Managing insect and mite pests in vegetable gardens
The content of this article was written by John A. Jackman and posted by the Texas A&M Agrilife Extension
For a fresh, satisfying and tasty way to help lower your grocery bill and provide personal satisfaction, nothing is better than vegetable gardening. But growing vegetables is not always easy, especially when it comes to controlling insects and other pests.
About 30,000 species of insects are found in Texas; fewer than 100 routinely cause problems in vegetable gardens. Most garden insects are either incidental or beneficial when they help with pollination, recycle organic matter or keep pests under control. A garden with many insects may be quite healthy and productive. However, insect pests can reduce the vegetable crop’s quantity and/or quality or transmit diseases from one plant to another. When that happens, control measures may be necessary. When dealing with insects in the garden, first identify the species to determine whether they are beneficial or pests. Learn to recognize the common insects in the area, especially common pests, and the signs of pest damage. Inspect the garden for pests at least once a week.
Whether they arrive by walking or flying, insect pests can take up permanent residence in the garden. Flying insects are highly mobile and can migrate in large numbers. In addition, pests such as aphids and mites can have a complete life cycle in about a week under good conditions, so their populations can increase rapidly. When many pests seem to appear overnight, they have either flown in or are reproducing rapidly.
As insects grow, they change size, shape and color in a process called metamorphosis. In some insect species, both the immature and adult stages damage plants. Because their forms change, insects can be difficult to identify; the damage they cause to plants also can change with their forms. Size matters too—small caterpillars may barely scrape the surface of a leaf, while larger caterpillars may eat great chunks.
Damage to plants depends on the insect’s mouthparts. Insects with sucking mouthparts feed by piercing leaves or fruit, leaving pock marks or mottled leaves. Insects with chewing mouthparts chew holes in plant tissues. Recognizing how an insect feeds can help a gardener select the proper insecticides; choose stomach poisons for chewing insects or contact poisons for sucking insects.
When planning a vegetable garden, consider possible pests and how to manage them before they cause problems. Implement the management plan in plenty of time to deal with pest problems.
Integrated pest management
Integrated pest management, often called IPM, uses a combination of pest control techniques that balance economic production and environmental stewardship. IPM is the overriding strategy for most of production agriculture today and is being adopted in urban environments as well.
Monitoring crops for the presence and absence of pests is an important part of IPM. In situations where a pest is present and could cause significant damage, management is justified.
Although many practices can be implemented as part of an IPM program, the use of insecticides is a control option. When non-chemical control practices are used, the IPM approach is much like organic gardening.
The following sections list many control practices available for home vegetable gardeners.
Gardening practices that reduce pest numbers or impact are called cultural control. These practices include variety selection, crop rotation, cultivation, weed management, water management and fertilizer use. For some pests, the best choice is to interrupt their life cycle by leaving the land fallow and weed-free for a period or by rotating crops. Plant debris can harbor pests; always remove it from a garden area. Weeds can attract insect pests and also must be controlled.
Host plant resistance
Through a natural process called host plant resistance (HPR), vegetable varieties can continue to produce in spite of the presence of insects and other pests. These plants show tolerance, nonpreference or antibiosis. Tolerance is the plant’s ability to grow and produce even with pest damage. Nonpreference is exhibited when a plant has structures, such as plant hairs, repellent odors or colors that cause insects to choose other plants. Some plants produce chemicals that kill or slow development of a pest—this is called antibiosis.
Extensive trials are needed to understand the host plant resistance of vegetables. Most variety selection emphasizes appearance, taste and production volume without regard to pests. The host plant resistance status of many varieties has not yet been tested.
Recently researchers have altered the genetic material of some vegetables including tomatoes, potatoes and corn—these are called transgenic plants. Dramatic results can be achieved when genes for insect resistance are incorporated in the new varieties. Most of the insect-resistant transgenic vegetable varieties incorporate genes of the bacterium, Bacillus thuringiensis, making them resistant to some caterpillar pests. This resistance inhibits the growth of caterpillars feeding on these plants. Resistant transgenic vegetable varieties are expected to become increasingly available to homeowners.
Biological control uses one organism to control another. Three successful approaches to biological control are importation, conservation and augmentation.
Importation requires bringing a parasite or predator from a foreign country to control an introduced exotic pest species. Because it is highly regulated by state and federal agencies, importation is not available to home vegetable gardeners, although they do benefit from successful importation research programs.
Conservation encourages natural enemies already in the area. Conservation methods include:
- Planting nectar-producing flowers that provide food for parasites.
- Avoiding unnecessary pesticide applications.
