Caterpillars are the larvae of moths and butterflies, and like their winged metamorphoses, they come in all shapes and sizes. While all species share a common appetite for plant foliage, the good news is that the methods for controlling them are basically the same. Before deciding upon a control method, take the time to identify which caterpillars might eventually become the gorgeous butterflies and lovely nocturnal moths we usually welcome to our gardens. These larvae should be protected, and fortunately, most are colorful and harmless.
Caterpillars inflict damage by eating the foliage and stems of just about any plant in your flower or vegetable garden, as well as your fruit and shade trees. Furthermore, they are voracious eaters, and can defoliate a plant in a short period of time. Telltale symptoms of a caterpillar infestation are holes in leaves and chewed leaf edges, as well as leaves that are rolled up or fastened with silk.
If you are able to catch the infestation early on, the best strategy for controlling caterpillars is to handpick them, then crush them or drop them in a pail of soapy water. Make sure you also remove rolled or folded leaves that shelter the caterpillars. (Make sure you wear gloves before handling saddleback caterpillars, as their sting is severe.) To prevent inadvertently killing desirable butterflies- and moths-to-be, use a good field guide such as Peterson First Guide to Caterpillars and pick only those caterpillars which become drab adults.
Cleanup and disposal of garden debris throughout the growing season, followed by a thorough end-of-season cleanup, will help reduce the size of future generations of caterpillars. Furthermore, many insects--such as fireflies, ground beetles, soldier beetles, stink bugs, and tachinid flies-- prey upon or parasitize caterpillars, and will gladly assist you in your quest to keep caterpillar numbers under control.
Specific recommendations for flowers. An effective but less selective control for caterpillars is to dust or spray all parts of the leaves on which they are feeding (especially the undersides) with BT (Bacillus thuringiensis). Apply at 3-5 day intervals, reapplying after rain, until caterpillars cease to be a problem. For an infestation that is out of control, spray all sides of the leaves with pyrethrum; usually two applications spaced 3-4 days apart will solve the problem.
Specific recommendations for vegetables. There are some species of caterpillars that are particularly troublesome in the vegetable garden. Cabbage loopers, as their name suggests, are very fond of cabbage (including the Chinese variety), and will also feed on lima and snap beans, broccoli, brussel sprouts, cauliflower, lettuce, parsley, radishes, rutabagas and turnips. They damage crops by chewing small to large ragged holes in the foliage, and by boring into developing heads of cabbage family crops. The best control strategy is to spray plants with BT (Bacillus thuringiensis) every 2 weeks until pests are under control, or until heads begin to form in the case of cabbage family members. Other effective controls are handpicking; spraying plants with liquefied and strained cabbage loopers, or hot pepper spray; and planting trap crops such as celery or amaranth. For serious infestations, spray leaves with pyrethrum.
Another caterpillar that shares the cabbage looper's taste in plants is the imported cabbage worm. Symptoms of infestation include ragged holes in the leaves and bits of green excrement; you will also notice white butterflies skipping about from plant to plant. Like cabbage loopers, imported cabbage worms eventually bore into the developing heads of cabbage family plants, turning them into mush in the process. The most effective strategy for controlling this pest is to use an agricultural fleece or net barrier and a preventive BT (Bacillus thuringiensis) spray early in the season, then to use more BT for later infestations. A thorough fall cleanup is essential to prevent recurring infestations, since these pests overwinter in the soil or on garden litter. After harvesting crops, promptly remove all old plants and overgrown weeds, leaving the soil completely bare, then cultivate soil to a depth of 6-8 inches. Make sure old plants are actively composted or destroyed.
European corn borers are most notorious for boring holes into the stalks and ears of the corn plant, but also damage chard, peppers, potatoes and tomatoes. Squash vine borers affect crops such as cucumbers, muskmelons, pumpkins, squash (summer and winter), and watermelons by burrowing into their vines and causing them to wilt. For more information on these pests, see Caterpillars - Borers and Miners: Specific recommendations for vegetables.
Parsleyworms are 2 inches long, and are a brightly colored green with yellow-dotted black bands across each body segment. They give off an odor and project orange horns when they are upset. They will someday become lovely, black swallowtail butterflies, so carefully weigh their aesthetic value against the damage inflicted on your carrots, celery, dill, fennel, parsley and parsnip plants before deciding to destroy them. If you decide the parsleyworms are eating excessively into your harvest, reduce their numbers by handpicking them, or try attracting birds such as northern orioles, barn swallows, bluebirds, chickadees, flycatchers and kinglets into your garden.
The tomato hornworm is a large (3 to 5 inches long), green caterpillar with a horn projecting from its rear; it will eventually become a moth whose diurnal feeding habits and ability to hover have earned it the name of "hummingbird moth." The hornworm chews holes in the leaves of dill, eggplant, pepper, potato and tomato plants, and will sometimes also damage the fruits of the latter three. Check damaged plants for dark colored droppings, and spray them with water; this should cause the hornworms to thrash about and enable you to handpick them more easily. If you notice papery cocoons on their backs, remove but don't destroy the caterpillars: they are already doomed by natural parasites, which should be allowed to reproduce. You can also control hornworms with a Bacillus thuringiensis (BT) spray, or a pyrethrum spray if the infestation gets out of hand.
Specific recommendations for trees. This section will address three pests that primarily affect trees: codling moths, spongy moths, and tent caterpillars. The larvae of codling moths primarily damage apple, apricot and pear crops, both when they are immature and again when they have ripened. Symptoms of infestation include holes in the fruit skin, tunnels through the fruit, and fecal waste in the core; check also for mounds of excrement around the tunnel entrances and for cocoons in the bark crevices. To control this pest, use a preventive spray of Bacillus thuringiensis (BT) early in the season and light horticultural oil later in the season to get the newly hatched larvae before they bore into the fruit. In addition, set out sticky traps to snare the moths during their emergence times (check with orchard growers in your area).
Spongy moths are imported pests that primarily affect the eastern United States, and their populations fluctuate between sheer nuisance to plague proportions. Their larvae damage host trees by devouring their leaves, often leaving them (and sometimes entire forests) completely defoliated. When the larvae are young, they chew only around leaf edges; by the time they are an inch long, large holes begin to appear in leaves. In July, the mature caterpillars encase themselves in brown shells to pupate. To control this voracious pest, particularly when its population is waxing, spray trees with Bacillus thuringiensis (BT) every 10 to 14 days from late April to mid-June. In June, wrap trunks of vulnerable trees with a piece of burlap a foot wide, hung about chest high; tie it at the center with heavy twine, letting the top fold over to form a skirt. When caterpillars descend from the tree each morning, they will hide under the fold. In the late afternoon, put on garden gloves and sweep the caterpillars off into a container of detergent and water. To prevent future spongy moth problems, check for egg clusters on lawn furniture before putting it away in the fall, and stone walls, woodpiles, fences, garages and outbuildings throughout the winter. Destroy egg masses by scraping them into a can of kerosene, gasoline or water, or by burning them. Reduce debris on the ground that can serve as protection for egg masses, larvae and pupae.
Tent caterpillars form large, tent-like, silken nests in the forks of tree branches, and can defoliate their woody hosts if given the chance. Remove the nests and destroy the caterpillars by hand. Spray the tree with Bacillus thuringiensis (BT) as soon as you see the nests begin to reappear, and repeat every 5 to 7 days until the pest is gone.