Making Your Garden Vegetables Less Susceptible to Insect Damage
Integrated pest management or IPM is the judicious use of multiple pest management tactics when pest populations reach above the action threshold. The action threshold is simply the level of pest populations when control should be implemented to avoid damage to the crop. Action thresholds help determine both the need for control actions and the proper timing of such actions. Homeowners and gardeners can practice IPM by using cultural control practices and pest exclusion methods, by employing alternative pest management strategies, and by conserving natural enemies. There are many ways to reduce or eliminate the need for insecticides; some of these are described in the sections that follow. To stay informed about pest occurrences and IPM recommendations, readers can subscribe to the Alabama IPM Communicator newsletter, an official publication of the Alabama IPM Center. To subscribe, visit the newsletter website at www.aces.edu/ipmcommunicator. For pest alerts, subscribe to the Alabama Vegetable IPM channel on Facebook.
Healthy soil will result in plants better able to resist insects and diseases. Before planting, the garden soil should be turned over and organic matter, such as manure or compost, should be added to supply essential nutrients. Organic nutrients are released slowly, in contrast to synthetic fertilizers, which provide “quick-fix” nutrients. When possible, good compost should be purchased from a trusted local source; buying poor quality compost may result in insect infestations like maggots and grubs.
Sustainable Pest Management Practices
The USDA National Organic Program Standards for Pest Management provides some excellent steps that can be used in vegetable gardens. These standards are three-tiered recommendations that are as follows: Level 1: Systems-based practices. These include cultural tactics such as variety selection, crop rotation, water management, sanitation (starting with clean fields), trap cropping, and companion planting. Level 2: Mechanical and physical practices. These include installing barriers like insect netting and row covers; using lures, traps, and repellents; regular mulching; and hand removal of insects from the foliage. Level 3: Biorational and other insecticidal materials.
Only approved insecticides can be used in commercial organic farming. Home gardeners can also use these organic insecticides some of which are very expensive; thus, cost could be a limiting factor. Home gardeners who wish to garden more sustainably may also use conventional insecticides for spot applications. Prevention of pest establishment is more important than therapeutic treatment of insects in the garden. For a specific list of organic insecticides, refer to the Alabama Extension publication “Insecticides for Organic Commercial and Backyard Vegetable Production” (ANR-1428). For tips about insect monitoring using pheromone traps, refer to the Alabama Extension publication “Pheromone Traps for Monitoring Insect Pests” (ANR-1431).
Most insects encountered in nature are beneficial and have a critical role in the natural food chain. Therefore, a gardener should be able to identify garden insects and determine whether they are harmful or beneficial. Table 4 provides a short list of insect predators that help seek and destroy insect pests and that are commonly seen in the garden. There are also many insect parasitoids that are too small to see individually. Refer to the Alabama Extension publication “Pest and Beneficial Insects” (ANR-2031) and “Beneficial Insects, Spiders, and Mites in the Southeast” (University of Georgia circular 1055). Applying conventional insecticides can destroy the natural balance by eliminating beneficial insects. Switching to one early season application of systemic insecticides on labeled garden vegetables can provide early season insect control and allow establishment of natural enemies. Use selective insecticides later in the season to protect natural enemies. Many vendors now provide natural enemies for use; examples include Arbico Organic and GardenersAlive.com.
Planting vigorous vegetable varieties and using proper crop rotation are two basic IPM tactics. Planting vegetables in a different section of the garden from year to year may help reduce pest infestation by disrupting insect life cycle. Some insect pests overwinter in the garden soil and emerge in the spring and begin searching for food. If the plant they prefer to eat is located several yards away, the insect must move to the source. Many will die along the way or will fall prey to birds and other predators. Also, many vegetables remove nutrients from the soil; by rotating vegetable crops each year within the garden, the soil in a particular section will have the opportunity to rest and regenerate.
A common practice among home gardeners is to plant a single crop in a straight row. This encourages pests because it makes it easy for them to travel from one host plant to another. By intermingling different types of plants and by not planting in straight rows, an insect is forced to search for a new host plant, thus exposing it to predators. This approach also works well with companion planting where insect control benefits occur with good placement of different crops.
If given a choice, some insects will opt to feed on one plant type over another. For example, pickleworms prefer squash to cucumbers, and leaffooted bugs prefer certain varieties of soybean and sorghum over tomatoes. Trap crop arrangement or layout is critical to the success of this IPM method. Trap plants should be planted 2 to 3 weeks ahead of the main crop. Once the trap plants have become infested, the target insect can be picked off and dropped in soapy water or the entire plant can be disposed of. For stink bug management, addition of earlyplanted trap crop (such as okra, beans, or sunflowers) followed by insecticidal or cultural pest management tactic can also be beneficial. In large gardens or farm settings, trap plants can be planted within the main crop in strips to attract and retain insect pests. For more information, watch the trap crop training module with videos and publications on the Alabama Vegetable IPM website.
