Fall and Yellowstriped Armyworms
- Scientific Name: Spodoptera frugiperda (Smith) is the fall armyworm, and S. ornithogalli (Guenee) is the yellowstriped armyworm. [Phylum Arthropoda: Class Insecta: Order Lepidoptera: Family Noctuidae]
- Common Names: fall armyworm, yellowstriped armyworm
- Climatic Zone: tropical and subtropical with populations developing each year in temperate zones.
- Geographic Distribution: Fall and yellowstriped armyworms are native to North and South America. These semitropical species fly from the Gulf States each spring into northern areas and Canada. Damage can occur year around in the Gulf States but is restricted to mid- to late summer in northern states.
- Damaging Stage: larvae
- Hosts: All species of turfgrasses may be attacked. Fall and yellowstriped armyworms most commonly damage the short cut turf found on golf course greens and tees but they can build up immense populations by mid-summer that literally mow off grass in roughs, fairways and around tee banks and bunkers. These species are especially troublesome in newly sodded or springed turf.
- Damage Symptoms: In keeping with their name, armyworms may feed on grasses any time of the day and are known for their habit of moving and feeding, en masse, from one turfgrass area to another. They commonly eat everything green, leaving only a few stems. On golf course greens and tees, fall and yellowstriped armyworms behave like black cutworms by grazing on the grass blades of short cut turf causing circular or finger-shaped sunken marks. When large populations occur, the turf may first appear silvery as the early instar larvae eat the leaf surfaces. This progresses to a ragged and unkept look, and finally, complete removal of all green tissues.
- Description of Stages: Fall & yellowstriped armyworms are the larvae of moths and thus have complete life cycles with egg, larval (caterpillar), pupal and adult stages.
Eggs: Armyworms lay masses of eggs, 100 to 300 in two to three layers, usually on weeds within the turf or on trees and shrubs overhanging turf areas. Individual eggs are round, 1/64 inch (0.4 to 0.45 mm) in diameter, flat on the lower surface, bluntly pointed at the top, and each has sculpturing, lines and ridges, on the surface. Eggs are greenish-gray when first laid but turn dark brown as they mature. Each mass is covered with a layer of buff colored scales from the abdomen of the female. On golf courses, females seem to have a habit of attaching egg masses to the flags on greens, tee markers and signs.
Larvae: The larvae have generally hairless bodies except for a few scattered bristles. Besides the three pairs of true legs, these caterpillars have five pairs of fleshy prolegs on the abdomen. Most armyworms have characteristic markings on the head and stripes on the body that aid in species identification. Full grown armyworm larvae are 3/16 inch (6.0 mm) wide and 1 to 1 1/4 inches (25 to 32 mm) long. Armyworms coil into a spiral briefly when disturbed. The fall armyworm is often green to brownish with dark brown stripes. The head capsule has an inverted, white Y-shaped mark. The tip of the abdomen has three sets of distinct black spots from which arise longer hairs. The yellowstriped armyworm can range from green to brownish, but most have a characteristic yellow stripe running down each side of the top of the body. Between the two yellow stripes are darker, diamond-shaped spots.
Pupae: The pupae are droplet-shaped and brown, reddish brown or black and 5/8 inch (13 mm) long. The antennae, wing pads and legs are firmly joined together, but the abdomen is free to twist around if the pupa is disturbed.
Adults: Males and females of both the fall and yellowstriped armyworms have different markings. The males have distinctive patterns of lines and patches of gray, white and brown on a generally gray-brown background. The females are generally mottled gray-brown with a single drop-shaped mark. Adults range from 3/4 to one inch (17 - 25 mm) long with wingspans of one to 1 1/4 inch (25 - 30 mm).
- Life Cycle and Habits: Fall and yellowstriped armyworms are a constant threat to southern turf. They overwinter as larvae or pupae only in the southern states and migrate as moths to the northern states in early spring through summer. Because of this, they are rarely a threat to cool season turf.
All stages are present throughout the year in the areas of the Gulf States within 100 miles of the coast. With the arrival of warm spring tempeartures, adults lay eggs on preferred grasses and small grain crops. The adults often attach egg masses to the leaves of trees, shrubs, and flowers. If the larvae can not feed on these hosts, they drop to the ground and feed on grasses and weeds. The eggs take seven to ten days to hatch in cool weather and can hatch in two to three days in the heat of July and August. The young larvae eat the egg shells before turning to plant foliage. The tend to feed together until they reach the fourth instar. At this time, they may become cannibalistic. Larger larvae feed during the day, and if disturbed by a show or touching, they will fall from the plant and remain tightly coiled where they fall. They soon recover and return to feeding. The larvae take as little as 12 days to mature in July and August but may need 28 to 30 days to mature in October. The larvae consume and average of 13 cm2 of leaf tissue, of which 12 cm2 is consumed in the last two instars. This is why they appear to suddenly strip the turf down within a couple of days. Upon maturation, the mature larvae move into the lower thatch and form pupae within some loosely spun silk webbing. The pupa takes nine days to mature in summer and up to 20 days in cool fall and spring weather. The adults actively feed on nectar and overripe fruits. They can live for about two weeks, and females lay three to five egg masses. The adults have strong migratory tendencies and are commonly swept into northern states with storm fronts. They most commonly arrive in northern states in June and July and they may complete one to two generations. Since the larvae and pupae can not withstand freezing temperatures, none survive the winter in the north. Their principal northern hosts are corn and soybean but they often appear on golf course greens and tee surfaces. In these locations, their damage resembles black cutworms.
