By: John A. Jackman
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.
Putting it all into practice
Plant a garden of manageable size
Garden size directly affects the feasibility of control measures. Removing pests by hand and swabbing pests with alcohol may not be feasible in larger gardens. The larger the garden, the lower the likelihood that non-pesticidal controls will be practical.
Leave the garden fallow for a time before planting
Insect pests such as white grubs, wireworms and cutworms overwinter in the soil and feed on abandoned plants or weeds. Removing these food sources during the off season reduces pest numbers before spring planting.
Practice good sanitation
Remove dead leaf piles, boards, railroad ties and other objects where pests such as cutworms, slugs, snails, pillbugs and sowbugs can congregate. Mulches help maintain moisture and provide shelter for spiders and predatory insects; however, mulch also provides shelter for pests.
Select pest-free transplants
Inspect plants before purchasing to be sure they have no pests. Most common insect and mite pests can be found on the undersides of leaves. Purchase only healthy pest-free transplants.
Select pest-resistant vegetable varieties
Some vegetable varieties are unattractive or resistant to certain pests. Planting resistant varieties adapted to your area can dramatically reduce the need for insecticides. For example, the sweet corn variety “Seneca Sentry” is resistant to corn earworms and is adapted to Central Texas. The leaves that wrap around the corn ear tip are much tighter around the silk than in more susceptible varieties. Unfortunately, the pest resistance status of only a few vegetable varieties is known.
Practice good horticultural methods
Properly prepare the soil before planting. Thoroughly till the soil to kill many insects and provide good growing conditions for seedlings and transplants. Healthy plants will be less susceptible to pest damage. The composition of the soil and spring growing conditions also affect pest populations. Soils with high organic matter are more likely to support white grubs, root maggots, pillbugs and sowbugs, even though these soils may promote better plant growth.
Keep a weed-free garden. Weeds supply food for insect pests and compete with vegetable plants for soil nutrients and water, which can decrease vegetable yield considerably. A weedfree garden and grass mowed short around the garden will discourage insects such as grasshoppers and armyworms.
Fertilize properly. Plants need adequate nutrients to grow well. However, using too much fertilizer can produce lush, green plants that attract aphids and other insect pests. A soil test will determine which nutrients may be lacking and which are at adequate levels.
Water properly. Either too much or too little water can be unhealthy for plant growth. Drought-stressed plants are more likely to attract spider mites.
Inspect plants and properly identify pests
Learn to identify the various insects and other creatures in the garden. Many of them are beneficial. Extension agents can help identify plant pest problems. Don’t treat undiagnosed problems.
Pests can attack garden plants from seed to maturity. Inspect the plants weekly or more often for pests, monitor natural enemies and evaluate the effects of control tactics. Check the undersides of leaves for aphids, whiteflies, spider mites and egg clusters of armyworms, Colorado potato beetles and squash bugs.
To detect low populations of spider mites and thrips, beat the plants on a piece of off-white paper; the pests will fall off the plant onto the paper, where they can be identified. Although yellow sticky cards are occasionally promoted as insect control devices, they are best used to monitor pest activity. These cards attract the winged adult stages of aphids, leafminers, thrips, whiteflies and many flies. Cards should be inspected and replaced regularly so pests can be detected early and their numbers monitored. Sex-attractant chemicals called pheromones are available commercially to monitor many insect pests, especially moths.
Consider all pest suppression methods
When a pest outbreak occurs, consider prevention methods and the best method of reducing pest numbers. Some mechanical suppression methods are:
- Reflective mulches such as foil paper, which can slow infestation by some pests such as aphids.
- Barriers to protect young plants or transplants from cutworms, sowbugs or pillbugs. Place a barrier made from cardboard, plastic or metal cans with the tops and bottoms removed around the base of each plant.
- Screens over the garden. Fine-mesh screens or fabrics can provide a barrier that even tiny insects such as thrips cannot cross. Several products are available. When barriers are properly maintained, insects can be excluded; however, plants should still be monitored regularly, which requires removing the barrier. The temperature inside barriers often exceeds that outside, so remove them before the plants experience heat stress. This method works best in early spring or fall when the temperatures under the screen are moderate.
- Cages and trellises. Plants growing on the ground are susceptible to soil pests. Vine plants such as cucumbers and tomatoes are easier to manage when grown on trellises or in cages. Monitoring for pests and spraying plants thoroughly is easier when they are held up off of the ground.
- High pressure water sprays. Small pests such as aphids and spider mites can be dislodged from plants with high-pressure water sprays directed to the undersides of leaves. Commercial spray devices (Water Wand® and Jet-All Water Wand®) are available, but similar devices also can be made at home. Be careful not to harm the plant or to distribute pests around the garden. Repeated treatments may be necessary.
Conserve natural enemies and protect bees
The first line of defense against insect pests is their natural enemies. Spiders, praying mantids, lady beetles, ground beetles, green lacewings, ambush bugs, assassin bugs, minute pirate bugs and some wasp species prey on other insects. However, the most effective natural enemies are tiny parasitic wasps and flies, together with bacteria, fungi and viruses.
Use pesticides only as a last resort; allow natural enemies to suppress the pest infestation. If a pesticide is required, select the least toxic, most target-specific varieties that decompose quickly.
