Several kinds of caterpillars may cause damage to all of our turf grasses. These caterpillars are the immature or larval stages of moths.
The most damaging is the tropical sod webworm. Adults of the sod webworm are small, dingy brown moths with a wingspread of about 3/4 inch. Larvae are small, greenish caterpillars which range from 1/25 inch long when they first emerge from the egg to 3/4 inch long at maturity. Eggs are deposited on the grass blades by the moths. These moths will fly up from the grass when the lawn is walked upon or mowed. The eggs hatch in about one week.
The larvae begin to feed on the grass blades and cause noticeable damage within two weeks. Development from egg to adult takes about 12 weeks.
The newly hatched webworm larvae chew away tissues from the surface of the grass blades, leaving a colorless, membranous area on the leaves. As larvae mature, the grass is progressively chewed off and becomes ragged with notches chewed along the sides of the blades, and yellowish or brownish in color. Damaged areas are often first noticed along hedges and flower beds.
The foliage may be almost completely stripped off in patches which soon become yellowish to brownish.
Injury normally begins in a few spots, with injured areas only two to three feet across. These spots enlarge, fuse, and may encompass large areas of the lawn when you have a heavy infestation. Grass under stress from hot, dry weather may be killed. However, healthy grass not suffering from lack of water can recover from a large amount of webworm feeding.
The webworms may be found by parting the grass in suspect areas and looking for small, green worms curled up on the soil surface and small green pellets of excrement. A flashlight at night will reveal the caterpillars feeding on the grass blades. Newly hatched caterpillars cause very little visible damage to grass. It is not until they are almost full grown at 3/4 inch long that their feeding becomes noticeable. It appears to show up almost overnight.
Cultural practices can influence the susceptibility of lawn grasses to webworms. The lush succulent growth, caused by frequent applications of highly water soluble inorganic nitrogen fertilizers attracts egg-laying female moths. Use of slow release nitrogen reduces damage from these pests.
A thick thatch layer is an excellent habitat for lawn caterpillars, and also chemically ties up insecticides, thereby reducing their effectiveness.
A number of beneficial insects and spiders are extremely efficient in reducing lawn caterpillar populations. The predacious earwig, several spiders, ground beetles and the ichneumonid wasp are some of the more common predators and parasites which attack lawn caterpillars.
Often I have noticed that one to two weeks after a lawn has been treated for chinch bugs it will have to be treated for sod webworms as their feeding increases to damaging levels. Probably because the beneficial insects, which had been keeping the webworm population in check, were no longer alive to do the job.
When it is determined that a pesticide needs to be applied for control, there are several to choose from. Two good ones are Sevin or Dipel, which is Bacillus thuringiensis, a biological control which only kills caterpillars. Worms treated with Bacillus thuringiensis may require two to five days to die, but they are unable to feed after the first day.
To further avoid the reduction of beneficial insects, spot treatments of Dipel can be applied when infestations are first noticed and the damaged area is very small.
Eileen Ward and her husband, Peter, own and operate Greensward of Marco Inc., a lawn maintenance and landscaping company. Besides completing horticultural courses from the University of Florida, she has a commercial maintenance spray license and is a registered dealer in agricultural products in Florida.