- Control Approaches:
Cultural Control - Use Resistant Turfgrasses - Perennial ryegrasses and fescues that contain endophytes are known to slow development or are toxic to fall and yellowstriped armyworm larvae. In order to significantly reduce armyworm populations, the turf stand must contain 40% or more endophytic plants.
Natural Control - Diseases, Predators and Parasites - There are numerous bacterial and fungal diseases that are known to attack armyworms and these may be some of the important natural controls, especially in wet conditions. Common predators include ground beetles, rove beetles, ants and birds. Many parasitic wasps and flies attack the eggs, larvae and pupae. These natural controls appear to have a significant affect on armyworms in turf, but their action is often too late to prohibit visible damage.
Biological Control - Entomopathogenic Nematodes - Products containing the insect parasitic nematodes, Steinernema carpocapsae and S. feltiae, are available and effectively control armyworms. Close attention to label recommendations, especially pre- and posttreatment irrigation and soil moisture requirements, is essential for success. A regular program of applications, beginning when the first eggs begin hatching and continuing at a 14 to 21 day interval, thereafter, has been shown to be successful. Affected armyworms often die in the thatch, not on the turf surface.
Biological Control - Use BT - Products containing the toxin derived from the bacterium, Bacillus thruingiensis (BT), are commercially available and adequately control caterpillars, especially armyworms, if enough material is ingested by young larvae. Check the label to see if the particular BT strain is recommended for “Spodoptera” armyworms. Use of these products has had mixed results in turfgrass management because of treatments are usually applied to a population consisting of large and small caterpillars. Use of this microbial toxin is best when the caterpillars are in the first to third instars. Irrigation should be omitted for at least 12 to 24 hours after application. Affected caterpillars rarely surface.
Chemical Control - Insecticides - The principle of controlling grass eating caterpillars with insecticides is to apply the treatment to the turf and omit irrigation for 12 to 24 hours until the insect consumes the foliage and makes contact with the treated surfaces. Both preventive and curative approaches can be successful. Monitoring to determine population level and size of larvae before treatment and after a treatment is recommended. Keep in mind that young larvae are far more susceptible to insecticides and other forms of control than are larger larvae.
Curative Treatments. Numerous organophosphate, carbamate, pyrethroid and diacylhydrazine insecticides effectively control armyworms in turf. On golf course greens and tees, it is recommended that spraying a band of turf surrounding the affected area (a minimum of one boom or spreader width) will help keep larvae from moving back onto the greens from surrounding turf.
Preventive Treatments. Field tests with neonicotinoid insecticides (usually long residual systemic insecticides) have shown that some applied to golf course greens or tees provided suppression of armyworms for about 30 days. Other systemic insecticides may have effective residual effects of 14 to over 90 days.
Regular monitoring of turf areas for evidence of armyworm infestation and applying treatment only when larvae are present and/or damage seems eminent, is part of the control process. Monitoring includes looking for larvae, damage and/or evidence of birds (starlings) probing the turf for larvae. Often, early evidence of armyworm attack is a frosting or silvery appearance of the turf surface due to the small larvae feeding on leaf surfaces. To determine if larvae are actually present, an effective monitoring method is to use a solution of liquid soap and water (two tablespoons of liquid Joy® dishwashing detergent in two gallons of water) spread over a one square yard area to flush larvae to the surface. When using preventive biological or insecticide strategies, sample every 14 to 21 days to detect early stage caterpillars, before significant damage is evident.
Determination of whether treatment is needed depends upon the type of turf area concerned and the standard of turf quality desired or required. For golf courses, damage in roughs is more tolerable than damage in fairways, and damage on tees and greens may not be tolerated at all. Many books and extension publications on turf insect management often mention “population thresholds.” Thresholds are the numbers of pest insects present in a given area (usually per square foot or yard) that warrant control. The concept behind using thresholds is to reinforce the IPM principle that application of a pesticide is not always warranted simply if pests are present. Pests must be present at populations high enough to eventually cause damage. In high cut turf, 5 to 10 armyworms per yd2 may just begin to show damage. However, two to three armyworm spots on a high quality golf green may be more than can be tolerated. Thresholds will have to be established for each golf course depending on the standards desired.