Most gardeners learn a great deal about their plants' growth needs, but know little about the insects in their gardens. Most of the insects in a garden are not harmful pests. The vast majority of insect species in North America are either beneficial or harmless to humans and garden plants. It is important that the gardener be able to distinguish between beneficial and pest insects. Frequent monitoring of insects in a garden is a key to using IPM or biological control in a garden. You need to know what to look for (and when and where to look) during your garden inspections to discover pests before they develop into problems. Learn as much as you can about the pests that call your garden home. Read books or extension publications, study pictures, watch what the bugs are doing in your garden. Attend extension meetings or organic gardening conferences. Once you have a good feel for what the pests are doing you'll be able to use non-chemical controls more effectively.
Beyond having a good knowledge of beneficial and pest insects, a well-designed garden can make insect management easier. Select vegetable varieties that are well-adapted to your conditions and follow proper maintenance to ensure vigorous healthy plants. Check the plants regularly to detect the beginnings of pest infestation. Remove any severely infested plants or plant parts to prevent the spread of the pests to other plants. Pick off large insects, such as cabbage loopers, tomato hornworms, and squash bugs and destroy them. Rotating plants to different areas can reduce certain pest problems. Various screening products, such as cheese cloth or floating row covers, are useful for keeping insects off plants. Put them over the crop at planting for maximum protection. But be aware that such covers will keep the plants warmer than they would be otherwise, so they will mature more quickly. The weeds will also grow faster under a cover, so mulch the plants well to suppress weed seed germination. If the fabric is secured with a shovelful of soil to prevent it from blowing away (a likely occurrence in most of the Midwest) the fabric tends to deteriorate, and it is inconvenient to get the cover on and off. To make getting under the cover easier, one gardener built a rectangular wooden frame of 2 x 4's, added a couple of pieces of lightweight PVC pipes bent into arches to support the fabric, and stapled the fabric onto the wood frame.
There are a number of traps that use color or odors to lure certain insects to their demise. Aphids and many other insects are attracted to the color yellow; yellow sticky traps are very useful for capturing aphids, whiteflies, spotted cucumber beetles, and other insects. Yellow plastic dishpans filled with soapy water may also be used to attract and kill certain insects, such as aphids. Other types of traps use chemical baits to attract specific insects. These chemicals lure insects to a sticky trap because the pests mistake the smell for food or a potential mate (pheromone-baited traps). These traps can be used in a home garden to determine if a specific insect species is present. For example, a pheromone-baited trap is available for the cabbage looper, which will trap adults flying in a garden in search of cabbage and broccoli. But because pheromone traps lure and capture only males, they won't eliminate the egg laying females that fly into the garden.
Eliminating insecticide applications will have a dramatic effect on the diversity of insects in a garden. Naturally occurring predators and parasites that benefit the gardener will increase in numbers. Lady beetles, green lacewings, spiders, and many other beneficial insects will arrive to attack garden pests and help reduce their numbers. The activities of these natural enemies may be encouraged by planting a variety of plants, including flowers, that provide alternative food sources (pollen and nectar) for the beneficials. Sweet alyssum, yarrow, dill, fennel, and many other cultivated plants are highly attractive to beneficial wasps, beetles, and flies, as are many "weeds", such as Queen Anne's lace and wild mustard.
If naturally occurring predators and parasites are not sufficient to control garden pests, augmenting them with releases of commercially available natural enemies may be effective. Many companies mass-produce predatory and parasitic insects and mites that can be purchased for release in the home garden.
An excellent option for aphid control in a home garden is the use of green lacewings. Green lacewings occur naturally, but often not until aphid populations have begun to build. These predators can be purchased and released early in the growing season for earlier and more effective control. Making releases of green lacewings as eggs is very easy in a home garden. The eggs are shipped in small containers mixed with bran (or another filler, such as vermiculite) that protects the eggs during shipping. All you have to do is sprinkle the contents of the container on the plants! Although these predators prefer to eat aphids, they will also eat caterpillar eggs and other soft bodied insect pests. Lacewing adults, beautiful insects with very delicate wings, add to the insect diversity of a home garden but are not predatory. The use and encouragement of these aphid predators, in combination with yellow pan trapping, and spot treatments with insecticidal soap provide an excellent management program for aphid pests in a garden.
Trichogramma wasps are being mass produced by several commercial firms and are available to home gardeners for control of cabbageworms, cabbage loopers, tomato hornworms and other caterpillars. In a garden, release Trichogramma when caterpillar eggs are found. The timing of release of these parasitic wasps is critical, since they only attack the eggs. The wasps are shipped as immatures inside moth eggs glued to small cards. Just place the cards between the cabbage leaves, or hang them on a lower branch of the tomato or broccoli plant, so they'll be protected from direct sunlight. Try to release the wasps in the early evening, when temperatures and winds are moderate. Eggs parasitized by Trichogramma will turn black and produce a new generation of these beneficial wasps in the garden.
Purchasing lady beetles and praying mantids for release in the home garden is not recommended. The species of lady beetles sold for use in gardens is collected at overwintering sites and stored in a refrigerator until shipped. When released, these adults will fly away from a garden, because even though they have been physically removed from their overwintering site, they still will attempt long distance dispersal flights. Praying mantids are highly cannibalistic and the insect species most commonly found in a home garden will not provide adequate food for this species, so they too will often leave the garden, if they survive attacks from other praying mantids! Several commercially available beneficial nematodes have a wide host range that includes many garden insect pests. These nematodes should not be confused with plant-parasitic nematodes which can be harmful to certain garden vegetables. Nematodes are especially useful for insects living in or on the soil surface, such as cutworms, squash vine borer, armyworms, cabbage root maggots, wireworms and white grubs. These nematodes are easy to handle and apply, and they present no risk to humans or other beneficial species in the garden, including earthworms. Nematodes move through films of water, so they should not be applied to dry soil. Make applications in the evening following a good rain or thorough watering for maximum efficacy.
Another option for biological control in the home garden are commercially available microbial insecticides. The most commonly used microbial is a bacterium, Bacillus thuringiensis, or Bt. New developments with this bacterium have expanded its host range and lengthened its period of activity in a garden. Several Bt products are available under various trade-names which are formulated as liquid concentrates, wettable powders, and ready to use dusts. Because Bt does not persist and multiply in the home garden environment, repeated applications are usually necessary. Certain Bt products are very effective against caterpillar pests of vegetables including cabbageworm, cabbage looper, and diamondback moth. Another type of Bt is active against leaf feeding beetles, including the Colorado potato beetle and the bean leaf beetle. In all cases Bt is more effective against young insects than older ones. Monitor your plants frequently so you'll know when the insects are just hatching from their eggs and are most susceptible, and make applications at that time.
As a last resort, use selective insecticides as spot treatments when other options are not available or when they have failed to provide adequate control. The home gardener has two environmentally friendly spray options for many insect pests, insecticidal soap products and oil sprays. Soaps act selectively on many pests including aphids, squash bug nymphs, leafhoppers, and thrips. These soaps break down quickly and do not cause any long term environmental contamination. Due to the selective action of these products, beneficial insects are not harmed by soap sprays. Oil sprays act to suffocate insect pests, and also have the advantage of rapid breakdown. These compounds are not toxic to humans and will not cause environmental contamination. However, they are not selective in their action, so overuse may have detrimental effects on beneficial insect species.
- John Obrycki, Iowa State University and Susan Mahr, University of Wisconsin
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