Greenhouse Insect and Mite Pest Management
Insect and Mite Pests Commonly Found in Greenhouses
Whiteflies
The most common and perhaps most difficult to control insect pests in greenhouses are whiteflies (See Figure 27.10). Whiteflies are tiny, sap-sucking insects usually smaller than 2 millimeters long. Several species of whiteflies attack greenhouse crops, especially vegetables. The most notable species are the sweetpotato whitefly or silverleaf whitefly (Bemisia tabaci), and the greenhouse whitefly (Trialeurodes vaporarorium). Occasionally, the bandedwinged whitefly (Trialeurodes abutilonia) may enter greenhouses from outdoors in the fall, but it not a serious pest and control is rarely needed. Species identification is critical because different species respond differently to control strategies, both chemical and biological. It is not uncommon to find several whitefly species in the same greenhouse.
Life Cycle
Whitefly life stages include the adult, egg, three nymphal instars, and the fourth instar or pupa stage, all occurring on the underside of the leaves. The first nymph stage is referred to as the crawler stage, which moves moves only a short distance before settling down to feed. The final stage (pupa) develops a waxy fringe and appears elevated on the leaf surface. The major diagnostic differences between greenhouse whitefly and the sweetpotato whitefly appear in the pupal stages. The greenhouse whitefly pupa is white or cream-colored, is raised off the leaf surface and is surrounded by a fringe of hairs, while sweetpotato whitefly pupae are more yellow in color, sit flat on the leaf and do not have a fringe.
Types of Damage
Nymphs and adults are typically located on the underside of plant leaves. Both nymphs and adults have piercing-sucking mouth-parts which they use to feed on plant fluids. High populations can cause plant foliage to become yellow. Nymphs secrete honeydew (sugary sap) that serves as a growing medium for black sooty mold fungi. Sooty mold can affect plant yields by reducing photosynthesis, and residue on fruit requires extra handling time. Whiteflies may also serve as vectors for several viral diseases
Monitoring
Whiteflies should be monitored by using yellow sticky cards and by inspecting the undersides of leaves for eggs, nymphs and pupae. Yellow sticky cards used at a rate of 1 per 1,000 sq. ft should be positioned just above the crop canopy and replaced each week. Use more sticky cards when growing susceptible crops and use additional cards under benches, and near doors and vents. The sticky cards can be moved vertically as plants increase in height.
Cultural Management Strategies
The first objective is to eliminate all possible sources of residual whitefly infestations. Totally eliminate all weeds and plant debris inside and immediately outside the greenhouse. They can harbor immature or adult whiteflies. Collect the weeds and debris in covered containers or seal them in plastic bags. Infested plant debris stored in open containers may continue to produce adult whitefly that may quickly migrate back onto crops inside the greenhouse. When plants arrive, before putting them in the greenhouse, randomly examine the plants. Look for nymphs, pupae and eggs as well as the adults. Because all stages of the whitefly life cycle are dependent on plants, allowing the greenhouse to stand empty of plants can also break the cycle of re-colonization and infestation. Adult whiteflies cannot live without feeding on a host plant (for more than a few days), so even a week at warm temperatures without plants should be sufficient to remove potential whitefly colonists from the greenhouse.
Biological Management Strategies
Two parasitic wasps—Encarsia Formosa and Eretmocerus eremicus—are commercially available for whitefly control. The predatory mites Amblyseius swirskii and Amblydromalus limonicus are reported to be very effective against all species of whiteflies, as well as thrips. A small black beetle called Delphastus pusillus is also available commercially, but is likely only effective where there are heavy populations of whitefly (a situation that most growers would want to control more quickly with pesticides, although in crops such as cut gerbera, it has proven to be very effective).
Predatory Wasps. Encarsia formosa, a small wasp that attacks the larval stage of whiteflies, is considered the primary natural enemy of greenhouse whitefly and banded-winged whitefly, but also provides some control against the sweetpotato whitefly. E. formosa kill whiteflies either by laying eggs into the immature stages (mostly the third and fourth larval stages) or by directly feeding on the young whitefly nymphs. Upon hatching, the wasp larva feeds on its host, killing it in its pupal stage. The wasp completes its development inside the whitefly pupa, and a new adult wasp will emerge through a round hole that it chews in the whitefly pupal skin. Parasitized greenhouse and banded-winged whitefly pupae are black in contrast to sweetpotato whitefly pupae, which turn brown when parasitized.
Predatory Mites. Amblyseius swirskii, a predatory mite feeds on thrips and is also a very effective whitefly predator. A. swirskii feeds on whitefly eggs and crawlers. As a result, it is very compatible with the parasitic wasps used for whitefly control, which target later life stages.
Black Beetle. The black beetle (Delphastus pusillus) has a ravenous appetite for whitefly eggs, nymphs, and adults. These predators can control all species of whiteflies and can be used with parasitic wasps.
Biorational Management Strategies
Microbials. The fungus Beauveria bassiana is effective against eggs, immature whiteflies, and adult whiteflies. Applications must be initiated before whitefly populations are high. Beauveria bassiana is appled as a foliar spray and good coverage of the undersides of the leaves is important to reach the immobile nymphal stages of whiteflies. Several applications will be needed.
Insect Growth Regulators. Whiteflies typically have several generations each year, especially in a greenhouse environment. They take about 21 to 25 days to develop to an adult, depending on temperature. Because the nymphs are on the plants for a long time and passing through many molts, whiteflies are susceptible to pesticides that act as insect growth regulators (IGRs) like Distance (pyriproxifen) and Pedistal (novaluron).
Chemical Management Strategies
Greenhouse whitefly nymphs and eggs are generally laid on the underside of foliage making contact by insecticide sprays difficult. Therefore, adult whiteflies are often easier to control than the more damaging nymphs with insecticides presently registered for use in greenhouses. Systemic insecticides can be very effective, if a sufficient amount is translocated to sites at which whiteflies are feeding. Systemics are usually most effective when applied early in the crop’s development. If necessary, applications should be repeated every 3 to 4 weeks. Because whitefly populations can develop resistance to insecticides, it is best to switch among products from different chemical classes (i.e., pyrethroids, organophosphates, insect growth regulators, etc).
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