Detection and management of thatch dwelling worms such as webworms, armyworms and cutworms are essentially the same. Because the brown patches caused by these worms can superficially resemble those caused by other pest insects or diseases, accurate diagnosis is important. Mature armyworm caterpillars are 1 1/2 - 2 inches long, and vary in color from grey to yellowish green. Light stripes run the length of their bodies. Monitoring or detecting armyworms can be accomplished by visually examining the thatch layer or by using a soap drench to flush them out. Thatch-dwelling insects such as armyworms can be found in the layer of dead grass just above the soil line. They are present if: grass blades in the damaged area are actually missing and not just dead; there are green fecal pellets in the thatch; you find larvae in silk-lined tubes in the thatch. You can use a small hand trowel to loosen the thatch. Soap drenches are a useful tool for monitoring and detection. To do this: Mark off two or three 2 feet square sections of lawn in both damaged and undamaged areas. Mix 2 tablespoons of liquid soap or detergent in 1 gallon of water in a sprinkling can, then pour the mix evenly over each area to be sampled. The soap irritates the caterpillars, causing them to crawl to the surface. Keep a close watch on each test area for about 10 minutes. Brief movements alone may indicate the worms are present. Where the thatch is thick, it may be necessary to pour several more gallons of soap solution on the test area to reach the worms. If the thatch is saturated but no catapillars appear, the damage is probably due to disease or another type of insect.
If your lawn is in good condition and growing vigorously, the presence of 2 or 3 worm larvae per square foot is probably not cause for concern. If your lawn is under stress, the presence of as few as 1 larva per square foot usually indicates the need for treatment.
Indirect strategies for armyworm control include planting resistant grass varieties, managing horticultural stresses on your lawn and conserving native biological controls. Kentucky bluegrass variety "Windsor" and "Park" show some tolerance for thatch-dwelling worms. If thatch dwelling worms are a chronic problem, however, you should consider replacing grasses with Indophytic grasses (grasses that contain beneficial fungi within their tissues). Grasses with endopytic fungi that repel caterpillars or worms include the perennial ryegrass types: Citation II, Commander, Penant, Regal, Repell, Sunrise, turf type tall fescue grass types include: Apache and Kentucky 31. Check with your local distributor for others. Reducing the horitcultural stresses on lawns inlude reducing the thatch level in your lawn if it is thicker than 1/2- 3/4 inch. Over fertilizing, fertilizing at the wrong time or poor watering techniques can cause thatch build up. Natural organic fertilizers are preferable because they provide food for both soil and plants over time, build up the soil water infiltration rate, increase water holding capacity, and make your lawn healthier. Good lawn management should encourage the presence of the natural enemies of thatch-dwelling worms. Certain ants, flies, spiders, vespid wasps, native earwigs, bettles and birds prey on these pests.
Direct control includes soap drenches and raking to remove moderate populations. For higher populations beneficial nematodes may control the infestation or an application of the microbial insecticide Bacillus Thuringensis (BT).
The key lies in maintaining a healthy soil. Pesticides and synthetic fertilizers kill natural soil organisms that are essential for healthy soil. Without a healthy soil, your grass will become dependent on expensive, toxic quick fixes that cause a host of problems for your lawn. Synthetic fast release chemical fertilizers are problematic because they: leach from the soil quickly, run-off into streams or groundwater, burn grass because their nitrogen is too hot, provide no food for soil microorganisms or earthworms, repel worms because they acidify the soil, slow down bilogical activity in the soil, increase susceptibility to pests and disease, increase soil compaction, lesson organic material so that soil structure declines, reduce water holding capacity of the soil, and lead to chemical addiction where more chemicals must be added with every application. Using organic lawn care methods will make your lawn care job easier over time because it will make your lawn healthier, less weedy, and more disease and pest resistant.
Sources: 1.Common Sense Pest Control, William Olkowski, Sheila Daar & Helga Olkowski
2. Preserve Your Lawn's Health with Organic Lawn Care
by Charyn Grandau, Education/Resource Conservationist; Soil and Water
Conservation District, St. Louis County
PO Box 27
Morse Mill, Missouri 63066-0027