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Lawn caterpillars are about to become active in St. Augustine lawns. When you walk on the lawn the moths can be seen flying up and around you.

There are several kinds of caterpillars which can cause damage to all of our turf grasses. These caterpillars are actually the larval stage of moths.

The most damaging is the tropical sod webworm. Adults of the sod webworm are small, dingy brown moths about 3/4 inch across. These moths will fly up from the grass when you walk on or mow the lawn. Larvae are small, greenish caterpillars which are also about 3/4 inch long at maturity. Eggs are deposited on the grass blades by the moths and will hatch in about a 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 yellow to brown in color. Damaged areas are often first noticed along hedges and flower beds. The foliage may be almost completely stripped off in patches.

Injury normally begins in small areas, approximately 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 their 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.

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Cultural practices can influence the susceptibility of lawn grasses to webworms. Lush succulent growth caused by frequent applications of highly water soluble inorganic nitrogen fertilizers attracts the 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 earwig, several spiders, ground beetles and the ichneumonid wasp are some of the more common predators and parasites which attack lawn caterpillars.

Insecticides sprayed on lawns for chinch bug control also kills these beneficial insects. Many times lawns have had minor damage from sod webworms until after treating for chinch bugs. Within one to two weeks these same lawns have to be treated for sod webworms as their feeding increases to damaging levels. Probably because the beneficial insects, which were keeping the webworm population in check, are no longer alive to do the job.

When it is determined that a pesticide needs to be applied for control Dipel, which is Bacillus thuringiensis, is 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 avoid the further reduction of beneficial insects, spot treatments can be applied when infestations are first noticed and the damaged area is very small. Also remember the moth stage does not cause any damage so trying to treat them will be futile.

Eileen and Peter Ward have owned a landscape and lawn maintenance company for 35 years. Eileen can be reached at Gswdmarco@comcast.net or 239-394-1413.

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