Barriers and Traps
Barriers and traps can be employed to capture or impede the movement of pests. Gardeners can use pest exclusion fabric — such as SuperLight Insect Barrier and Agribon — for excluding early season pests from crops. A board or thick piece of paper painted bright yellow and coated with a sticky substance, such as Tanglefoot®, will attract and intercept aphids, whiteflies, and other small flying insects. Use of Superlight Insect Barrier wrapped around the base of squash plants or put over the plants to cover them completely can help reduce squash vine borer moths early in the season.
Mulching is the spreading of organic matter in the garden and around plants. It is an effective method to control weeds and serves as a refuge for predatory insects like ground beetles and spiders. Mulch also helps the soil to retain moisture, which promotes plant vigor and reduces abiotic stresses. Mulch should be added to the garden when plants are 4 to 6 inches high. Grass clippings, leaves, hay, sawdust, wood chips, and compost make excellent mulches. Some mulches may slow down insect pest movement or deter them early in the season. One drawback of using mulch may be increased numbers of slugs in the garden that feed on young and succulent plants.
Insect Sampling Techniques
Both pest and beneficial insects can be sampled by home gardeners to assess the level of insect activity before and after an insecticidal spray. Some simple techniques to frequently sample insect populations include the following: • Yellow sticky cards: Many insects and spiders can be intercepted on colored sticky cards as they migrate long distances. Yellow sticky cards can be placed at several locations around a garden to monitor the aphid migration while blue cards can be used to monitor thrips. • Drop cloth/shake cloth: Gardeners can use a drop cloth spread between the rows of vegetables to gently shake off slow-moving insect pests (such as caterpillars) and beneficial arthropods for counting. • Sweepnet sampling: Gardeners can use a sweep net for collecting insects off the plants by vigorously moving the net through crop canopy. A relatively easy way of viewing insect collection is to empty the contents of the net into a gallon-size Ziploc bag. The bag can then be frozen in a refrigerator to preserve specimens for later viewing or identification. • Pitfall traps: Pitfall traps can be placed in the ground and are excellent for estimating the population/activity of soil-dwelling insect pests and predators (like tiger beetles, ants, etc.).
Proper Use of Systemic Insecticides
Systemic insecticides (imidacloprid-containing products) are now available for home gardeners; these insecticides should be used early in the season so that the insecticide is not present at flowering. The product should be applied as a drench soon after transplanting or when the first sucking pests are noticed on leaves. A systemic insecticide also requires a few days to get absorbed and translocated throughout the vascular system of plants. For more information about proper use of the new insecticides, contact a Regional Extension Agent at the county Extension office and read the insecticide label very carefully.
Insect Monitoring Using Pheromone Traps
Moths can be detected using pheromone traps around the garden. Research at Auburn University and elsewhere indicates that detection of the first flight of moths may indicate the need to scout crops for other life stages of the insect pest; for example, if 7 to 10 corn earworm and fall armyworm moths are found in separate pheromone traps each week, then vegetable crops should be examined immediately for larval feeding damage. To a limited extent, it may be possible to “trap out” or intercept moths in an area using multiple traps (e.g., in small greenhouses).
Pheromone traps are available for almost all major insect pests of vegetables including corn earworm (tomato fruitworm), fall armyworm, cutworm, tobacco budworm, and squash vine borer. Pheromone traps can be purchased in bulk from vendors like Great Lakes IPM (Vestaburg, MI) or in the form of kits from Internet sites like Arbico Organics (Tucson, AZ) and Planet Natural (Bozeman, MT). Place pheromone traps away from the main crop because the lure is attractive to target insects over long distances. Most lures attract male moths but some attract both sexes of the species.
General Recommendations for Managing Soil Pests in the Garden
Many soil insect pest populations reach high numbers in grass or turf. Home gardens are often established in areas previously covered with grass. To reduce soil insect problems, thoroughly till or spade the area well in advance of planting (30 days or more) and again just before planting. This will bring soil pests up near the surface where their chances of mortality are increased. A broadcast soil insecticide applied properly 2 weeks before planting will help to control some soil insects like cutworms, wireworms, and white grubs. See the note on applying broadcast soil insecticides at the end of Table 1 (page 14) for instructions on this topic.
Managing Pests That Feed Above Ground
This section provides some non-chemical methods for management of insect pests that feed above ground.
Aphids, Flea Beetles
- Control aphids in early stages when aphids are in low numbers.
- Use of SuperLight Insect Barrier can prevent aphids and the beetles.
- Knock aphids from plants with a strong spray of water from the garden hose.
- Spray plants with insecticidal soap. Test tender plants first to ensure they are not burned by the soap solution.
- Aphids can transmit plant virus disease; remove and destroy diseased plants to delay spread of the virus to healthy plants.
- Conserve and/or release insect predators like lady beetles and lacewings as soon as aphids are detected. For fast control of aphids, release larvae stages in the garden.
