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Armyworms in small grains

from Dr. Ian MacRae, UMN Entomologist, NWROC - Crookston


We are receiving calls regarding armyworms in small grains in NW MN.

At this time they are small larvae (1/2"-3/4" long) and feeding in the lower foliage.  Scout for armyworms at grassy margins of the fields, low, weedy areas in fields or in lodged grain; populations are more likely to develop in these areas first.  Armyworms prefer the edges of leaves first and are messy, wasteful eaters.  They generally retreat during the day under soil and plant residue on the ground and feed more often beginning at dusk, it’s easier to scout for armyworm damage than the armyworms themselves.  Look for leaves that have been notched/cut, partially eaten leaf material on the ground, and small round pellets (armyworm frass, i.e. poop) near the base of the plants.

Consider applying insecticide if: there are 4-5 armyworm larvae per sq. ft., caterpillars are ¾ - 1 ¼ in. long, leaf feeding or head clipping is found, and parasites are not evident.  By the time armyworms are more than 1 ½ in long, they have stopped feeding and are getting ready to pupate.  At this point the damage has already been done and control applications will probably not provide an economic return.

There are a number of insecticides registered for use against armyworms in small grains, check the label for rates.  At this time of year, be certain to check the PHI as well.

I've attached a small factsheet including detailed information.

AND, from Bruce Potter, IPM Specialist, SWROC - Lamberton

True armyworm

After the yesterdays newsletter was emailed, I heard about some treatment of armyworms on wheat in SW MN. Additionally, some consultant friends let me know that they have been treating some wheat for armyworm across the border in eastern South Dakota. One mentioned that areas with hail have had problems.

Why hailed areas? Like black cutworm, true armyworm move north with weather systems from the south and drop out with thunderstorms. Additionally, the lodged, tangled stems of hailed on wheat or other grasses provides a good environment for eggs and larvae.

Unlike black cutworm, it is difficult to predict where problems from armyworm flights will occur geographically. Minnesota is a big place but I would focus on lodged small grains and not-Bt corn with grass weed pressure first.

Bruce has a management overview of armyworm management he published earlier on June 9 and can be read here.

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