• Topics: Entomology and Nematology  | Turfgrass Pest Insects  |  Caterpillars  | Buss, Eileen A  | Meagher, Robert Leo 

Lawn Caterpillars¹

Are lawn caterpillars doing your mowing for you? Young caterpillars, or larvae, injure turfgrass by chewing notches along the edge of the leaves. This creates a ragged appearance (Figure 1) that may be hard to notice at first. Mature caterpillars eat a lot before they pupate and consume patches of turfgrass down to the crown. Because the turf looks scalped so quickly, people think that the damage occurs “overnight.” Several caterpillar species can be turfgrass pests, including the tropical sod webworm, the fall armyworm, and the striped grass looper.

Figure 1. 

Caterpillar damage to bahiagrass.

Identification

Tropical sod webworm larvae (Figure 2) are gray-green, have brown spots on each segment, and are the smallest of the three species. Mature larvae can be 3/4 to 1 inch in length, and they pupate in the thatch or on the soil surface. Fall armyworm larvae (Figure 3) can be green or brown, and mature larvae are 1½ inches long with four pairs of prolegs (fleshy legs on abdomen). As larvae grow, light stripes appear along the length of the body and dark spots appear on the top of each segment. Fall armyworm larvae have an inverted light-colored Y on the front of their heads. They pupate in the soil. Striped grass looper larvae (Figure 4) have longer and thinner bodies and “loop” like inchworms when crawling. They only have two pairs of prolegs. Their color ranges from cream to black, there is a light-colored narrow stripe down their backs, and many stripes on their heads. Striped grass loopers pupate on tall pieces of grass or small shrubs,

Figure 2. 

Tropical sod webworm larva.

Figure 3. 

Fall armyworm larva.

Figure 4. 

Striped grass looper larva.

Tropical sod webworm adults (Figure 5) are small, tan to gray moths with a wingspan of ¾ to 1 inch. They do not cause damage. Moths hide in shrubs and other sheltered areas during the day and fly low when disturbed. Females lay clusters of 6-15 white eggs on grass blades at night. Eggs darken to brown and hatch within 7 days. Fall armyworm adults (Figure 6) are larger, with a wingspan of nearly 1½ inches. Females are almost all gray, but males are shaded gray and brown and have white spots near the center of the wing and near the tip. Fall armyworm eggs are laid in clusters of 50-150 along grass blades or on non-plant surfaces. Eggs are gray in color and coated with moth scales (Figure 7). Striped grass looper adults (Figure 8) are the largest of the group, with a wingspan of 1½ inches. Wings are tan to yellowish-brown in color with vertical lines and round spots. Eggs are laid singly on grass blades.

Figure 5. 

Tropical sod webworm adult.

Figure 6. 

Fall armyworm adult male.

Figure 7. 

Fall armyworm egg mass on bermudagrass.

Figure 8. 

Striped grass looper adult.

Biology and Monitoring

Tropical sod webworm is most active from April through November in north Florida, but may occur year-round in south Florida. Three to four generations occur in Florida each year. Tropical sod webworm larvae feed on St. Augustinegrass, bermudagrass and zoysiagrass. Fall armyworm occurs year-round in south Florida and migrates northward each spring. This means that populations can be damaging in the spring in south Florida, but don't build up until fall in north Florida. Fall armyworm will feed on all turfgrasses, but prefers bermudagrass. Striped grass looper also occurs year-round in south Florida, and isn't a problem until fall in north Florida. Striped grass looper is primarily a pest on bahiagrass in pastures, but will readily infest other turfgrasses. Larvae of these species are active at night and will hide in a curled position near the soil surface during the day. Fall armyworm larvae may also feed during the early and later parts of the day. Green or brown pellets of frass may be visible on the soil surface, indicating that larger larvae are present. One generation of tropical sod webworm is about 6 weeks; fall armyworm and striped grass looper can develop in about 4 weeks under warm weather conditions.

