Insects should be controlled only when they appear in numbers large enough to be destructive. There are five simple strategies for managing insects in a vegetable garden, effective in either traditional or organic systems.
Be a sanitary gardener. Insects will often overwinter in old garden refuse, so it pays to clean up dead plant material at the end of the year. If you want to keep the organic matter in the soil, till or plow the garden after the last of the crops are harvested.
Maintain good plant health. Healthy plants can often partially defend themselves from pests and will recover from damage quicker.
Physical Control Methods
Use physical barriers and cultural controls. You can keep some types of insects at bay by preventing access to the plants. For example, collars around the lower stems of small plants will foil cutworms.
Biocontrol and IPM
Encourage or release beneficial insects. Encourage the presence of predator insects by allowing the presence of a low level of pest (prey) insects and by avoiding the use of broad-spectrum (kills all insects) insecticides in the garden.
Judiciously use pesticides as a last line of defense. For many insects, both organic and synthetic insecticide options are available. If insecticides are used, direct the application to the specific problem rather than broadcast an application across the garden. Also, use products that target the specific insect you are trying to control. Make an effort to identify and use the most ecologically friendly products available. All insecticides are not equal for either efficacy or impact.
Insects that commonly infest Idaho gardens at damaging levels
- Aphids: These stem and leaf feeders can often be controlled without the use of insecticides. Insecticidal soaps or a hard stream of water that simply knocks them off the plant will usually be sufficient to eliminate damage. Also, aphids will almost always be eliminated by predators if you are patient enough to let them do their work.
- Grubs and Wireworms: These soil-dwellers are difficult to control and may require the use of a soil-applied insecticide prior to planting. This means knowing the history of the garden plot and realizing the problem exists before you see damage.
- Cutworms: This pest often kills seedlings and transplants by chewing through the stems at ground level. Cabbage, broccoli, tomatoes and other soft-stemmed transplants are often the victims. Placing cardboard collars around the base of plants is an effective method to prevent damage.
- Corn Earworm: Control of this pest usually requires the use of an organic or synthetic insecticide product, applied to the green silks once or twice.
- Slugs: In dry climates, slugs and snails are typically not a problem unless too much water is being applied to the garden. If the problem cannot be solved by reducing irrigation, the use of baits and traps can be partially effective.
- Cabbage Worms: These slender green caterpillars chew holes and deposit webs on broccoli, cabbage, cauliflower, and kale. When obvious damage is present, they may require control with bacillus (the organic pesticide sold as Bt) or another insecticide.
- Tomato Hornworms: These large, ferocious-looking caterpillars can defoliate tomato or potato plants within a day or two. Physical control (pick them off the plants and step on them or squeeze them between two rocks) is the best method for eliminating this occasional pest
- Colorado Potato Beetles: These serious pests of potatoes, when present is small numbers, can be removed from the plants by hand. In large numbers they may require an insecticide.
To learn more about controlling vegetable insects, see the following resources:
From Cornell University: Managing Home Garden Vegetable Pests
From Oklahoma State University (very detailed): Home Vegetable Garden Insect Pest Control
From Organic Garden Pests.com (for tips on organic control of insects): Organic Pest Control