The southern chinch bug, Blissus inslaris, is the most prevalent and damaging insect pest of St. Augustine grass. The white grub may also cause serious damage. Army worms, mole crickets and sod webworms also attack the St. Augustine grass.
Chinch bugs, grubs and worms may all be controlled by preventing excessive thatch, a layer of dead plant material between the soil and the top of the grass. Thatch protects insects and makes the application of insecticides less effective. Thatch forms when soil microbes can’t break down mown grass as fast as it is added. Mow once a week during the growing season, removing no more than 35 to 40 percent of the leaf blades.
Rake thatch more than 1-inch deep with a vertical mower, a power rake with blades that cut into the turf, pulling thatch to the surface. Treat lawns with accumulated thatch using an aeration machine that removes 1/2-inch to 3/4-inch cores of soil using hollow tines mounted on a drum. Removing these plugs, generally 1 to 6 inches deep and 2 to 6 inches apart, increases the penetration of air and water to the grass roots. Excessive fertilizer creates thatch; use no more than 3 to 4 lbs. of nitrogen in an organic, slow release-fertilizer per 1,000 square feet in sunny locations.
Chinch bug nymphs are bright orange with a white band on the abdomen; they turn dark red then black. The black adults have shiny white wings. Feeding chinch bugs inject a toxin into the grass causing irregular patches of stunted or dead grass surrounded by halos of dying, yellowing grass. Damage develops rapidly during hot, dry weather. Over-watering destroys microbes necessary to decompose thatch; water to about 6 inches deep and let the grass dry between watering. For the home gardener, horticulturalists at Texas A & M University recommend insecticides containing the active ingredients acephate, chlorpyrifon, diazinon or permethrin, a synthetic pyrethroid. Apply granular insecticides with a fertilizer spreader and irrigate with 1/8 to 1/4 inch of water. Use a hose-end sprayer to apply 15 to 20 gallons per 1,000 square feet of grass.
White grubs are the larvae of the Japanese beetle and other beetles. Beetles lay their eggs on well-watered lawns in the summer. The grubs feed on grass roots, causing the lawn to turn brown. Horticulturalists at the University of Rhode Island recommend the application of insecticides containing imidacloprid and halofenozide in April before the beetles have laid their eggs. Apply halofenzoide that mimics insect hormones in early summer; in the fall, apply insecticides containing carbaryl or trichlorfon.
Army worms, caterpillers of the gray moth, eat grass blades. Apply Bacillus thuringiensis (Bt) on small worms or insecticides containing carbaryl, deltamethrin or permethrin. Irrigate before treating to move the worms out of the thatch. Bt is a bacterium found in the gut of butterflies and moths.
Nocturnal mole crickets have grayish-brown velvety bodies and front legs adapted for digging. They burrow into grass on warm, humid nights and feed on roots in the top 1 inch of soil. Use soapy water to flush them to the surface where you can rake them. For homeowners, the Alabama Extension Service recommends insecticides containing the active ingredient acephate in the spring and acephate chlorpyrifos or isofenphos in the summer.
The sod webworm, Crambus spp, the larvae of the white brown moth, feeds on the undersides of grass leaves causing dead patches. Flush them to the surface with soapy water and rake them. For larger infestations use Bacillus thuringeiensis.