White grubs are among the most destructive soil insect pests in New England. Grubs can cause serious damage to home lawns, golf courses, or athletic fields by feeding on the roots and root hairs of grass plants. Often the turf is weakened so much that the sod can be rolled up like a carpet. Sometimes raccoons or skunks will tear up clumps of turf to feed on the large soft-bodied grubs. This tearing of the lawn is often more damaging than the feeding of the grubs.
DESCRIPTION: Grubs are the larval (immature) stage of several species of scarab beetle, such as the Japanese beetle, European Chafer, Asiatic Garden Beetle, Oriental beetle, or the June beetle. Grubs are cream-colored or white, with a hard brown head capsule and three pairs of legs. They usually curl into a distinctive C-shape. When feeding actively, grubs will be found anywhere from the thatch to about four inches into the soil. They range from one-eighth of an inch to one inch long.
LIFE CYCLE: Most species of white grubs complete one generation per year. Adult beetles lay eggs in the soil from late June to early August, depending on the species and the location. The eggs hatch, and tiny (one-eighth inch) grubs begin feeding on root hairs. These larvae molt after about three weeks. Second stage larvae continue feeding for about three weeks and molt once more to the largest grub stage. Most individuals reach this stage by late September and feed into October. As cool weather approaches, the grubs migrate downward through the soil, remaining below the frost line. Grubs hibernate for the cold winter months, reducing their metabolism drastically. As the soil temperature rises again in the spring, they move upward again until they reach the root zone and resume feeding. Grubs feed for four to six weeks in the spring. Then they go into a “resting” (pupa) stage about two inches deep in the soil. About a week later, the adult beetle emerges, complete with wings, reproductive system, and new mouthparts. About ten months of the year are spent in the grub stage and these grubs can weaken or damage turf, particularly in the late April and May and again in September and October.
THRESHOLDS: Most properly maintained home lawns should be able to tolerate at least eight and perhaps as many as twenty Japanese beetle-type grubs per square foot. June beetle grubs, which are much larger, can be more damaging. NOTE, however that skunks and raccoons seem to be able to detect grubs at lower densities, so many people consider it necessary to control white grub populations when they reach eight to ten grubs per square foot.
BIOLOGICAL CONTROL: There is a bacterial disease (Bacillus popilliae, “milky disease”) which attacks Japanese beetle grubs only. However, in some areas of the Northeast, milky disease has not worked as well as one might hope. This is apparently partly because the soil temperatures in the New England summer do not remain warm enough long enough for the organism to be established, partly because the acidic soils may interfere with the organisms, and partly because some of the material applied to turf did not have adequate concentrations of the bacteria to take hold. While milky disease has many advantages, it is not a viable option for use in New England. Entomopathic nematodes (small microscopic worm-like animals which cause disease in insects) have shown some promise in controlling white grubs. However, the nematode which is most readily available commercially, Steinernema carpocapsae (sold as Vector&trade, Orthoganic™, and other trade names), has not been particularly effective against white grubs in field conditions. The nematode which works consistently on white grubs, Steinernema glaseri, is not readily available commercially. In any case, nematodes which are used to control insects are very sensitive to desiccation and must be used carefully. As a general rule, applications should be watered in as soon as possible, and should not be made during the hottest part of the day (for example, from ten in the morning until two in the afternoon.) There is a parasite which occurs naturally in parts of the Connecticut River valley. This parasite, a tiny fly called the “winsome fly,” lays eggs on the thorax (shoulder) of Japanese beetle adults in early July. The eggs hatch into tiny larvae which burrow into the body of the beetle and kill it within five days. Efforts are being made to introduce some of these parasites to the eastern part of Massachusetts. In the meantime, any Japanese beetle which has some small white specks on the shiny green shoulder region should NOT be killed. The parasite will kill it within five days, and then will be able to survive in the old shell until the next season.
CHEMICAL CONTROL: Grub populations can be reduced with properly applied insecticide. The timing of a grub application is very important, and depends in part on the material being used. White grubs are most susceptible to chemical control when they are very small (normally during August for most white grub species in New England), but they also are somewhat vulnerable in the spring when they have just returned to the thatch to feed (April and early May). Several insecticides are labeled for use against white grubs, either by commercial applicators or by homeowners. Some of these materials work rapidly while others take longer to affect grubs but last a little longer. As a general rule, the insecticides which work rapidly should be used later in the treatment period (for example, early August or mid April). Exact treatment dates cannot be predicted because they vary considerably from one year to another. Two insecticides which are currently available for white grub control work quite quickly and can be used later in the treatment periods, after small grubs have become apparent. These are trichlorfon (Proxol™, Dylox™) and isazofos (Triumph™). Both materials are relatively mobile and will penetrate the thatch more readily than most other insecticides. They will also be more likely to move, either through run-off or leaching, than less soluble materials, and so should be used very carefully near surface water. Also Triumph™ cannot be used on golf course fairways or applied by homeowners.