County Extension Agent
Armyworm outbreaks are difficult to predict when it comes to timing and location. But infestation seems to occur each year during the fall especially after cold fronts and rain. Armyworms can destroy agricultural crops such as hay fields and small grains, lawns, shrubbery, vegetable gardens and flowers over night. There are many species of armyworms present in Texas, but the fall armyworm is usually the species that causes the most problems in pastures, small grains and turfgrass.
All armyworms have four life stages: egg, larva, pupa and adult. Eggs are very small, white, laid in clusters of 50 or more and are covered with grayish, fuzzy scales from the body of the female moth. The eggs are seldom seen in grasses and are usually laid at the base of host plants. Lush plant growth is preferred by the adults for egg laying. Larvae (caterpillars) are very small when they emerge from the egg. Larvae will feed for 2-3 weeks and can be 26-37 mm. (1-1.5 inches) long with various color patterns depending on the species. The larvae have five instars (stages when molting occurs) and sometimes hide in debris on the soil surface in the middle of the day. When full grown, larvae enter the soil and form the pupal stage. Adult moths emerge from pupae. Moths mate and lay eggs, thus starting the life cycle over again.
Several generations (a generation is the development from egg to adult stage) occur each year and typically take about 28 days to complete. Generation time can be extended if cooler temperatures occur and can last up to several months. Armyworms in the spring and summer occur in more distinct groups than later in season. Fall populations of larvae often blend together several generations and may appear to be continually occurring.
When feeding, larvae strip foliage and then move to the next available food. High populations appear to march side by side to the new food. Thus, the name armyworm is derived.
Armyworms attack many different kinds of plants. When food is scarce, they will move to plants that are not normally attacked. Thus, armyworms can be found on nearly any plant as they migrate in search of edible foliage. Plants attacked by armyworms include: bermudagrass, fescue, grain, and forage sorghum, corn, small grains, sweet potato, beans, turnip, clover, spinach, cucumber, potatoes, tomatoes, cotton and cabbage.
Damage consists of defoliation. The small larvae will chew the green layer from the leaves, creating a “window pane” effect. The first three instars cause very little feeding damage while the last two instars consume 85% of the total foliage consumed.
Control-Although armyworm outbreaks are memorable when they occur, in reality, the outbreaks are usually small in scope. Weather and natural enemies usually act together to keep populations under control.
Parasites such as wasps and flies are very effective against armyworms. Predators, such as ground beetles, are also effective in limiting outbreaks. Birds, skunks and rodents also consume large numbers of larvae and pupae.
Armyworms should be controlled when they occur in large numbers or plant damage is becoming excessive. Preventive treatments normally are not justified because attacks are sporadic and egg mortality is usually high. A variety of natural enemies keep fall armyworm larvae down to moderate numbers. Early detection of larvae is the best management tool and is achieved by frequent, through inspection of plants. Outbreaks seem to occur shortly after a rain or supplemental irrigation.
Fall armyworms feed any time of the day or night, but are most active early in the morning or late in the evening. Susceptible fields or lawns should be scouted by counting the number of armyworms in a square foot area in 8 different sites. Divide the total worm count by 8 to find the average number of armyworms per square foot. Be sure to take samples in the interior of the field because this pest is often heaviest near field margins. Sometimes, only the field margins require treatment.
The threshold level ranges from two to three larvae per square foot for seedling wheat. For older plants, three to four larvae and obvious foliage loss justify control measures. Thresholds in improved pastures and lawns vary with conditions but treatment should be considered when counts average three or more worms per square foot.
Insecticide choices vary with the crop but the following (with product names and grazing restrictions in parentheses) are labeled for use in pastures include carbaryl (Sevin(r)) (14 days), malathion (o days), Methomyl (Lannate(r)) (7 days) methyl parathion (Penncap(r)-M) (15 days) and various biological such as Dipel(r) (0 days). Insecticide labeled in lawns and turf include halofenozide (Mach(r)2), bifenthrin (Talstar(r)), cyfluthrin (Tempo(r), Bayer Advanced(r)), carbaryl (Sevin(r)) permethrin (multiple brands) and spinosad (Conserve(r) and others).
For more information contact the Somervell County Extension Office at (254) 897-2809.