AgWatch

Alert For Missouri: Be Scouting For Fall Armyworm In Pastures

LAMAR, MO.
   There have been several reports of armyworm damage across Missouri which means producers should be scouting for fall armyworm in grass pastures.
   “Numbers of fall armyworm larvae often peak in late summer and early fall resulting in substantial damage to grass and legume crops,” according to Jill Scheidt, agronomy specialist with University of Missouri Extension.
   Fall armyworm larvae tend to feed on all tender green plant tissues which give infested pastures the appearance of drought. If heavy feeding occurs, grass plants may become severely defoliated, stunted or killed.
   “Damage may occur on 60 different hosts with tall fescue and orchardgrass being favored host plants,” said Scheidt.
   Damage in newly seeded alfalfa can be severe with complete stands of seedling alfalfa destroyed in a matter of a few days. They may also feed on soybeans.
   Several generations of fall armyworm occur in Missouri each year.
   Larvae produced in spring and summer are often light in color ranging from light green to tan. Larvae produced during fall generations often are dark and may be completely black or gray in color. Both color phases of this insect will possess stripes running the length of the body.
Identifying characteristics of the larvae include an inverted Y on the face of the insect and four black spots or bumps on the top of each segment. Spots on the last segment of a worm will be arranged in a square pattern, with three white lines located on the back of the segment located behind the head capsule. Sometimes the three white lines will extend to additional segments.
   Larvae typically grow through six “worm stages” often reaching a length of one and a quarter to one and a half inches.
   Damage in grass pastures often appears overnight as growing larvae become large enough to consume substantial amounts of forage in short periods of time.
   “Larvae are especially active both early and late in the day. Scouting is best accomplished during these periods to gain an accurate estimate of larval numbers,” said Scheidt.
   The economic threshold for this pest in grass pastures is to treat if three or more larvae are present per square foot.
   Pyrethroid insecticides are effective in controlling armyworm outbreaks. Two products that are recommended for fall armyworm in pasture are zeta-cypermethrin (example: Mustang Max) and lambda-cyhalothrin (example: Warrior II), both sold under various trade names.
   Suggested rate of Mustang Maxx is 2.8-4.0 fluid ounces per acre- with a zero-day haying and grazing restriction or 1.28-1.92 fluid ounces of Warrior II, with a seven hay restriction. Read and follow all label directions and be aware of pre-harvest and grazing restrictions. ∆

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