Identifying the Pest
Grubs are milky white with a brownish head. They typically curl into a “C” shape when disturbed. Depending on the species, they may have a one-year to a three-year life cycle. Annual white grubs (larvae of masked chafer beetles or Japanese beetles) begin their life cycle in mid to late summer, grow quickly, and do most of their feeding in late summer. Larvae of June beetles take three years to complete their life cycle, so their damage to lawns may appear in spring, summer, or fall.
When to Control Grubs
Controls are most effective on immature grubs. For most species and locations that means July or August is the prime time to treat with an insecticide. However, because grub populations vary from year to year, you may be able to save the cost of treatment if you first sample your lawn to estimate how many grubs are present. Dig up several pieces of sod about a foot square. If you find five or fewer grubs per square foot, you need not apply grub control. The lawn will withstand the amount of feeding these few grubs do. If 10 or more grubs are present, treat your lawn. If the average count is between five and 10, whether or not to control depends on the health of your lawn, your tolerance for damage to the lawn, and the presence of natural controls.
Natural Grub Control
Milky spore is a natural bacterium that can be an effective control of Japanese beetle grubs, but it usually takes several years to become established in the soil, and it requires some grubs to live on, so don’t use it in combination with chemical controls. Milky spore is only effective on Japanese beetle grubs. If you’re wondering how to stop grubs in the lawn, try nematodes. Beneficial nematodes are microscopic worms that live in the soil. They seek out grubs, and after entering their bodies, release bacteria that act as killers. The two major types of insect parasitic nematodes are Steinernema spp. and Heterorhabditis spp. The nematodes are applied as a live product, so make certain, if you use them, that they’ve been handled carefully, are alive, and that their shelf life has not expired. These two types of nematodes are effective on all types of grubs. Weather and irrigation practices can also work in your favor. Grub populations are usually highest when soil conditions are consistently moist. In dry years, or if you withhold irrigation water during hot summer months, many grubs will not survive. This strategy may work well for bluegrass lawns, which recover well from summer dormancy.
Chemical Grub Control
Apply curative chemical controls at the correct time, when the grubs are small and actively feeding near the soil surface. This is usually from early July until mid-August. Chemicals labeled for curative control are carbaryl, halofenozide, and imidacloprid. Because grub populations vary so much from year to year, preventive chemical controls are seldom justified. But if your lawn has been attacked by grubs consistently, a preventive insecticide application in late May or June may be warranted. Imidacloprid and halofenozide provide extended, preventive grub control. Water in the chemical controls so they penetrate into the soil and can act on the grubs. Watering the lawn after chemical application not only moves the product down into the thatch layer, it also stimulates the grubs to move upward in the soil, closer to the chemical.
