Overview
Grubs are the larvae of beetles. They are between 3/4" and 1 1/2" in length, white to grayish with brown heads, and commonly curl into a "C" shape when disturbed. They feed underground on roots (particularly those of turf grasses) for one year or more, depending on the species. Damage caused by grubs begins in early fall when the larvae begin to hatch from eggs laid in July and August. The larvae feed on roots until the ground freezes in late fall, then burrow more deeply underground to stay warm. In spring, they return to the surface and resume feeding on roots. Damaged sections of lawn appear burned and can be lifted with ease; upon doing so the culprits will be exposed. From your lawn, grubs may move into your vegetable garden and proceed to eat the roots and bases of plants, especially in early summer. They can cripple or ruin plants without being detected. Vulnerable crops include young apple trees, blackberries, corn, onions, potatoes, and strawberries. Other symptoms of infestation include the presence of moles, shrews, chipmunks, skunks, raccoons, and birds, all of whom relish grubs and will dig up your lawn or garden in search of a good snack. Large numbers of adult beetles being attracted to lights at night may also signal a problem.
The use of parasitic nematodes is a promising method of controlling grubs. The nematodes inject killing bacteria into grubs but leave earthworms and other beneficial soil life unharmed. Up to four applications may be required. Milky spore disease or Bacillus popilliae is an effective long-term control. While it is expensive and takes 3 to 5 years to be completely effective, an application can last 10 years or more when properly applied.
Lawn Grubs
The two most common grubs in St. Louis are the Southern masked chaffer, Cyclocephala immaculata, (also known as the "annual" white grub) and May or June beetles, Phyllophaga spp. Over 90% of the damage is caused by the annual grub. Japanese beetle larvae may also be lawn grubs. The adult beetles of the annual white grub do not feed on foliage. Adult May and June beetles feed primarily on tree leaves, but the damage is insignificant. White grubs are a favorite food of moles, shrews, chipmunks, skunks, raccoons, and birds, which may tear up the lawn in search of the grubs.
Symptoms & Diagnosis
Since the grubs live and eat underground, they are frequently not noticed. The first indication of damage is generally a gradual thinning and weakening of the lawn followed by small patches of dead or wilting grass even in the presence of adequate soil moisture. Bluegrass is especially susceptible. Damage is often most evident in the middle of August through September. Animals digging up the lawn for the grubs or large numbers of adult beetles being attracted to lights at night may also signal a problem. Diagnosis requires peeling back areas of the lawn with a sharp shovel and looking for the grubs. If fewer than ten grubs are present per square foot, it is unlikely that control is required as the amount of damage caused is minimal. Some grubs can be found in the healthiest of lawns.
Integrated Pest Management Strategies
1. Evaluate damage. Peel back patches of grass and count the number of grubs per square foot. If fewer than 10 per square foot are found, control is not required or recommended.
2. Keep the lawn healthy. Proper watering and fertilizing will not prevent damage from grubs, but it will allow the grass to recover more quickly and make damage less noticeable. Mow the grass high.
3. Do not overwater your lawn. Adult beetles look for moist lush lawn to lay their eggs. By keeping the lawn drier during egg-laying times, June through July, you can limit the number of eggs laid in your lawn.
4. Parasitic nematodes. The use of parasitic nematodes is yet unproven and experimental but looks promising. Homeowners who have a grub problem and do not want to use chemical sprays may want to give this a try. The nematodes kill the grubs but are harmless to earthworms and other beneficial soil life. Nematodes are living organisms and must be handled carefully. Follow suppliers’ directions carefully. Up to four applications may be required. Available only at specialty stores or by mail order.
5. If necessary, use chemical insecticides. Chemical controls for grubs or either preventative or curative. Preventive chemicals have a longer residual in the soil and include halofenozide (Mach 2) and imidacloprid (Merit) either of which should be applied from mid-June through July. These products should be applied to areas of turf that have a history of grub damage, irrigated lawns that are experiencing a high beetle flight, or turf areas with 10 small grubs per square foot in mid-July. Curative chemicals include carbaryl (Sevin) and trichlorfon (Dylox). These chemicals have a shorter residual in the soil and are applied in late summer after the eggs have hatched and the grubs are present (10-12 grubs/sq. ft.). They are typically applied from late July through August. Spring treatment of grubs is not recommended. See these turfgrass grub-control products: Dylox (Quick Kill), Mach 2 (Extended Season), Merit (Extended Season) - Imidaclopid.
Organic Strategies
Strategies 1, 2, 3, and 4 are strictly organic approaches.
Specific recommendations for gardens. Reduce overwintering grubs by removing all old plants and overgrown weeds as you harvest each crop in the fall. Cultivate the bare soil thoroughly to a depth of 6 to 8 inches. 2 to 3 weeks later, cultivate the soil shallowly to a depth of about 2". At this point you can plant a cover crop or lay down 4" to 6" of winter mulch. In early spring, about 2 weeks prior to planting, give the garden another shallow cultivation to about 2". Upon planting, give soil a final cultivation. This frequent tilling will expose overwintering grubs to cold and to their predators. Since crows, robins and starlings are avid grub eaters, their presence in the garden prior to cultivation should be encouraged.