There are several species of caterpillars called sod webworms that can be highly destructive pests of Ohio lawns. They may also become important pests of grass covered parks, cemeteries, golf courses. They have even been noted to cause damage in small grain crops such as corn, wheat and oats. Damage to grass is caused by the feeding of the larval or "worm" stage. The adult moth does not cause damage to turf, other plants or clothing.
The damage caused by sod webworms may first appear in early spring. The damage shows up as small dead patches of grass among the normally growing grass. The summer generation may cause general turf thinning or even irregular dead patches in late June into early August. Sod webworms prefer sunny areas and the larvae are often found on south facing, steep slopes and banks, where it is hot and dry. Heavily shaded turf is seldom attacked by the larvae.
The most severe damage usually shows up in July and August when the temperature is hot and the grass is not growing vigorously. In fact, most sod webworm damage is mistaken for heat and drought stress. Sod webworm-damaged lawns may recover slowly, without irrigation and light fertilizations. These thin turf areas allow weeds to establish in the lawn making it unsightly.
Sod webworms appear to feed on all the common turfgrass used in Ohio. However, common Kentucky bluegrass, perennial ryegrass and fine fescues are the ones showing damage the most. However, improved perennial ryegrasses with endophytes are highly resistant to sod webworms. Likewise, tall fescue, though often attacked, usually out grows the damage. On golf courses, bentgrasses are commonly attacked.
The general thinning of turf is usually not associated with sod webworm activity, and thus, goes undiagnosed. The sod webworm caterpillars live in tunnels constructed in turf thatch or extending to the soil under the turf. These tunnels are silk lined and the webbing joins soil particles and leaves together. The larvae emerge from these burrows to chew grass blades off just above the thatch line, usually at night.
In thick, green turf, injury appears as small brown patches about the size of a quarter to three inches in diameter. When many larvae are present in mid-summer, the small brown patches run together and form large irregular, thin and brown areas.
The surest way to tell if you have sod webworms is to find a suspected area of infestation (brown patches). Get down on your hands and knees, take your two index fingers and part the grass blades in the area between dead and live grass and look for an area with small green pellets. The pellets, called frass, are the excrement of the larvae and indicate that a larva is close by. Sod webworm adults are about 3/4-inch long, cigar-shaped and buff-colored moths. They typically roll their wings around the body when resting on a grass blade. Two small snout-like projections are visible at the front of the head.
The adult moths fly mainly in late June and again in mid-August though some species may be present any time during the summer. Seeing these moths fly up while mowing or walking around the lawn does not confirm that damage is, or will be done by the larvae. The adult moths can fly considerable distances and may be coming from other infested areas.
If you still suspect sod webworm activity but are unable to find the larvae or their frass, use a soap disclosing drench. Simply mix up two gallons of tap water with two tablespoons of liquid dishwashing detergent. Sprinkle this mix over a one square yard of the affected turf. Within a couple of minutes, the flesh-colored, spotted larvae will wriggle to the surface. If you get 10 to 15 larvae in a one square yard of turf, treatment is warranted.
Most sod webworm larvae are under one inch in length and are characterized by having a series of dark spots running down the body. However, some species have the spots light in color. The most common lawn damaging sod webworms in Ohio are the bluegrass sod webworm, Parapediasia teterrella (Zincken), the larger sod webworm, Pediasia trisecta (Walker), the striped sod webworm, Fissicrambus mutabilis (Clemens), and the cranberry girdler, Chrysoteuchia topiaria (Zeller). Occasionally, the vagabond sod webworm, Agriphila vulgivagella (Clemens), adults will emerge in large numbers in September that causes considerable alarm. However, the larvae of this species feeds only in the late fall and early spring when the turf is actively growing.
The bluegrass, larger and striped sod webworm usually have two generations per year with the summer generation of larvae causing the most damage. The other species have a single generation per year. All the sod webworm larvae spend the winter as a partially grown larva buried several inches deep in the soil. At the approach of warm weather in the spring, the larvae move upward and begin feeding on the lush spring growth of grass. The bluegrass, larger and striped sod webworms finish their feeding in late May to early June. At this time they burrow deeper into the thatch or soil to pupate. After 10 to 14 days, the new adult moths emerge at night to mate and lay eggs. Mated females lay most of their eggs on the second night by randomly dropping the eggs into the turf. The larvae from these eggs feed through July into early August before pupating again. The second generation of adult moths appears in late July through August. Since the turf is usually slowly growing at this time, sod webworm feeding can cause considerable damage. The second generation of larvae feed in September until October but damage is rarely detected because the turf has begun to grow rapidly again.
The sod webworms with a single generation usually emerge as adults in late June through early August and their larvae feed mainly in the late fall and early spring. These species rarely cause significant damage to Ohio turf.
Control should only be directed towards the feeding larvae, not the flying adults. Since drought stress and turf diseases can cause thinning similar to sod webworm damage, be sure to check the turf for frass or use the soap disclosing flush to determine that larvae are present.
Strategy 1: Cultural Control - Use Fertilizer and Water - Damage can often be outgrown if water is continually available. Considerable damage may occur if irrigation is not possible during periods of drought or close mowing is used.
Strategy 2: Biological Controls - Natural parasites occur but predaceous ground beetles and rove beetles seem to be the major control factors found in turf. Unfortunately, if the lawn has been regularly treated with insecticides, these predators may not be sufficient to keep the sod webworm population in check. The bacterium, Bacillus thuringiensis, or BT is registered for sod webworm management but it is only effective against the very young larvae. By the time that damage is evident, the larvae are too large to be managed with BT. Recent development of insect parasitic (entomopathogenic) nematodes has resulted in a useful biological control. The Steinernema nematodes are the most effective. Follow label directions closely but to have the best success, water the lawn a day or two before applying the nematodes and water the lawn again, immediately applying the nematodes. Since sunlight can kill the nematodes, they are best applied in the early morning or late afternoon.
Strategy 3: Use Resistant Turfgrass Varieties - Turfgrasses with fungal endophytes are generally resistant to sod webworm attacks. Look for perennial ryegrasses, tall fescues and fine fescues that have "Endophyte Enhanced" on their package. The seed of these grasses can be overseeded or slit seeded into existing lawns.
Strategy 4: Chemical Control - Use Contact and/or Stomach Pesticides - Most webworms are easily controlled if the pesticides are ingested when the larvae feed shortly after dark. Therefore, best control is achieved by spraying in the late afternoon. Late fall or early spring applications are often not effective because many larvae are hiding in deeper soil chambers. Check OSU Extension Bulletin L-187, Control of Turfgrass Pests, for the current listing of insecticides registered for sod webworm control.
NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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