- Selecting pesticides that are toxic to a pest but relatively nontoxic to beneficial insects.
Augmentation is the release of additional predators and parasites, such as lady beetles, praying mantids and parasitic wasps, into the natural populations. However, the benefit of additional releases may be marginal because many of these predators and parasites already exist in the environment.
Biological control is not an instant solution to pest problems. A sound biological control program must be supported by careful study, starting with proper identification in order to match pests and beneficial organisms. Increased monitoring is necessary. Many biological control agents are specific to certain pest species. Usually, biological controls are not available for a specific pest.
Mechanical control—including barriers, covers, high pressure water sprays and hand-picking pests—uses physical means to reduce insect numbers or damage.
Barriers, which prevent the movement of pests onto plants, include cardboard or plastic cylinders around the base of transplants or cloth or plastic screening to protect a newly planted garden. Screening may increase the temperature of a planting bed, often an additional benefit. Screening is most useful for susceptible young plants and seedlings and may provide some protection from frost as well.
High pressure water spraying, one of the few options available when vegetables are near harvest, is most effective against small, soft-bodied pests such as aphids. High pressure water sprays may help remove webbing, dissolve droppings and reduce the number of pests in a short time. However, water sprays may not kill all of the pests and may distribute pests to other hosts.
Hand-picking and destroying some pests may be feasible in small gardens, and can be successful for tomato hornworms and even squash bugs if persistently done. Obviously, hand picking is more feasible for larger insects than for small insects.
Pesticides in any form are regulated for safety by the Environmental Protection Agency (EPA); the sale and use of these products is regulated by the Texas Department of Agriculture. These agencies do not consider effectiveness in the registration process. Labeled insecticides may or may not be effective in killing pests that are mentioned on the product label. The number of products available for use in home vegetable gardens and the rapid turnover in the market makes the effectiveness of products difficult to determine. Their effectiveness can change as pests become more tolerant or as environmental factors interact with a chemical. Furthermore, a pesticide may fail if it is not applied according to the label directions.
The user is always responsible for the proper use of any pesticide. Using a product in a manner or situation not defined on a pesticide label is illegal.
Product labels list restrictions that must be considered, including limits on product rate, number of applications per season, specific crops the product may be applied to, method of application and number of days required from last application to harvest.
Some generic insecticides have several trade names; special restrictions may be noted on a specific label. Read the label for additional restrictions and follow directions carefully.
The EPA approves pesticides for use on a particular crop after evaluating safety data only, in most cases. If a crop is not listed on a product label, the pesticide can not be considered safe for use on that crop. The EPA considers greenhouses to be separate from crop lands; therefore products must specifically state for greenhouse use on the product label. These products may not be safe to use on some crops because of other factors such as phytotoxicity. Table 1 lists a summary of the chemicals registered for use in home gardens. This list was prepared from product labels; not all products have been examined by the Texas AgriLife Extension Service.
Product labels also list suggested target pests. Table 2 lists product labels reviewed for this guide, and includes most of the common active ingredients available for use in home gardens.
Pesticide registration status changes rapidly. In most cases, products are phased out with dates to stop both wholesale and retail sales. Usually provisions allow homeowners to use already purchased products beyond those dates.
Pesticides vary widely in their hazardous effects on humans and the environment. The key words on the label—CAUTION (least toxic), WARNING (more toxic), and DANGER (most toxic)—indicate toxicity of the product. Use this label information as a guideline on product use and potential hazard. Most of the products mentioned here include caution on the label. Mixing the product for use is one of the most hazardous steps in pesticide use—take special care during that step.
Insecticide classes provide a key to understanding how the product works and thus which pests are most likely controlled. When insect control is unsatisfactory, change to a product from another insecticide class. (See Table 3.)
Less toxic approaches
Instead of applying conventional chemicals, many gardeners prefer to use less toxic approaches to insect management, which can range from “soft” insecticides to natural control. Home gardeners have more of these products to choose from than ever before. Some of the less toxic products registered and sold as pesticides are included in this guide.
Less toxic chemicals are available under different legal registrations. Chemicals listed in the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) EPA Title 40, Chapter 1, Subchapter E, Part 152.25 are considered “minimum risk pesticides” and are exempt from FIFRA registrations. This list includes cedar oil, citric acid, citronella, cloves, garlic, lemongrass oil, mint, peppermint, rosemary, thyme and white pepper. For more information visit the Web at http:// www.epa.gov/lawsregs/laws/fifra.html.
Inert ingredients often are included in pesticide formulations to dilute the active ingredients and/or facilitate the application. Inert ingredients are also covered under the same FIFRA registrations in list 4A and are considered minimal risk.