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Pest Management in Greenhouses
Pest management programs should be designed to meet a specific production objective. In greenhouse production, this objective is usually to produce undamaged pest-free plants. However, in some situations the objective may be to maintain healthy plants. This allows for some tolerance of minor insect or mite pest damage. There also may be a preference for the use of nontoxic or low-toxicity pesticides. This is particularly important in regard to the potential contamination of surface and groundwater. Financial constraints also play a role in determining the overall objective. All of these factors significantly affect the selection of pest management tactics.
Start with a Clean Production Area
The first steps to take in a pest management program are preventative, and starting with a clean production area is essential. Greenhouses can be fumigated or otherwise treated prior to establishing a new crop to help eliminate pest problems from previous crops. Proper pruning and removal of clippings and other debris from the growing area will eliminate some of the sites that harbor pests. Weed control in and around production areas eliminates alternate host plants for pests.
Select Pest-free, Quality Plants
The selection of uninfested plants, plugs, cuttings or transplants is critically important. Carefully inspect all plants brought into the production area. Discard, treat or return infested plants and, if possible, hold the remainder in a quarantine area for a while. When possible, use pest-resistant or tolerant plants to reduce the need for pesticides. Becoming knowledgeable about the susceptibility to pests of a particular plant species or cultivar will help you anticipate problems throughout the production cycle.
Detect Pests Early
Once plants are established, there are several methods for monitoring the presence of pests.
Yellow sticky traps. Place yellow sticky traps in and around the production area to help detect early movement of adult whiteflies, thrips, adult leafminer flies, fungus gnats and aphids. Implement control programs when significant numbers of pests or related damage are first detected.
Regular inspection. Conduct regular inspections, paying particular attention to the underside of leaves. Try beating portions of plants against off-white paper to dislodge small pests onto the paper. This method is useful in detecting hard-to-see pests such as spider mites and thrips. In the greenhouse, regularly inspecting plants that are highly attractive to certain pests can be useful for detecting low populations of pests.
Pheromones. Pheromones are natural chemicals produced by animals to signal each other. There are three basic types of pheromones. Aggregation pheromones attract many individuals together; for example, at a site where food is plentiful. Sex pheromones attract one sex of a species to the other sex. Trail pheromones are deposited by walking insects, such as ants, so others can follow. Synthetic pheromones mimic these natural chemicals. They are used to attract pests into traps, disrupt mating and monitor pest populations. For example, in Christmas tree production, producers can use pheromone traps to monitor flight activity periods of the Nantucket Pine tip moth. Because they do not kill insects, pheromones are not true pesticides.
Consider All Management Tactics
Consider the economic, toxicological (worker safety, phytotoxic potential and pesticide residue) and environmental effects of all control options. When pesticides are necessary, select the least toxic, most target-specific, most effective and affordable material possible.
Types of Control:
Good horticultural practices constitute the first line of defense against pest problems. Select proper sites for growing plants. Plants are healthier and better able to tolerate pests when they are protected from sun scalding and given good growing conditions. The use of slow-release fertilizers at optimum rates and proper watering practices can make plants less attractive to pests. Weed control in and around production areas eliminates alternate host plants for pests.
Pest infestations can be physically manipulated with light, humidity and temperature. In interior plantings, light quantity and quality affect plant health and influence pest populations and their damage. Unlike conventional lights, yellow "bug" lights used at night around plants will not attract insects such as moths, crickets and June beetles. Temperature and humidity are important, although not always controllable. In greenhouses and interiorscapes, these factors can sometimes be manipulated to reduce pest survival or improve the success of natural enemy releases. Devices which use sound for pest suppression have not been shown to be effective.
Mechanical methods for pest suppression and exclusion can be helpful.
Screens. Screens can exclude plant pests from the greenhouse. Insect-exclusion screens are sold with instructions for proper use. When they are placed in front of intake fan vents, be careful not to reduce air flow. Proper maintenance of exclusion devices is essential.
High pressure water sprays. Devices that produce high pressure water sprays can effectively dislodge spider mites and some aphids from host plants.
Trapping. Light traps, yellow sticky traps and sticky-surfaced pheromone traps attract and kill certain insect pests. However, they do not provide absolute control. These devices are best used to monitor pest population densities over time and to detect infestations early. They also can be used to evaluate the impact of management practices.
For best results: a) use the same brand of sticky trap throughout the monitoring period so you can document an accurate picture of relative insect population densities; b) change traps weekly and count pests per card or per area (i.e., pests per square inch of card); c) in greenhouses, use one trap per 10,000 square feet and place them at least 150 feet apart; and d) hang traps just above the plant canopy over the center of the crop, and also in doorways and near vents.
The term "biological control" refers to the use of natural enemies to suppress pests. Biological control tactics include the conservation, augmentation and importation of natural enemies. Biological control is an environmentally safe method and is the basis for some integrated pest management programs.
Importation. Many pests are exotic and have no natural enemies in Texas. Reuniting pests with their natural enemies may provide the most dramatic and sustainable method of suppressing them. Importation of such natural enemies is called classical biological control. The search for exotic beneficial organisms that can control serious plant pests in Texas is a major mission of biological control scientists in the Department of Entomology at Texas A&M University.