Whether naturally occurring or deliberately released into the garden, these organisms should be encouraged. Natural enemies can be released in the garden to control pests. Lady beetles and green lacewing larvae eat aphids and whiteflies; predaceous mites eat two-spotted spider mites; and certain wasps parasitize specific insect pests. (Trichogramma species develop inside caterpillar eggs; and Encarsia species develop inside immature whiteflies.)
Companies that sell these natural enemies do not guarantee the results; factors such as the number of pests present, the environment, release times, prior pesticide use and the presence of ants can affect releases. Parasitic nematodes (Biosafe 100® and other products containing Steinernema carpocapsae) are available to control a variety of vegetable garden soil pests.
Bees are necessary for pollinating vegetables such as cucumbers, pumpkins, squash and melons, and should be protected. Do not apply pesticides during the hours when bees are active. Instead, treat plants when bees are not active—very early in the morning or late in the afternoon. Avoid using products or formulations toxic to bees. If a bee hive is located nearby, cover it during the pesticide application or protect the hive from pesticide drift.
Apply pesticides only when justified
If other measures have failed to control a pest population, a pesticide may be required. Because they are toxic and must be used carefully, pesticides are regulated by law and must be applied strictly according to label directions.
To control leaf-feeding insects, the pesticide must cover the undersides of leaves, which can be difficult to accomplish with dust-formulated products. When using liquids such as emulsifiable concentrates or wettable powders, mix the directed amount with water and spray immediately. Alkaline water may decompose the active ingredients if the solution is allowed to stand. Shake the mixture while treating. If spray droplets bead up and roll off the treated foliage, an additive called a spreader-sticker such as Hi-Yield® Sticker-Spreader may be necessary. Spreader-sticker products are sold in most pesticide outlets. After treatment, clean the sprayer thoroughly, store the pesticides properly and wash the protective clothing separately from other laundry.
Table 1 shows registered pesticides for use on home garden vegetables and common insect pests.
Product labels found in trade channels were the primary source of information in this table. This includes many, but not all, of the products available to the home vegetable grower.
A list of the products reviewed during preparation of the guide is provided in Table 2. Only a few products with a mixture of two or more active ingredients where reviewed for this guide. Because multiple active ingredients complicate the summary, these were not included in this table.
The column titled “Pesticides Registered” lists the common names of the chemicals with that particular vegetable listed on the label. “Pesticides Registered by Pest” are similarly the pesticides that have that pest on the label. The pesticide label may not specifically state the combination of commodity and pest on a particular label. The user is responsible for reading and following directions on the label.
Pests sometimes have several common names; additional common names are given in parentheses. Some labels have a general common name for a pest group listed under the pest name; specific examples are listed after the colon. In a few cases, the adult and immature forms have different common names and control measures. The control measures for each stage are indicated in the table, or both names are listed separately with different control measures. For example, immature cucumber beetles are called rootworms. This example includes several species that can be damaging in Texas, with adults that feed on flowers and foliage and larvae that feed on roots.
General use products
Some products have broad pest and site combinations on the product label. For example, the product label might just say “pests” on “vegetables.” Some of these products are Green Light® Tomato & Vegetable Spray Read-to-Use (neem oil), Green Light® Neem Concentrate (neem oil), Green Light® Neem II Ready-to-Use (neem oil and pyrethrins), Green Light® Bioganic® series, Bonide® Bio-Neem, and some insecticidal soaps. Monterey® Worm-Ender® is a Bacillus thuringiensis product that is simply labeled for caterpillars (often referred to as worms) on vegetables. These products are not included in the table but can be considered legal uses for vegetables as listed on the product label.
Products for specific pests
Ants: Several ant species—fire ants and Texas leafcutting ants being the most severe—can be found in vegetable gardens. Control ants outside the garden if possible. Baits or mound treatments are preferred. Never apply a pesticide for an ant treatment inside the vegetable garden if vegetables are not listed on the product label. A mound drench of boiling water can be used inside the garden to control fire ants; be careful to avoid burning the applicator or the plants. For more information on fire ant management see http://fireants.tamu.edu.
Snails and slugs: Products containing metaldehyde are the primary control measures for snails and slugs. These products are granular baits, meal or pastes. Use metaldehyde products with caution around pets. Some snail and slug baits contain carbaryl and include other pests on the label. Iron phosphate and orthoboric acid also are active ingredients in some snail and slug baits. Most snail and slug baits may not be used on vegetables or in the garden; however, Green Light® Snail & Slug Bait states on the label that it can be used in the garden.
Grasshoppers and crickets: Grasshoppers and crickets may move into gardens rapidly, especially when winged. Protecting foliage with an insecticide may not be very successful if these insects invade in large numbers. Insecticides including carbaryl, esfenvalerate, malathion and azadirachtin are labeled for use on grasshoppers; bait formulations with carbaryl and some with a combination of carbaryl and metaldahyde also are available. Treating the grounds outside of the garden may help. Use barriers to protect the most valuable plants.
Sowbugs, pillbugs, millipedes, centipedes, mole crickets, cutworms, root maggots, mole crickets, wireworms, springtails and earwigs are soil pests that can be avoided by using winter or summer fallow periods. These pests may also be controlled with preplant treatments of insecticides in the soil. Some formulations of synergized pyrethrins and carbaryl are labeled for these pests. Some bait products may have these pests on the label. No home vegetable pesticide products were found with white grubs or wireworms on the label.
Additional pesticide precautions
Oil products may damage plants, especially in hot weather. Bacillus thuringiensis products for caterpillars work best when the caterpillars are small.
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