To find larvae, part the grass in suspect areas and look for chewed leaves, silken webs, green or brown frass, and larvae. Use soap flushes (2 TBSP liquid dishwashing soap mixed in 2 gallons of water) to drench larvae or other potential pests out of the ground. Pour the mixture on 1 square yard of damaged grass, and observe after 5 minutes. If nothing emerges, examine several other areas.

The adults (moths) of all three species are active from dusk till just after dawn. A commercially available sex pheromone lure can be used to monitor fall armyworm. The sex pheromone for striped grass looper is known but not available. Researchers are currently determining the sex pheromone for tropical sod webworm.

Management

Damage Thresholds

The number of larvae that can cause significant damage depends on the turfgrass variety, how stressed it might be, and how tolerant the turfgrass managers or their clients are to damage. In general, most turfgrasses can tolerate the notching damage that young larvae cause, but three 1/2 inch, mature fall armyworm or striped grass looper larvae per square foot may justify a treatment. About 10 to 20 tropical sod webworm could warrant treatment. Keep in mind that large larvae may pupate quickly, which may make an insecticide application unnecessary or ineffective.

Cultural Control

Cultural practices can influence turfgrass susceptibility to these caterpillar species. Turf can recover from damage if properly irrigated and kept healthy. Drought or low mowing heights may reduce or prevent grass recovery. Because many eggs are laid on grass blades, removing and destroying cuttings after mowing might reduce infestations.

Application of water-soluble, inorganic nitrogen fertilizers causes rapid leaf growth, and increases the chance of caterpillar problems. Female moths that are ready to lay eggs are attracted to the succulent leaves. Responsible use of slow-release fertilizers may reduce turfgrass susceptibility.

Many insects, including lawn caterpillars, live in thatch, which is a layer of accumulated dead plant roots, stems, rhizomes, and stolons between the live plant and soil. Over-watering or over-fertilization can cause turf to develop a thick thatch layer. Excessive thatch should be mechanically removed (vertical mowing, power raking, etc.) to minimize insect habitat and minimize binding of pesticides to organic matter.

Biological Control

Ants, ground beetles, rove beetles and spiders are predators of caterpillars in lawns. One parasitic wasp, Horogenes sp., attacks tropical sod webworm larvae, and other wasp species attack fall armyworm (Aleiodes laphygmae and Cotesia marginiventris). Parasitic flies attack striped grass looper larvae (Sarcodexia sternodontis and Chetogena sp.). Preventive pesticide use in lawns can reduce natural enemy populations and reduce their ability to naturally minimize pest populations.

The bacterium Bacillus thuringiensis (Bt) is registered for use against all three species and is most effective in controlling young larvae. Some commercial products include Dipel, Green Light Bt Worm Killer, Safer Caterpillar Killer, and Thuricide. By the time damage is apparent, however, the larvae may be too large to control with Bt. So, monitoring is important. There are also species of fungi and viruses that affect these caterpillar species which might be commercially available.

Chemical Control

Control should only be directed against the feeding larvae, not the non-feeding, flying adults. Time applications to control young larvae, if possible. Spot treatments may be applied when infestations are first detected and the damaged area is small. A treatment might be most effective if applied in the early evening when larvae begin feeding. Examples of products that can be used by commercial applicators are in Table 1.

All directions and insecticide labels should be read and understood before a product is used, particularly the dosage rates, application procedures, and precautions. Those products mentioned are only examples; other effective products could be used.

References

Brandenburg, R. L. and M. G. Villani. 1995. Handbook of Turfgrass Insect Pests. Entomological Society of America, Lanham, MD.

Niemczyk, H. D. and D. J. Shetlar. 2000. Destructive Turf Insects, 2nd ed. H.D.N. Books, Wooster, OH.

Potter, D. A. 1998. Destructive Turfgrass Insects: Biology, Diagnosis, and Control. Ann Arbor Press, Chelsea, MI.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.

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