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Posted 26 August 2011. PMN Crop News.

Yellowstriped Armyworm

Source: University of Illinois Press Release.

Urbana-Champaign, Illinois (August 1, 2011)--I recently noticed an unusual insect feeding on tomatoes in my garden. The pest was a yellowstriped armyworm, and because tomatoes are a staple of locally grown food – this pest oddity is probably worth noting.


While some might be able to pick out this species as an armyworm, the exact name might be hard to come by because even insect keys make identification of yellowstriped armyworm a little difficult. The larva of the yellowstriped armyworm, Spodopter ornithogalli, varies in color from a dark black to gray. Typically, the "worm," which can reach just over 1 and three-fourths inches when fully grown, has a yellow-colored line running down either side of its body. Good eyes can also pick out a black, "diamond-shaped" marking just above each yellow line on each segment of the insect's back. Examination of the head capsule will show the yellowstriped armyworm to have an inverted, white, "V-shaped" marking. This "V-mark" also makes identification confusing because many keys claim no such marking exists on the insect's head. The eyes appear brown when looked at under a hand lens but usually appear black with the unaided eye.

Yellowstriped armyworm moths, which have a wingspan of about 1 and one-fourth inches, usually fly into the area from tropical areas. Occasionally, the insect survives the winter as a pupa beneath the soil surface. When pupae overwinter in the area, adults emerge the following spring.

Once mated, the female moth begins to deposit numerous eggs in small clumps on the base of upright objects. The eggs are initially green in color, but as they accumulate enough heat units to hatch, the eggs turn pink to brown. Emergence happens about six days after the eggs are deposited on the plant, and the resulting larvae feed on just about any vegetation available.

While the wide host range of yellowstriped armyworm typically provides this larva with an abundant food supply that scatters the population throughout an area, larva occasionally appear in corn fields at high enough levels to cause noticeable damage. However, yellowstriped armyworms are not of concern until 50% defoliation occurs. Economic injury in row crops is therefore rather rare making yellow striped armyworms little more than an interesting "find" in the cornfield. In commercial tomato production and around the home, the larvae can actually feed on fruit while also damaging green beans.

Yellow armyworms don't usually congregate and feed in large numbers. They are not the same species that plagued some Fulton and Mason County producers earlier in the growing season. That pest was actually the true armyworm, which does move and feed in mass causing extensive damage. Often a gray or brown color with a black colored "V" on the head, true armyworms also migrate into the state and do not survive the winter in Illinois. They can strip a field virtually overnight. Yellow armyworms are more often noted in smaller numbers. Chemicals probably are not necessary if you see them – simply pick them and heel/stomp….it's amazing how well that works.

Matt Montgomery