whiteflies.

The use of ventilation fans or the constant use of circulating fans and the use of airblast sprays also discourage the settlement of adult whiteflies and leafminer adults in the tops of plants. Obviously, extreme high or low temperatures between crops will kill certain pest stages.

Use Of Controls

The decision as to the necessity of control must be based on insect and mite species involved, type and location of feeding on a particular crop, the age of the plant, amount of damage caused by given numbers of pest, and the potential for disease infection.

Remember, however, that it is essential to identify the pest properly before using any crop protectant. Since some insecticidal formulations for use in greenhouses are not readily available in some areas, plan ahead so that you will have the proper materials on hand when needed.

Growers can use aerosols, mists, smokes, fogs, dusts, sprays, drenches, and granules to control pests. Since most pesticides are sensitive to ultraviolet rays, the later in the evening a treatment is made, the more effective it will be. Afterward, the house must be ventilated before workers can safely enter it.
Fogs, smokes, and aerosols are generally applied per cubic foot and must be applied when the greenhouse is closed. Dusts and sprays are applied with conventional dusting or spraying equipment. Necessary safety clothing must be worn.

Apply granular insecticides with a hand-held shaker or some other device that does not grind up the granules. Wear boots, long pants, a long-sleeved shirt, rubber gloves, and a respirator. The plants and potting mix should not be handled until the granular pesticide has been washed from the foliage and watered thoroughly.

Current EPA policy interpretation of greenhouse registrations is that if the pesticide label does not restrict or prohibit use in the greenhouse, then it can be used. The IR-4 Minor Use registration program has been more active in obtaining greenhouse prioritization and studies on efficacy, residue, and phytotoxicity with insecticide labels. As a result, several new registrations have been obtained. Read and follow label directions for best results.

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