Pseudaletia unipuncta (Haworth)
The armyworm, or “true” armyworm, feeds on a variety of plants, preferring grasses. It is a sporadic pest that can be found throughout the Great Plains, but significant infestations in wheat are most likely to occur in the southern plains (Kansas to Texas). Most damage occurs during warm, moist periods in late spring, but there also is a danger of migration out of maturing grain fields and into adjacent fields of corn and sorghum.
Identification / Life Cycle
Armyworm moths are tan to light brown with a tiny white spot centered on each forewing (Figure 7.26). Adults lay their eggs in large clusters on lush vegetation. The larvae are green to black with stripes of various colors (Figure 7.27). The head capsule is medium brown with dark markings. Armyworms pupate in a brown earthen shell just below the soil surface.
Figure 7.26 Adult armyworm moth.
Figure 7.27 Armyworm larva.
Plant Damage and Response
Each larva, feeding mostly at night, can consume 43 linear inches of wheat leaf, or the equivalent of three whole plants, in the course of its development. However, 80 percent of this damage occurs during the last three to five days of larval feeding. Wheat is likely to suffer yield loss if the flag leaf is destroyed before the soft dough stage is completed. Head clipping in barley is serious and should be prevented, and while it is less likely in wheat, worms should be watched closely if present after heading. As wheat plants mature, armyworms may feed on beards and clip heads to complete their food requirements.
When leaf feeding is observed, look for larvae curled up on the ground under litter, especially in patches of lodged plants. Treatment is usually not necessary below levels of four or five larvae per foot, but may be justified at infestations of five to eight per foot depending upon larval maturity in relation to crop maturity.