Many grub worms are beneficial. Only about 10% of the grubs in the soil are plant root eaters. The rest are feeding on dead and decaying organic matter and aerating the soil as they travel. The grubs that are as large as your thumb are the beneficial and interesting rhinoceros beetles.
COMMON NAMES: Grub, Grubworm, June Beetle, June Bug, May Beetle, May Bug, White Grub
SCIENTIFIC NAME: Order Coleoptera, family Scarabaeidae, Phyllophaga crinita
IDENTIFICATION: White grubs are the larval stage of May or June beetles. Larvae are characteristically C-shaped with a white body and tan to brown head. The last abdominal segment is clear, allowing dark digested material to be seen. Larvae vary in size with age and species. Adults are medium to dark brown.
BIOLOGY AND LIFECYCLE: Females lay up to forty eggs, which hatch 2 to 5 inches inches deep in the soil in three to four weeks. Three instars. They feed the first summer on decaying vegetation, hibernate through the winter, and feed on organic matter and plant roots the second summer. Adults emerge in the spring. The process can last from one to three years. The adult beetles show up in late March. There are over 100 species of June bugs in Texas, but this one is responsible for almost all the damage to lawns. In Texas this four-year cycle is cut to two years and even down to one year in the southern part of the state because of warmer soil conditions. Green June bug larvae are primarily organic-matter eaters and actually beneficial. Complete metamorphosis.
HABITAT: Adults fly around and land on door screens. Larvae (grubs) live in the soil. Pest grubs are found primarily in turf. Those found in planting beds are usually feeding only on decaying organic matter and are not troublesome. Beneficials and pest grubs are hard to tell apart when found in the soil, but if you lay the grubs on a smooth surface, the larva of the green June beetle will turn over on its back. With its feet in the air, it scoots away at a surprisingly fast pace.
FEEDING HABITS: Larvae feed on plant roots or decaying organic matter. Feeding decreases as soil temperatures decrease in the fall when the grubs migrate deeper into the soil. Adult beetles chew leaves at night but are not highly destructive.
ECONOMIC IMPORTANCE: Can cause reduced plant production and even plant loss. Damaging to lawns. Grubs are rarely a problem for organically maintained gardens with healthy, biologically alive soil.
NATURAL CONTROL: Grow nectar and pollen plants to attract native predators and parasites. Beneficial nematodes, cats, skunks, opossums, armadillos, raccoons, foxes, coyotes, and other insectivorous animals.
ORGANIC CONTROL: Beneficial nematodes, compost, molasses, and light traps for adults. Heterohabditis nematodes seem to be the most effective nematodes for grubworms.
INSIGHT: According to entomologists, only one in 100 grubs is destructive to plant roots