Armyworm and Fall Armyworm
Contents:
Introduction
Armyworm, Pseudaletia unipuncta (Haworth) and the fall armyworm, Spodoptera frugiperda (J.E. Smith) are in the Lepidoptera order known as moths and within the Noctuidae family. The larvae or caterpillars cause defoliation damage that results in turfgrass thinning. Armyworms are known to mass together and move in large groups to new food sources causing defoliation damage along the way.Identification
Armyworm caterpillars will vary in length from 1/16" (2mm) as first instar larvae to 1 1/2-2" (35-50mm) as mature larvae. The larvae have stripes running the length of their body; one strip is present on each side and another stripe runs down the middle of the back. They are hairless caterpillars having a base color ranging from yellow-green to a dark brown to gray color.Fall armyworm larvae will also have stripes but differ in having a yellow-white Y-marking on the head and four dark circular spots on the upper side of each abdominal segment. They will reach lengths of 2 inches when mature.
Distribution
The armyworm is a native species which occurs throughout the United States east of the Rocky Mountains and is also found in New Mexico, Arizona, and California.The fall armyworm occurs throughout the United States in the warmer months, but is found all year in the southern states. This species overwinters in the Gulf Coast states and Florida and continuously migrates north during the spring and early summer.
Hosts
Armyworms mainly attack grasses such as small grains. However, other grasses such as pasture grasses, turfgrasses and corn are fed on. Within turf and pastures, armyworms inhabit the thatch layer. Under extremely dry conditions they will seek harborage inside soil cracks and under ground litter.Fall armyworm attacks grasses, corn, cotton, alfalfa, clover, peanuts, tobacco, and many garden plants. The microhabitats in turf, pastures and meadows is very similar to those selected by the armyworm.
Life Cycle
Armyworms pass the winter as a partially-grown larvae in the soil or under debris in grassy areas. Activity and growth are continuous except during very cold weather. Larvae which successfully overwinter feed during the following spring. When fully grown, they stop feeding for four days, then pupate over a 15-20 day period. Adults emerge in May and June. Mating takes place at night especially during the 5th hour after sunset (Pfadt 1978); multiple matings usually occur. Females feed for 7-10 days on honeydew, nectar, or decaying fruit before laying eggs. Eggs are laid at night in clusters of 25-134 on grass or small grain leaves. A single female may live as an adult for 17 days and produce up to 2000 eggs. Eggs hatch in 6-10 days.Young caterpillars begin feeding on leaves, especially at night or during cloudy weather. They usually hide in the thatch during daylight hours. Six larval instars are passed in 3-4 weeks; the last instar consumes 80% of the foliage eaten during the insect's lifespan. Full grown larvae pupate in a flimsy silk cocoon under leaf litter or in earth cells 2-3" below the soil surface. Following pupation, in August or September, the emerging adults mate and lay eggs. Larvae develop partially before winter.
The number of generations produced each year increases as latitude decreases; 3-4 are produced in the central states, while 5 or more generations are produced in the South. Outbreaks are most common after cold wet spring weather. In seasons of unusual abundance, larvae may crawl in large groups from one food source to another (hence their common name). Armyworms cannot survive exposure to temperatures above 88°F.
The fall armyworm life cycle is similar to that of the armyworm. Although primarily a pest in the south, fall armyworm adults migrate northward each year, and have reached pest status as far north as Minneapolis (Niemczyk 1981). Successful over wintering occurs only in the South.
Damage
Defoliation damage is nearly identical to sod webworm injury. Only it proceeds at a faster rate because of the large sized caterpillars and synchronous egg laying and subsequent population growth. Both species may be active throughout the growing season and outbreaks may coincide with sod webworm activity.Armyworm - Treatment may be triggered when May larval populations reach 1/sq. ft.
Fall armyworm - Tall fescues can suffer severe late summer damage if larval populations reach 1/sq. ft. Other grasses may be more tolerant.
An irritant or detergent flush can be used to determine larval thresholds. Apply 2 ounces of liquid dishwashing detergent per gallon of water to a 4-5 sq. ft. area. If larvae are present they will come to the surface within a few minutes. Because these two moth species are important agricultural pest, most states also monitor the seasonal flight activity with black light traps. Outbreak predictions for corn and other agricultural crops may also be a strong indicator that problems may occur in landscape turf situations.
Biological-Biorational Control
Both of these species are susceptible to a wide variety of pathogens that occasionally become epizootic during major outbreaks. Bacillus thuringiensis products such as Steward and Dipel will provide effective control if applied to young 1st-3rd instar larvae. Other biorational control products include the entompathogenic nematodes Steinernema carpocapsae (Vector) and Heterorhabditis bacteriophora (Cruiser).A new product which mimics the insect molting hormone, ecdysone, and causes premature molting is Mach 2 (halofenozide). This product has provided good control of both fall armyworm and armyworm caterpillars.
A plant extract from the Neem tree and known as azadiractin has been developed to control several leaf feeding caterpillars of turfgrass including armyworms. Turplex Bioinsecticide is a commercial preparation of this material.
Another recent introduction of a biorational control product is a bacterial fermentation product within a chemical family known as sinosyn which has insecticidal properties. It is marketed as Conserve SC and is labeled for armyworm control of all larval instar stages.
Armyworm - larvae may be effectively controlled by the parasite tachinid fly Winthemia quadripustulata. Other insect parasites include Telenomus minimus (an egg parasite); the braconid wasps Apanteles laeviceps, A. marginiventris, and A. militaris; Predators include: ground beetles; sphecid wasps; birds; toads; domestic fowl; and small mammals (e.g., skunks).
Fall armyworm - egg and larvae parasites include the ichneumon wasp Ophion bilineatus; the braconid wasps Chelonus texanus, Meterous laphygmae, and Apanteles spp.; Trichogramma minutum; Euplectrus wasps; and the tachinid flies Winthemia quadripustulata and W. rufopicta. Predators include ground beetles, birds, and many small mammals.
Cultural Control
Since armyworm damage is similar to scalping the turf with a lawn mower, watering and fertilizing will quickly stimulate regrowth. Turfgrass varieties with high levels of fungal endophytes are highly resistant to both species.
Chemical Control
Similar to sod webworms both species prefer to feed at night, so most insecticides labeled for sod webworm control will also control both armyworm species. The following insecticides have labeling for armyworm controlPyrethroids: Astro, Battle, Scimitar, Tempo, and Talstar
Carbamates: Sevin
Organophosphates: Dursban and Proxol
by Dr. Lee Hellman, Department of Entomology and Dr. J. Kevin Mathias, Institute of Applied Agriculture