Regional Gardening Reports :: National Gardening Association

In the Garden:
New England
July, 2003
Regional Report

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A cabbageworm is the larva of a common white butterfly. It feeds on the leaves of cabbage, broccoli, and other cole crops.

Diagnosing Plant Problems

All gardeners face the challenge of identifying the sources of problems with their plants, but the task is most daunting for the beginner. The first step toward diagnosing problems is frequent observation. By getting to know what your plants look like when they are healthy, you'll be quicker to recognize when they are struggling. And the sooner you recognize that a problem exists, the easier it will be to remedy it.

Determine the Culprit
When you see a problem, consider the simplest and most common cause. For example, if you see a wilted plant, check soil moisture first. Although some diseases can cause wilting, a more common cause is lack of water.

Don't spray an insecticide until you've identified the culprit. Not all insects are pests! If you suspect an insect is causing problems, examine the plant for insects. 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 if you need to, so you can research the possible culprits. Or try to collect insects you find so you can use a reference book to identify them. Wait to spray until you've made a positive ID. Many insecticides will kill not only pests but also beneficial insects such as honeybees.

Gardeners sometimes mistake symptoms of nutritional deficiencies -- yellowing leaves, stunting, weak growth, poor production -- as signs of pests. If you see symptoms like these, consider testing your soil to rule out nutritional deficiencies and/or improper pH levels.

Once you've identified that you indeed do have a pest problem, determine whether control is really necessary. Are there just a few spots on the leaves of a plant you'll be harvesting in a week or two? Control measures may not be warranted.

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; copper strips form a barrier to slugs and snails.
2. Repellents deter pests from attacking your plants. For example, neem oil may repel Japanese beetles, and garlic 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, non-toxic way to control pests. For example, slug traps can significantly reduce their numbers.

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.

Synthetic insecticides should be used only as a last resort. Your Cooperative Extension office has the latest recommendations. You may find, however, that you can achieve adequate control using careful cultural controls, without resorting to sprays -- either organic or synthetic.

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