- Parasitoids such as Aphelinus abdominalis and Aphidius ervi are effective against potato aphids. Aphidius colemani is effective against green peach aphid.
Cutworms, White Grubs, Wireworms
- For wireworms, plan your crop rotation.
- Most can be excluded by using fine insect mesh or fabric (SuperLight Insect Barrier).
- If you suspect larvae in soil, then keep garden weed free and turn soil to expose larvae to natural enemies.
Cabbage Loopers and Cabbageworms
- Cover plants with a floating row cover to reduce pest establishment and egg laying.
- Handpick and destroy the worms. Green worms are much easier to spot on red-leaf cabbage varieties.
- Spray plants with Bacillus thuringiensis when worms are still small. Repeat spraying at 5- to 7-day intervals until insects are gone.
- Remove alternate host weeds such as wild mustard and shepherd’s purse from the garden.
Colorado Potato Beetles
Potato beetles are infamous for insecticide resistance to synthetic pyrethroids; therefore, use insecticides sparingly and try the following alternative management methods.
- Potato beetles prefer to feed on potatoes, eggplants, and tomatoes. Rotate these preferred hosts with other crops in alternate years.
- Interplant or rotate with non-host crops (e.g., beans) with preferred host crops.
- Handpick and destroy orange egg masses, which occur on the underside of leaves, reddish-brown larvae, and yellow and black striped adults. Two to three generations of the insect occur in Alabama and throughout the South.
- Place straw mulch around plants to hinder movement of the beetles.
Corn Earworms (Tomato Fruitworms)
- Plant and harvest corn as early as possible to avoid infestations.
- Select tight-husked corn varieties for planting; this inhibits worm movement into the ear.
- Beginning at the flower stage, check upper tomato leaves once per week for the small, round, white fruitworm eggs. Also look for the greenish caterpillars on foliage and fruits. Spray plants with Bacillus thuringiensis weekly as long as larvae are detected.
- Use New England Hubbard squash as trap crop for cucumber beetles and squash bugs. Watch the IPM training module on the Alabama Vegetable IPM website for more information about the effectiveness of trap crops.
- Cover young plants with floating row or SuperLight Insect Barrier covers until the vines start to run.
- Eliminate weeds in and around the garden; some weeds are hosts for bacterial wilt disease, which is spread by the cucumber beetles.
- Grasshopper populations are most damaging in late summer; a floating row cover can be used to protect late-season plantings.
- Fall plowing of the garden, including fence rows and garden borders, exposes the grasshopper eggs to the weather and to insect predators.
- Avoid planting the garden near lawns or in previously grassy areas.
- Sunflower and sorghum trap crops also seem to deter Japanese beetles feeding on main crops.
- Protect plants with floating row covers.
Mexican Bean Beetles
- Handpick and destroy the egg masses and beetles in early morning before they become active.
- Interplant nonhost crops like potatoes among bean plants to disrupt egg-laying.
- Soybeans are preferred by bean beetles over snap or lima beans; thus, soybeans planted nearby can be used as a trap crop.
- Mites don’t like moist conditions. Keeping the foliage wet can deter mites, but it can also encourage plant diseases.
- Keep plants adequately watered and stress free. Do not use synthetic pyrethroid insecticides too often to avoid mite outbreaks.
- Do not mow grass located close to the vegetable garden during hot and dry weather.
- Use New England Hubbard squash as trap crop for attracting squash bugs and cucumber beetles. Watch the IPM training module on the Alabama Vegetable IPM website for more information about trap cropping.
- Remove and destroy any clusters of oval, orange-brown squash bug eggs.
- Remove and destroy vines and unused fruit after harvest to eliminate overwintering sites.
- Place shingles or boards near squash or pumpkin plants in spring or early summer to attract squash bugs. Then, collect and destroy the adult bugs that gather under the boards each morning.
Squash Vine Borers
- Pest exclusion is critical in managing vine borers. To delay infestation, cover small plants with a row cover until female flowers appear. They have a bulge between flower and stem that is absent in the male flowers.
- Squash varieties with long vines may continue to grow despite borer damage.
- Cut open borer entry holes in the stem with a knife. Then, remove the worm and pack moist earth around the stem. This is a more labor-intensive method.
- Handpick worms from plants during evening hours.
- Bacillus thuringiensis will be effective if sprayed when the worms are small.
- Dill planted next to tomatoes will serve as a preferred trap crop; handpick worms off the dill.
- Do not destroy hornworms that have small, white cocoons attached to their bodies. These are parasite cocoons from which small beneficial parasitic wasps will emerge.
- Turning the soil in the fall will expose hornworm pupae to weather and predators.
- Whiteflies are rarely a problem in outdoor gardens. Make sure any purchased transplants are not infested with whiteflies (or other insects).
- Use bright yellow sticky cards to monitor aphids and whiteflies.
- Use beneficial insects like Encarsia, which are very effective in enclosed areas.