Quick Facts...
- Sod webworms and cutworms are common insects in lawns throughout Colorado.
- Most sod webworms and cutworms chew on grass blades.
- These insects rarely become abundant enough to cause noticeable injury.
- Because sod webworm injury is infrequent, always check suspected areas of infestation before applying an insecticide.
Sod webworms are one of the most widely recognized groups of turfgrass pest insects in Colorado. Adult sod webworms are common lawn moths that may be disturbed to flight during mowing. The caterpillar stages feed on grass blades and cause injury when abundant.
Various species of cutworms also are common on Colorado lawns, feeding on grass roots or blades. Occasionally in spring, cutworms also may cause localized problems.
Sod Webworms
Several species of sod webworms occur in Colorado. The damaging stage is the caterpillar or larval stage. Typical sod webworm caterpillars have a dark head. Their bodies are light brown or gray with dark spotting. Webworm-infested thatch has silken tunnels produced by the caterpillars. During the day, they hide within these webbed tubes, and at night they move out a short distance to feed. Sod webworms are typically 1 to 1 1/2 inches long when full grown.
Adult sod webworms are the most commonly observed stage of the insect. They are small, buff-colored "lawn moths" that often rest on grass or shrubbery. Several sod webworm moths have attractive markings with silvery striping. They are weak fliers. When disturbed from their resting places, they may fly short distances across a lawn. (Several other moths found on lawns are not lawn-damaging species, including the alfalfa webworm and lucerne webworm.)
The most common sod webworm species (the larger sod webworm, Leach's crambus) overwinter as partially grown caterpillars. When temperatures rise in spring, the caterpillars resume feeding and become full grown in May and early June. Most lawn injury occurs during this time. The sod webworm caterpillars then change to the pupal stage within the silken tubes. Because they resemble a small clod of earth, sod webworm pupae rarely are observed.
In approximately one week, adult moths emerge from their pupal cases to mate and lay eggs. Flights are most common shortly after dark. The female moth flies low over the lawn area and scatters her eggs. Adult moths have a short life span, usually less than two weeks. Females can lay up to 60 eggs per evening.
Sod webworms have one to two generations per year in Colorado. When a second generation occurs, peak larval feeding and damage is in July.
One common species of sod webworm, known as the cranberry girdler, feeds on grass roots rather than blades. Cranberry girdler caterpillars feed primarily in September and October, and cause later injury than is typical of other sod webworms. Cranberry girdler caterpillars have an orange head with indistinct body markings.
Cutworms
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| Figure 1: Sod webworm larva. |
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| Figure 2: Sod webworm adult moth. |
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| Figure 3: Bronzed cutworm larva. |
Cutworms are the caterpillar stage of "miller" type moths. (The nuisance
miller moth of eastern Colorado, the army cutworm described in fact sheet
5.572, Moths in the Home, occasionally occurs as a spring cutworm
of turfgrass.) The adult moths are inactive by day and may be attracted
to lights at night. Cutworm caterpillars reach 2 to 3 inches when full
grown. Some species are dull gray or white. Others, such as the bronzed
cutworm, may be marked with stripes and distinctive coloring.
Bronzed cutworms, the most common cutworm in lawns, overwinter as eggs that hatch in late winter. The cutworm caterpillar stage feeds on the leaves or roots of the grass in spring. In May, bronzed cutworms become full grown, stop feeding and pupate. Several months later, the adults emerge and lay eggs in August and September.
Occasionally, infestations of the armyworm, a "climbing cutworm," may occur. Because the armyworm does not overwinter in Colorado, infestations depend on flights of the adult moths into the state during the summer. Even though the armyworm is a fairly important pest of wheat and barley, it rarely damages turfgrasses.
Damage
Most sod webworms and cutworms clip and feed on emerging grass blades. During low-level infestations, this feeding injury appears as a general thinning of lawn areas. With cutworms, indistinct circular areas of feeding injury may be noticeable. Heavy sod webworm or cutworm infestations can result in death of grass that has repeatedly been clipped back.
The number of caterpillars required to cause significant turf injury varies. Lawns that are adequately watered and grow vigorously can tolerate high populations of sod webworms and cutworms with little observable injury. Thin lawns that grow poorly may be seriously damaged. It is not uncommon for sod webworm injury to be more noticeable along sidewalks and other areas that are warmer and under more drought stress.
Various thresholds of sod webworm numbers that threaten turf damage have been proposed and range from one to 12 per square foot. The general guideline for most Colorado sod webworm infestations is likely to be in the middle of this range. Because cutworms feed more heavily, suggested treatment thresholds for cutworms would be in the lower end of the range.
Natural Control
Natural controls of sod webworms effectively reduce sod webworm and cutworm populations. Ants, ground beetles, rove beetles and other insect predators are highly effective in controlling sod webworms. Parasites of sod webworms -- parasitic wasps, microsporidia and fungal diseases -- are also important.
Birds are the most obvious predators. Starlings, while rearing young, feed on tremendous numbers of sod webworm and cutworm larvae. Flocks of starlings and blackbirds can eliminate infestations in a short period of time.
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| Figure 4: Life stages of the larger sod webworm in Colorado. |
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| Figure 5: Life stages of the cranberry girdler in Colorado. |
Chemical Controls
Consider insecticide treatments for sod webworms and cutworms only when caterpillar populations are sufficiently abundant to threaten serious injury. Routine sod webworm treatments are not recommended. Sample the thatch for larvae to determine the number of sod webworms. Carefully examine areas suspected of sod webworm or cutworm activity for the webbing, green fecal pellets and clipped grass associated with the insect.
Irritants, or "disclosing solutions," also can be used to sample sod webworms and many other turfgrass insects, such as cutworms and billbug adults. These irritants cause the insects to move to the surface of the lawn where they easily can be counted. The most effective irritants involve drenches of the garden insecticide pyrethrins at a dilution of 0.0025 to 0.005 percent applied at a rate of 1 gallon of water per square yard. A less effective, but more available, substitute is 1/4 cup of dry laundry detergent or 1 ounce of liquid detergent per gallon of water.
If infestations of sod webworm or cutworm larvae threaten a lawn area, the insecticides listed in Table 1 can provide control. These insects are controlled relatively easily with insecticides. Some attention to application technique can improve effectiveness. Ideally, mow lawns prior to treatment so less grass is present to dilute the treated area. Apply insecticides later in the day because sod webworms feed at night. If granular insecticides are used, a light (less than 1/8 inch) watering is required to move the insecticide off of the granule.
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