Caterpillars That Are Large & Feed on Grape Vines

Grapevines (Vitis spp.) add a touch of drama to your yard with their bold leaves and twisting branches, and also provide you with colorful, edible fruit. Grapes thrive in sunny sites in U.S. Department of Agriculture plant hardiness zones 2 through 10, depending on the variety. Grapevines attract moth larvae of all sizes, but the sphinx caterpillars, sometimes called hornworms, are the largest you'll encounter. If left unchecked, the pests can make your vines look ragged and unsightly. You can control sphinx caterpillars fairly easily using biological and mechanical control methods.

The larvae of sphinx moths are often called grape sphinx caterpillars because the smooth-bodied, green, brown or yellow-brown pests enjoy feeding on grapevines from early summer through fall. The largest culprits include the larvae of achemon sphinx moths (Eumorpha achemon), whitelined sphinx moths (Hyles lineata) and Pandora sphinx moths (Eumorpha pandorus), all of which reach 3 1/2 to 5 inches long. Some sphinx caterpillar species, including whitelined sphinx larvae, have a dark, hornlike spine protruding from the tops of their hind ends. Other species, such as the achemon and Pandora sphinx moths, lack the "horns," but sport a single distinctive "eyespot" on their rear ends instead.

Adult sphinx moths are fast flyers that hover above flowers like hummingbirds when drinking flower nectar, which is why they are sometimes referred to as hummingbird moths. You'll rarely see these nocturnal insects feeding, but you might spot them collecting around outdoor light fixtures on warm evenings. The moths have green or brown bodies with bright pink or red markings and wingspans up to 4 inches. The moths emerge in late spring through early summer to mate. The females lay light green eggs on the tops and undersides of grapevine leaves, and the small larvae hatch in one to two weeks.

Sphinx caterpillars are typically considered minor pests because they usually appear cyclically, with several years of severe infestations followed by a several-year break. But the pests can strip a grapevine of its leaves whenever they do appear. Young caterpillars chew small circular holes in the grape leaves, but the mature sphinx larvae are voracious eaters that consume about nine grape leaves a day. Large populations quickly strip grapevines, and the pests will then crawl long distances to find and feed on healthy grapevines. In addition, you might pick the pests along with your grapes, which taints your fruit harvest.

You can successfully control small populations of grapevine caterpillars by picking them off the plant and dropping them into a pail of soapy water. Inspect your vines carefully because the large pests have coloring that makes them hard to spot. Colorado State University Extension suggests searching the outside parts of grapevines around dawn or dusk when you can see the feeding pests more easily. Checking the soil beneath the vines for large, dark droppings can indicate where the big beasts are hiding. Check your vines daily during a severe infestation. Once the populations are under control, check your vines about once a week for the rest of the growing season to ensure the caterpillars don't make a comeback.

If you see any large caterpillars with small, white cocoons on their backs, Missouri Botanical Garden recommends leaving them on your plants. Those moth larvae are being parasitized by small wasps (Cotesia congregatus), and those cocoons will soon hatch and release more caterpillar-killing wasps into your garden.

A biological pesticide containing Bacillus thuringiensis can quickly and safely control large caterpillars feeding on your grapevines. Bt is a natural soil bacterium that affects only moth larvae, causing caterpillars to stop eating and die within a few days. Although lethal to sphinx caterpillars, Bt won't harm people, plants, pets or beneficial insects, including parasitic wasps. Bt is even enough to spray on your grapes up to the time of harvest. Carefully read and follow the mixing and application instructions on the manufacturer's label. One product recommends mixing 1 tablespoon of Bt concentrate in 1 gallon of water. Use a small, handheld garden sprayer to thoroughly cover the grapevine, including the tops and undersides of all the leaves. Spray your plants as soon as you first spot the large caterpillars, then repeat applications every five to seven days until the pests disappear. Treat your vines again after a heavy rain.

Although nontoxic, Bt sprays can still irritate your eyes and skin. Reduce your risk of exposure by wearing goggles, a face mask, waterproof gloves, long sleeves, long pants and shoes with socks. Keep the biological pesticide in the treatment area by spraying vines only on calm days when no rain is expected for at least 24 hours.