Every vegetable garden has resident and visiting populations of insects and bugs. Most are beneficial, such as honeybees, helping the gardener in some way. Indeed, the garden would not survive without them. Some are at least neutral, perhaps doing some minor damage but also performing useful tasks. A few are pests, with no redeeming qualities where the gardener is concerned --- except perhaps to provide prey for beneficial insects.
Know Who Your Friends Are
There are gardeners who see an insect --- any insect--- crawling or buzzing around their garden and they run to get bug spray to zap it. Unfortunately, they don't stop to realize that they might be killing a friend. To them, all insects are enemies. Wrong. Most insects are beneficial, or at least neutral. When you kill one of them, you are actually helping the real pests.
Most insects are so different from one another that it is easy to tell them apart. Get an easy-to-use field guide, such as "Good Bug, Bad Bug" by Jessica Walliser (St. Lynn's Press, 2008). Before you zap something, learn to ID it. You'll be glad you did.
Two of the most destructive garden pests are beetles, simple to recognize because of their flashy coloring. What's worse is that the Colorado potato beetle and the Japanese beetle actually live in your garden from year to year --- if you let them. It's up to you. They emerge when the soil warms up a bit, feed voraciously on your plants, and mate. The female burrows down into the soil, lays her eggs, then dies. The eggs hatch, the grubs eat the roots of your plants, then develop into adults. They emerge, mate, and the cycle begins again.
Japanese beetles larvae --- the grubs that live in the soil --- are killed by the thousands by a bacterial disease called milky spore. If you buy milky spore powder and spread it, strictly following the directions, you can get rid of Japanese beetles. Dead grubs release billions more spores, so it keeps working for years. It is really that easy, and best of all milky spore bacterium does not harm beneficial insects, birds, bees, pets or man, and it survives the coldest weather.
Colorado potato beetles are controlled by another safe insecticide, generically known as Bt, which stands for Bacillus thuringiensis. They have to eat it, which isn't hard to accomplish --- just spray it on your plants and watch them eat a fatal dose. They stop eating within minutes, and die shortly afterward. You must get the right kind of Bt for Colorado potato beetles, which is either Bt. ssp. san diego or Bt. ssp. tenebrionis. Other strains don't work.
Some insects don't do much damage by eating your plants, but they spread deadly plant diseases. Good examples of these bad critters are striped and spotted cucumber beetles. Small yellow beetles with black markings, they feed by piercing the stem and sucking the sap of your plants. They attack most of the squash family --- cucumbers, melons, summer and winter squash, etc. Because they feed on healthy and sick plants, they spread bacterial wilt and cucumber mosaic virus, both of which can rapidly kill your plants. Control them by hiding your plants under floating row cover fabric, or, if it is too late for that, by spraying with an insecticide containing pyrethrin. They overwinter in garden trash, so cleaning up your garden at the end of the season denies them winter quarters.
Caterpillars can do a huge amount of damage in a short time. If you have the time, patience, and aren't too squeamish, handpick them off your plants, drop them in a bucket of soapy water, then flush them down the toilet. Alternatively, spray your plants thoroughly---the tops and undersides of all leaves --- with a solution containing Bt. var. kurstaki, sold under the trade name Thuricide.