Gardening Columns

Insect Pest Primer

First published on garden.org on July 3, 2008, by Suzanne DeJohn

Insect Control Rule Number 1: Never spray an insecticide until you've identified the culprit! Not all insects are pests -- many are beneficial, most are benign. Don't assume that any insect crawling around on a plant is there to cause trouble. Case in point: Ants on peony buds are harmless.

If you suspect an insect is causing problems, examine the plant. Check the leaves, top and bottom, looking for insects, caterpillars, and egg masses. As you touch the leaves, watch for scurrying or flying insects. Jot down notes, take a photo, or collect a sample so you can research the possible culprits. Wait to spray until you've made a positive ID. Many insecticides will kill not only pests but also beneficial insects, including predatory insects that eat the pests and pollinators like honeybees.

If a plant is struggling, consider non-pest causes first. For example, if you see a wilted plant, check soil moisture. Gardeners sometimes mistake symptoms of nutritional deficiencies -- yellowing leaves, stunting, weak growth, poor production -- as indications of pests. If you see symptoms like these, consider testing your soil nutrients and pH levels.

Types of Insect Pests
Entomologists (insect specialists) often categorize insects by how they feed.

Chewing insects eat leaves. Symptoms include holes, ragged edges, and "skeletonizing" -- eating the tissue between leaf veins. Examples include weevils, caterpillars, flea beetles, and Japanese beetles. Look for the telltale frass (excrement) of the larger of these pests.

Sucking insects pierce a hole in plant tissue and suck out the fluids. Signs include stippling on foliage or silvery bronze leaves and discolored blooms. Examples include spider mites, aphids, thrips, and leafhoppers. These pests often leave behind moltings -- the outer skin they shed as they grow.

Other insects, such as wireworms, feed on roots. Cutworms feed at ground level, girdling young seedlings.

Monitoring
Once you've identified that you indeed do have a pest problem, determine whether control is really necessary. Is the damage located on the leaves of a plant you'll be harvesting in a week or two? Control measures may not be warranted. However, many pests multiply quickly so keep a close eye on pest populations.

Once you've identified the pest, research its life cycle and habits. Some pests, such as leaf miners that tunnel into plant tissue, will not be affected by sprays. Controls will be more effective if you catch the insect in the most vulnerable part of its life cycle.

Simple Controls
If you feel control is necessary, use the least invasive control first. Insecticides should be used only as a last resort. Here are some other options:

1. Use barriers to exclude pests. For example, row covers exclude cabbage loopers and flea beetles.
2. Repellents deter pests from attacking your plants. For example, neem oil may repel Japanese beetles, and garlic- and clove-based sprays have shown promise in deterring pests.
3. Hand-picking can keep certain insects populations in check. Learn to recognize pests' eggs and larvae and destroy them. You can use a spray of water from the hose to dislodge pests, such as aphids, from sturdy plants.
4. Trapping can be an effective, nontoxic way to control pests. For example, yellow sticky traps lure whiteflies away from your plants.

Insecticides
If you've tried barriers and deterrents and still have problems, you may feel the need to use a pesticide. Here are some options. Be sure to apply materials evenly and thoroughly, according to label directions.

Insecticidal soap can control some soft-bodied insects such as aphids. It can also affect beneficial insects, however, so use with care.

Horticultural oil also can be used to smother some insects, and it's especially effective on scale and mites.

Biological controls generally target one or a group of pests. Bacillus thuringiensis, or Bt, is a bacterial control for certain caterpillars, such as cabbageworms, cabbage loopers, corn earworm, and tomato hornworms. Bt doesn't affect insects from other families and is considered safe for use around pets and people. However, Bt will kill butterfly larvae as well as pest larvae, so use only when necessary.

Botanical insecticides, such as pyrethrum and neem, are substances derived from plants. These insecticides generally break down quickly when exposed to air and light, so they are effective for a limited time after application. This is important because it means that they don't persist in the environment as some synthetic pesticides do. But most botanicals are broad-spectrum, meaning they will harm both pest and non-pest insects.

You may find, however, that you can achieve adequate control using careful cultural controls, without resorting to sprays, even organic ones.