Conservation. Pesticides kill beneficial predators, parasites and pathogens as well as pests. They can cause outbreaks of secondary pests, or rapid resurgence of pests, that were initially suppressed. Using nonchemical control methods, or pesticides that kill only the target pest, protects natural enemies. Some easily seen predators are spiders, lacewings, lady beetles, ground beetles, rove beetles, syrphid flies, flower flies, hover flies, true bugs (including minute pirate bugs, big-eyed bugs and damsel bugs), predatory mites and even fire ants. However, many important natural enemies are rarely seen, such as parasitic wasps and flies (more than 8,500 species), nematodes, and pathogenic bacteria and fungi.
Augmentation. Natural enemies can be released all at once or over time to suppress pests or keep their numbers low. Also, the environment can be enhanced to favor natural enemies. Releases of natural enemies for controlling certain pests can be very effective in greenhouses and interiorscapes such as the two-spotted spider mite controlled by predacious mite releases. Outdoor releases are affected by unpredictable environmental conditions. Furthermore, if a second pest is unaffected by the released organism, pesticides used to control the second pest often eliminate the natural enemy of the first pest. Specific recommendations for Texas are still being developed.
The application of microorganisms in a manner similar to conventional pesticides is a type of augmentation. These products are referred to as "microbial insecticides." Several products available contain varieties of the bacterium Bacillus thuringiensis, which controls certain caterpillars, beetles and flies, but does not affect other arthropods. Microbial insecticides are relatively slow acting and are most effective if applied when pest numbers are low and pests are in early stages of development.
The term pesticide can be defined as anything that kills pests. This would include chemicals, biologicals, or even your shoe.
Broad spectrum. Broad spectrum insecticides kill a wide variety of insects, usually by affecting a system common to all, such as the nervous system. These "broad spectrum" pesticides are general purpose killers used when several kinds of insects are a problem. Examples of broad spectrum insecticides include pyrethroids and those containing acephate, chlorpyrifos, diazinon and carbaryl.
Target-specific. Target-specific, or narrow spectrum, insecticides are much more selective to certain pests or groups of pests. Use these insecticides when you wish to kill only one insect pest and not other beneficial insects. For example, chitin inhibitors affect insects only at certain stages in the development of their exoskeleton. Growth regulators are even more specific. They affect only insects that have particular hormones.
Chitin synthesis inhibitors interfere with the growth and molting of immature insects. Chitin is the primary structural chemical in an insect's body wall. An immature insect treated with a chitin inhibitor dies the next time it tries to molt. An example is cyromazine-containing products which only affect certain fly larvae.
Insect growth regulators or IGRs mimic an insect's natural juvenile hormone. They interfere with certain normal processes and prevent immature insects from becoming reproductive adults. Growth regulators act slowly. Their effects include abnormal molting, twisted wings, loss of mating behavior and sometimes death to embryos in eggs. Because IGRs attack a growth process found only in insects, there is a great margin of safety for humans and other vertebrates. An example is kinoprene-containing products which affect primarily sucking insects.
Short-term vs. residual. Pesticides also vary in how long they last. Some break down quickly into nontoxic by-products. These short-term chemicals are best when the pest will not return and when long-term exposure could injure nontarget plants or animals. For example, short-term insecticides often are used in homes and dwellings where people and domestic animals might be exposed. Examples of short residually active products are those containing insecticidal soap, pyrethrins or resmethrin.
Other pesticides remain active killers for a long time. These residual chemicals are very useful for a persistent pest problem, if used where they will not become an environmental or health hazard. Examples of long residually active products are those containing pyrethroids, chlorpyrifos or imidacloprid.
Miticides. Miticides are chemicals that control mites (tiny spider-like animals). The chemicals usually must contact the mites to work. These animals are so numerous and small that you must take great care to completely cover the area where they live. Miticides and insecticides are similar and some products affect both groups. Examples of specific miticides include products containing avermectins.
Fungicides. Fungicides control the fungi that cause molds, rots and plant diseases. They are best used as preventive treatments rather than a curative treatments. All fungicides work on contact. Most are sprayed over a large surface area to hit every fungus directly. Some fungicides are systemic. These are fed or injected into the plant to be protected. The chemical then moves throughout the plant, killing the fungi.
There are two types of fungicides: protectants and eradicants. Protectants prevent plant diseases and eradicants cure them.
Protectants. Fungicides used as protectants are similar in purpose to vaccinations for humans. You apply them before disease starts. This is very useful when a particular disease or group of diseases is likely to occur year after year. For example, protectants often are used as a routine precaution on fruit and vegetable crops.
Most protectant fungicides are "fungistatic". This means they prevent or inhibit fungal growth. Once the fungistatic action stops, the target fungus may grow again or produce spores. You may have to apply fungicide regularly to maintain disease protection.
Eradicants. Eradicants are like penicillin and other antibiotics that cure human diseases. They are less common than protectants because fungus is hard to destroy after it infects a plant. Eradicants often are used when protectants are unavailable, too expensive or applied too late, or when a disease appears unexpectedly. For example, orchardists use eradicants to combat diseases such as pecan scab.
Nematicides. Nematicides are chemicals that kill nematodes. Nematodes are tiny hair-like worms, most of which live in the soil and feed on plant roots. Soil fumigants can control nematodes in the soil. However, a few contact insecticides and fungicides also work well against these tiny worms.
Molluscicides. Molluscicides control snails and slugs. Usually the chemicals must be eaten by the pest to work. Baits often are used to attract and kill snails or slugs in a target area.
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