There are a number of reasons why turfgrass can thin out. The first is too much shade. As the amount of shade increases, the grass has less potential to produce the food that it needs to survive, so the grass will thin out. Another reason is disease.
But I want to concentrate on insect problems such as mole crickets or white grubs.
Right now, I have seen a number of lawns that have grubs in them. Thinning grass is one way to tell white grubs are present, but there are other ways.
The presence of moles or armadillos is another indicator. These animals eat grubs and earthworms, so they will tunnel or dig in your grass looking for them. If you have a severe grub infestation, you might have raccoons, skunks and birds digging up your lawn looking for grubs. Crows have even been known to pull pieces of sod out to get to the grubs.
White grubs are the larval stage of beetles. There are four main beetle grubs that cause lawn problems -- May-June beetle, masked chafers, Japanese beetle and green June beetle.
The life cycle of these beetles are very similar, but a few have some differences. These differences are mainly in how many years it takes for them to complete their life cycle.
- In Georgia, most May-June Beetles take two years to complete their life cycles. Some take only one year, while others can take three years. Depending on the species, adults emerge from the soil between March and July. They are active at night, when they feed and mate, and then return to the soil in the daytime. Eggs are laid in the soil near the preferred host and hatch in three to four weeks. Grubs feed on roots and overwinter in the soil as second or third instars. They resume feeding in the spring and either continue to feed and grow through the summer or pupate in the soil and emerge as adults later that season.
- Masked chafers and other chafers found in Georgia have one-year life cycles, with adults emerging in early summer. The adults are quite active night fliers, but they do not feed and are not pests themselves. They return to the turf in the daytime to lay eggs. These eggs hatch in two to three weeks. Grubs feed on a mixture of plant roots and organic matter in the soil. They grow quickly, reaching full size by late August. Damage to turf is heaviest in September and October. Grubs overwinter deep in the soil, returning to the surface in the spring to feed for several weeks before pupating a few inches deep in the soil.
- Japanese beetles have a single generation each year. Adults appear in late May and might be active into July. Adults live four to six weeks, and females lay eggs during most of their lives. Eggs hatch in about two weeks. After maturing in the fall, the grubs overwinter in the soil and resume feeding in the spring. After two to four weeks, the grubs mature and pupate in cells in the soil. They emerge three to four weeks later and remain in the cells for several days before digging to the surface. Adults are active during the day, returning to the turf in late afternoon.
- The green June beetle has one generation per year. It spends the winter deep in the soil as a large grub. Grubs resume feeding in the spring, then pupate in cells in the ground in May and June. About three weeks after pupation, adults emerge. Eggs are laid in the soil during July and August and hatch in 10 to 15 days. Grubs feed actively during late summer and fall and may be active in warm periods throughout the winter.
Green June beetle grubs are unusual among Georgia pest species in that their primary food is dead and decaying plant material, not live grass roots. The damage they inflict on turfgrass is mechanical. They loosen the soil and damage roots as they tunnel, and leave mounds of dirt on the surface when they emerge at night to feed. The grubs spend the daytime resting in vertical tunnels in the soil.
Finding white grubs in your lawn doesn't mean that you have to treat for them. Find out how many grubs are present. Treatment likely won't be needed unless you find three or four grubs per square foot.
Cut out a square foot of sod and count the grubs. If enough grubs are present to require treatment, there are insecticides available to control the grubs. The insecticides trichlorfon, imidacloprid 1.47 percent carbaryl (Sevin SL) and halofenozide are available for homeowners. Some of these are fast-acting contact insecticides, and some provide seasonlong control. Early to mid-September is a good time to treat for white grubs.
Columbia County Extension Agent Charles Phillips can be reached at (706) 868-3413 or by e-mail at email@example.com. The Extension Web address is www.ugaextension.com/columbia
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