How to Stop Caterpillar Damage in Your Garden

A first step in managing a caterpillar infestation in your garden can be identifying the moths also hanging around the crime scene.

By: Debbie and Mark Wolfe

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Napa Cabbage With Insect Damage

When you learn to identify these pests in their adult stage, it makes controlling caterpillar damage a bit easier. Organic treatments for caterpillars, such as Bt, are most helpful when the target species is actively present. By watching out for the breeding-stage adult you won’t waste time or money spraying ineffectively at the wrong time. Another strategy to deal with many caterpillars is to grow plants with small clusters of flowers that are attractive to the predatory wasps whose larva will destroy caterpillars.

Cabbage Looper Caterpillar (Trichoplusia ni)

The cabbage looper caterpillar is not only a pest of the cabbage family. This garden generalist may also be found on tomatoes, spinach, lettuce or other garden crops. It moves like an inchworm, walking up with its rear legs until its midsection forms a "loop" before raising its front end and reaching to move forward. Often, cabbage loopers feed on the undersides of leaves, or bore into heads or crowns.

Cabbage Looper Moth (Trichoplusia ni)

Cabbage looper is one of the top pests of the brassica family. Watch for the brown moth at dusk, or even at night, from spring through fall. Keep the garden free of cabbage plant debris to reduce the cabbage looper caterpillars’ abundance of hiding places.

Cabbage White Caterpillars (Pieris rapae)

The caterpillar of the cabbage white butterfly, also known as the imported cabbage worm, is a specialist of the brassica family. It especially loves cabbage, broccoli and cauliflower foliage. Look for chewed leaf margins with dark colored "frass" (excrement) deposits nearby.

Cabbage White Butterfly (Pieris rapae)

The cabbage white butterfly is near the top of the list of garden pests. It can produce several generations of offspring per year, as long as the cabbage family is growing. When you see these black dotted white butterflies, it’s time to treat.

Diamondback Moth Caterpillar (Plutella xylostella)

Relatively tiny diamondback moth larva stick to the undersides of leaves where they chew away the bottom layer of tissue. What’s left is a translucent leaf skeleton that soon turns dry and brown.

Diamondback Moth (Plutella xylostella)

Found around the globe, diamondback moth is highly dispersive, appearing during the growing season even in regions where it cannot overwinter. Because the adult is the overwintering stage, damage begins relatively early in the season where it tolerates the cold.

Tomato Hornworm Caterpillar (Manduca quinquemaculata)

If your tomato plant goes from lush to denuded overnight, it is likely the work of the voracious tomato hornworm. Although they prefer tomatoes, any crops of the solanaceous family are fair game, including potatoes, eggplants and peppers.

Five-spotted Hawkmoth (Manduca quinquemaculata)

The five-spotted hawkmoth is the parent of the tomato hornworm. It lays its white or greenish eggs on the undersides of leaves. Two to eight days later the eggs hatch and the larvae begin to feed. Watching for the brown and grey moth is key to prevention. By the time you notice damage, you may have lost a plant.

Squash Vine Borer (Melittia cucurbitae)

Squash vine borers are almost never seen outside their plant tunnels. They hatch on squash stems near ground level and immediately eat their way into the stems of the plants. By the time the mushy stems and frass deposits are detected, the damage has been done. Monitor exposed vining crops regularly for the earliest sign of infestation: frass deposits at the base of stems. When detected early, a wire may be inserted into entry points to destroy the larvae inside the stem, before it kills the plant.

Squash Vine Borer Moth (Melittia cucurbitae)

The moth parent of the squash vine borer is easily mistaken for a wasp because of its size, movements and coloration. Upon closer observation, these small clear-winged moths have thick bodies unlike any wasp. Minimize their contact with your vining crops by using row covers until flowering. Once the vines begin to run, bury the stems wherever they touch the ground to promote rooting and hedge against damage.

Tent Caterpillars (Malacosoma americanum)

Unlike the others on this list, tent caterpillars attack woody plants. They spin their webs in twiggy crotches of nearly any deciduous tree or shrub. The web is a shelter at night and in foul weather. Remove these "tents" by pruning them out of the tree when the caterpillars are at home. Destroy them by burning or hot composting.

Eastern Tent Caterpillar Moth (Malacosoma americanum)

The eastern tent caterpillar moth has a single brood per year. Eggs are laid on branches in fall where they overwinter. In spring, the eggs hatch and the caterpillars congregate to spin their tent which shelters them for up to two months. They then pupate for a week and a half before emerging as breeding adults. Destroy egg masses in fall or winter.

Black Swallowtail Caterpillar (Papilio polyxenes)

The caterpillar of the black swallowtail butterfly has a voracious appetite for carrots, parsley, dill and fennel. Butterfly lovers may want to plant extras of these crops to make up for losses. Otherwise, the caterpillar is susceptible to the same treatments as the other caterpillars on this list.

Black Swallowtail Butterfly (Papilio polyxenes)

Black swallowtail butterflies may do little or significant damage to garden crops, depending on the number of caterpillars and their stage of maturity. If you wish to prevent damage without killing the caterpillars, simply move them to a weedy patch that includes Queen Anne’s lace, a primary black swallowtail host plant in the wild.

Head of Green Cabbage

By looking out for the breeding adult moth form of garden caterpillar pests, you will have the best indication of when to use row covers or spray to prevent damage. Knowing your enemy will help you defeat him.