Safely Managing Insects in the Vegetable Garden - June 8, 2011
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Experienced vegetable gardeners are very aware of the various insects that feed on their crops and how to manage them. Many of these gardeners have learned to manage garden insect pests without using conventional pesticides. After all, growing healthier and fresher food is the primary reason most people garden. With the expanding interest in growing vegetables in home gardens, many gardeners lack pest management experience and may appreciate vegetable pest management basics.

First and foremost, you must correctly identify the pest. This begins by closely observing the damage and looking for the cause. Many times, insect pests are nocturnal or blend in with their surroundings making this more challenging. Other times, it is so obvious that insects are present, but determining the extent of the damage (if any) can be difficult. Observe the plants to see if damage is present and visit the garden at many times of day and night to watch for activity.

Once you have zeroed in on a pest causing observable damage, ask yourself the following questions. Can the pest be manual controlled (trapping, handpicking, squashing, shop vac, etc.)? Would physical barriers such as floating row cover or cardboard collars at the plant base protect the crop? Have you applied excessive quantities of readily available nitrogen that might unduly attract plant feeding insects? Do you know of any “least-toxic” pesticides that may help reduce harmful insect populations while conserving beneficial or predatory insects?

This may seem like an uphill battle, but your knowledge and experience will improve over time. Also, maintaining a garden journal listing crops, pests, and treatments applied will remind you of what to expect and how successful a given strategy may have been in the past. What follows is a shortlist of common garden pests and some cultural, physical, biological, and least-toxic pesticide solutions to employ on them.

Aphids tend to be present in the early season, but as the weather warms, their predators (ladybird beetles, syrphid flies, and lacewings) become more numerous and aphid populations naturally decline. In the meantime, their numbers can be reduced by wash them off with a high pressure hose or directly applying insecticidal soap.

Caterpillars are larval stages of moths and butterflies and have chewing mouthparts that create visibly chewed leaves, stems, and fruit. A few tomato hornworms are usually found each year in the garden. When small, they are difficult to locate, but as they grow, they can often be located by looking for damaged foliage and excrement below their feeding areas. Hand picking and destroying is the best control. Cutworms are larvae of several species of night-flying moths in the family Noctuidae. They feed at night and are often present in large numbers. Cardboard collars buried one inch below ground and three inches above create a physical barrier and will protect newly planted or recently germinated seedlings. Cabbage loopers eat leaves of cruciferous crops (broccoli, cabbage, turnips, radish, etc.) and can be managed by applying a biological insecticide containing Bacillus thuringiensis (called Bt). Because Bt kills caterpillars but does not kill other insects, it allows natural enemies to survive and contribute to pest suppression.

Flea beetles are small insects that feed on tomato, potato, and eggplant leaving tiny holes in the leaves. You may not observe them directly, but the damage to young plants can slow their growth and subsequent production. A relatively new product made from kaolinitic clay called Surround is being used by several local growers to manage a host of foliar feeders. It forms a physical barrier which decreases damage by repelling these insects. Some growers also use rotenone mixed with insecticidal soap when populations are high.

Grasshoppers are often problematic for Yavapai County vegetable gardeners. When they arrive in large numbers, it can be difficult to manage them without conventional pesticides. Some strategies include covering or wrapping crops with floating row cover (light, spun fabric designed for horticultural use) and applying a bait which contains Nosema locustae: a protozoan microbe that causes disease in grasshoppers. Under the best conditions, these products can provide 30-40% mortality of grasshopper populations. However, when grasshoppers are large and temperatures are high, it is not as effective. Poultry can also provide effective grasshopper control, but they will also eat the plants so access should be limited by fencing or individual plant cages.

Naming of companies or products is neither meant to imply endorsement by the author nor criticism of similar companies or products not mentioned.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: May 31, 2011
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