The best defense against garden damage from insects and disease is a long-term program of soil building. Healthy soil will produce healthy, resistant plants.
When insects (like the Japanese beetle pictured above) and diseases do strike, it doesn't foretell the end of your garden, it's just a message that something isn't in balance. The next step is to get reliable information -- a knowledge of just exactly what is causing the problem -- so you can shape a plan of action appropriate to the situation.
Pest damage may appear before you see the culprits themselves. Look under the leaves or go out at night with a flashlight to catch nocturnal varmints, such as slugs, at work.
Those white butterflies hovering over your vegetable garden may well be there to lay eggs on broccoli and cabbage that will produce well-camouflaged cabbageworms.
When insect damage does appear, it probably can be attributed to one or two types of pests. Many chemical and some organic pesticides kill a broad range of insects, including beneficial ones. If you can identify the insect, you might be able to identify a selective control. Here are some potential pest controls to consider:
Hand picking. Knock the villains off into a can of soapy water. This can make a big difference, particularly if you act before they have a chance to multiply.
Bacillus thuringiensis (Bt). This is a powder or spray that kills moths, butterfly, and sawflies in their caterpillar stage. Tent caterpillars and cabbageworms are among those it kills.
Milky spore disease. Incorporate this product into lawn soil to kill Japanese beetles while they are in the sod-eating grub stage.
Insecticidal soap. These sprays are effective on a variety of insects, especially aphids and other soft-bodied pests. The fatty acids in the soaps attack break down the insect's cell membranes.
Insect traps. These use attractive colors like yellow and red, as well as a scented lure, to attract insects onto a sticky pad. They are often used in orchards. A sticky material is sometimes spread around trees to trap insects on the move. Be careful with placement, though: Japanese beetle traps may attract beetles from the entire neighborhood into your yard.
Organic pesticides. These pesticides are made from plant materials. While potent, they break down into harmless materials faster than conventional pesticides. Pyrethrum, made from the flowers of a chrysanthemum, is an example. They are sometimes sold in combination. Follow label directions carefully. Just because a spray comes from plants doesn't mean it isn't dangerous.
Predators. Birds, insects, toads, and bats all thrive on the bugs that damage your garden. Ladybugs, lacewings, and certain wasps are among the beneficial insects. Garden supply houses sell such insects, as well as martin houses to attract birds and bat houses to encourage visits by bats.
Conventional pesticides. The government controls pesticide labeling and home garden use. Labels are very specific about what insects they control and their known hazards and degree of risk. Pesticides that were common only a few years ago, such as Dursban and diazinon, are being removed from nursery shelves. If you do use conventional pesticides, follow directions to the letter.
There are many plant diseases, but rarely do they get so severe in the home garden that gardeners do anything about them. The most common are fungi that turn the bottom leaves of tomato plants blotchy and spotted, then yellow. These can be treated with a spray. Another common tomato problem is blossom end rot, which causes the end of the tomato to turn black. Uneven watering, not disease, is the culprit here. Home gardeners can control both pests and diseases through good gardening practices. Rotate crops -- particularly potatoes and tomatoes -- so that problems that reside in the soil don't come back year after year. Clean out the garden and till the soil in fall so insects don't winter over in plant materials. Dispose of infected plants away from the garden, and burn them if you can.
Try to stay out of the garden when it is wet, since you can spread diseases from plant to plant via moisture.
Some gardeners use a strategy of starting with the least drastic measure -- knocking bugs into soapy water, for example -- and use other methods only as necessary.