Authors: Buyung Hadi, Jeffrey Bradshaw, Jan Knodel
There are three species of armyworms that may act as pests in soybean production: fall armyworm, Spodoptera frugiperda, beet armyworm, Spodoptera exigua, and yellowstriped armyword, Spodoptera ornithogalli. These species belong to a large family of moths, the Noctuidae. The feeding activity of larva, or caterpillar, stage of many members of the Noctuidae, including the armyworms, can be injurious to crops.
Identification
A newly hatched fall armyworm caterpillar is light-colored with a black head. The body color darkens as it ages. Fall armyworm caterpillars can be identified by a prominent white inverted Y-shape on the head. Additionally, four dark spots are usually visible on the upper side of the caterpillar's eighth abdominal segment.
Newly hatched beet and yellowstriped armyworm caterpillars may look fairly similar and thus indistinct to fall armyworm caterpillar. In later instars a beet armyworm caterpillar is green to black in color. The body is smooth with fewer hairs or spines than fall aryworm caterpillar. Usually, a black spot can be noted on the side of the second thoracic segment, near the head. Unlike the fall armyworm, no prominent white Y-shaped marking is found on the head of beet armyworm caterpillar.
A later-stage yellowstriped armyworm caterpillar is usually dark with yellow or light colored bands running along the sides of the body. A black spot can usually be spotted on each side of the first abdominal segment.
Life Cycle and Seasonal History
Armyworms undergo a complete cycle of metamorphosis with egg, larva (caterpillar), pupa and adult stages and the length of their life cycle is dependent on temperature. A fall armyworm caterpillar grows from egg hatch to the time of pupation in 19 days at 77˚F (25˚C). Under the same temperature, it takes another 12 days for the adult moth to emerge from the pupa. Adults mate and lay their egg masses at night, usually on the underside of the leaves. At 70-80˚F (20-30˚C) the eggs hatch in 2-4 days. The caterpillars disperse as they hatch and feed on plant foliage. The caterpillars grow in size and molt a couple of times to accommodate the new size. The stages between molts are termed 'instars'. The sixth instar caterpillar drops to the ground and pupates about 1-3 inches deep in the soil. In northern latitudes fall armyworm is limited to one generation per year. Fall armyworm overwinters in areas where hosts are available year-round and temperatures rarely fall below 50˚F (10˚C). Due to this restriction, fall armyworm usually overwinters in southern states and migrates northward in summer months. At least two strains have been reported for fall armyworm: the rice and corn strain. The rice strain of fall armyworm feeds on a broad range of plants whereas the corn strain is more host specific.
The whole life cycle of beet armyworm can be completed in about 36 days. Eggs are deposited in a cluster covered by white scales. Eggs hatch within 2-5 days. The first instar caterpillars initially feed together near the egg cluster and disperse as they mature. In about three weeks, the caterpillars develop through five instars. The last instar falls to the ground and pupates in the soil. Typically the fall frost in the northern states is capable of killing beet armyworm. This insect overwinters in the southern states with possible summer migration to states as far as Kansas and Nebraska. In the northern states, beet armyworm usually produces one generation per year.
Yellowstriped armyworm life cycle requires 23-25 days to complete although in cooler climates it may take a month or more. Mated female adults lay a cluster of 200-500 eggs on the underside of leaves. The egg cluster is typically covered with scales. Like beet armyworm, newly hatched yellowstriped armyworm caterpillars feed in a group and disperse as they mature. Yellowstripped armyworm survives the winter in Iowa as pupae, and together with fall armyworm are the most common armyworms on Iowa soybean. In northern states, yellowstriped armyworm can have as many as three generations per year.
Plant Injury and Damage
Generally, armyworms caterpillar injure plants by feeding on the leaves. All armyworms are general feeder, capable of infesting many crops and vegetables. On soybean, armyworms may feed on both leaves and pods.
Fall armyworm caterpillars infesting seedling soybean may cut the stems and reduce the crop population below the optimal level. Early instar caterpillars of beet and yellowstriped armyworms feed in groups producing skeletonized leaves with the fleshy leaf tissues eaten and the leaf veins intact. As the caterpillars mature, they disperse and the resulting patches of leaf defoliation become more irregular.
Armyworms are occasional pests of soybean. Soybean plants are capable of compensating for foliage loss, especially if it occurred before flowering. However, high level infestations resulting in severe defoliation can retard further plant growth. Fall armyworm infestation of soybean is associated with grassy fields around soybean fields. All species of armyworms described here rarely reach economically important population levels in the Northern Plains.
Management Approaches
Scouting and Threshold
In seedling soybean, examination of whole plants in rows may be the best method to scout for armyworms. Cut seedling stems may be a hint of fall armyworm infestation. Fall armyworm treatment in seedling soybean is warranted if the number of cut stems renders plant population below the recommended stand density. In older soybean plants, armyworms can be scouted using a sweep net or by shaking plants and catching the dislodged insects on a cloth placed between rows. As leaf feeders, armyworm populations should be considered together with other defoliating pests such as the soybean looper and velvetbean caterpillar to determine the need for action. If only fall armyworm is present, then 8 caterpillars per row-ft (26 cacterpillars per row-m) is a reasonable action threshold.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com
