It's hard to know if your browning lawn or drooping flowers are suffering from disease, lack of water, poor nutrition or something else entirely. The symptoms of root-eating insects, including loss of plant vigor, stunted growth, wilting and discoloration, often resemble plant diseases or deficiencies. If you suspect root-feeding insects are damaging your plant, dig the soil around the plant and sift through it for insects. You may have to dig up the entire plant to fully inspect its roots and properly identify the pest.
Immature beetles, called grubs, are lightly colored with dark, hard heads. Most have six legs, grow up to 1 inch long and typically lie in a "C" shape. Japanese and June beetle grubs, along with the legless grubs of long-nosed beetles, called weevils, are common root-feeding pests of vegetables, turfgrasses and ornamental plants. The presence of adult beetles and evidence of digging by raccoons and skunks are indicators of root-eating grubs. Pick adult beetles off plants and drop them in a bucket of soapy water. Treat larvae underground with the parasitic nematode Heterorhabditis bacteriophora, available from garden suppliers in granule or other forms. Mix 1 teaspoon of the nematodes, which come packaged in inert powder, in 1 gallon of water. Spray during overcast conditions onto moist soil or turf. Keep the area moist for the next seven days.
Root-feeding fly larvae, or maggots, are pale, legless, tube-shaped creatures measuring approximately 1/4-inch long. Common pests of root vegetables and other plants, root maggots cause wilting, yellowing and stunted growth in host plants. Young plants are most vulnerable to root maggots in cool, wet springs. No effective treatment exists for these pests once they are established in your plants. Cultivate the soil well in the fall to remove all plant debris, including roots, where root maggot pupae hide for the winter.
With their clear wings and striped markings, adult Sesiidae, or clearwing moths look similar to wasps and bees. Caterpillars of these moths are frequent feeders on the roots of trees, shrubs and vines. Keep roots healthy with loosely packed soil for oxygen and proper irrigation for the particular plant. When cultural methods fail and plants show serious signs of distress, apply parasitic nematodes to kill pest caterpillars.
Cicadas and Aphids
Large, noisy cicadas lay eggs in twigs, which then die and fall to the ground. The eggs hatch, and nymphs burrow into the soil where they feed on roots for several years. The feeding inflicts only minor damage and is rarely a cause for concern. Aphids are tiny, pear-shaped insects that suck the sap out of plants. While most species favor leaves and stems, some aphids attack fruit tree and vegetable roots. Aphid treatments usually work on contact with the pest. No current treatments are available for underground aphids, but these pests rarely cause significant damage. Rotate vegetable crops to avoid increasing populations of root aphids.