Thick, succulent caterpillars may be appealing to hungry birds, but they can quickly ravage your vegetable garden or defoliate your trees. Some larvae efficiently munch on plant-destroying aphids, while others are passing time in your garden pond or rain barrel, only to emerge as annoying, blood-sucking mosquitoes. Larvae are the immature stage of several groups of insects, with many different forms and feeding habits -- some destructive, some helpful. Treatment for unwanted larvae depends on the type of insect and their location.
Under the Lawn
Grubs are the larvae of various beetles that feed on turf roots, leaving dry brown patches. Typically light-colored, grubs are “C”-shaped with three pairs of legs behind a dark, hardened head. To confirm a grub infestation, pull back a small patch of turf at the edge of a damaged area. If you have 6 or more larvae in a 1-square-foot patch, treatment is necessary. Parasitic nematodes are a safe, non-chemical treatment for grubs, available at garden centers in powder and sponge forms. For an area up to 3,200 square feet, soak the nematode sponge in two quarts of water. Squeeze the sponge to extract the nematodes. Pour the mixture into a garden sprayer and add another gallon of water. Spray the lawn while periodically shaking the mixture to avoid settling. Sprinkle the lawn with water after applying the nematodes and keep the lawn moist for the next seven days.
Larvae that feed on leaves of trees and other garden plants are often the caterpillars of moths and butterflies. Mature, healthy plants can tolerate some defoliation by caterpillars, but younger plants, or those with large masses of the pests, need treatment. Where possible, hand-pick caterpillars off plants and drop them into a bucket of soapy water. For caterpillars out of reach or in large groups, spray the pests with the naturally occurring, commercially available soil bacterium, Bacillus thuringiensis var. kurstaki (Btk), which targets the insect’s digestive system. Mix 1 tablespoon of concentrated Btk in 1 gallon of water into a garden sprayer and shake well. For large shade trees, mix 4 teaspoons per gallon. Spray the plant on cloudy days or in late afternoon, since sunlight breaks down the bacteria. Repeat the application weekly as needed. Protect your eyes and clothing and avoid inhaling the mist as you apply the treatment.
Mosquito larvae live in standing water. Eliminate potential mosquito breeding sites in your yard by keeping gutters free of debris, turning children’s wading pools and toys upside down when not in use, frequently changing the water in bird baths and storing other items that accumulate water in sheltered areas. Add a cover to your rain barrel or treat it with the israelensis strain of Bacillus thuringiensis (Bti). Sold in puck-like tablets, Bti kills mosquito larvae in water. One-quarter tablet provides protection for up to 5 square feet of surface water. A full tablet covers up to 100 square feet. Do not use these tablets in or near drinking water. Wash hands thoroughly with soap and water after handling.
Some adult beetle, moth and wasp species lay eggs on or inside injured, diseased or drought-stressed trees. The eggs hatch into wood-boring larvae that tunnel through the wood, destroying the tree from the inside out. Eliminating wood-boring larvae is difficult once they are inside the tree. Keep trees well watered and prune off injured or infested branches to prevent further larval damage. Always disinfect pruning tools with a mix of 1 part bleach and 9 parts water before and after cuts to prevent the spread of disease.
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