Scouting for True Armyworms Is Highly Recommended in Small Grains and Early Corn

During the week of April 15 to 21, 2017, there were 301 (Princeton) and 4 (Lexington) captures of true armyworm (Mythimna unipuncta) moths in pheromone-based traps in those locations. For Princeton, this is a 1/3 fold increase in 1 week (KPN), while in Lexington, there was no change in the number of moths captured. Although there is only one trap in each site, the captures in Princeton is an indicator of increased activity by armyworms (egg oviposition and hatching of larva) in wild grasses, small grains, and early corn in western Kentucky. Consultants and farmers are also confirming the presence of flying moths in their fields.

There have been armyworms flying since the beginning of March (Beware of True Armyworms).  The temperatures were favorable for mating and egg oviposition during most of this time, except during the freeze of mid-March. Based on historical data and accumulated degree day (Acc.DD) models, in all probability, armyworm larvae may have been feeding on wild grasses or wheat fields in undetectable numbers. The required Acc.DD of 612 for larval development and subsequent feeding (Table 1) was completed on April 20 [630 Acc. DD for Caldwell County (Calculating Degree Days in KY)]. So far, there are no reports on caterpillar damage. Armyworms need at least 1 week to start laying eggs, then eggs will hatch 6 to 10 days later (Table 1). Larvae will start feeding immediately after hatching in both wheat and early-planted corn. Predictions on insect numbers are not 100% accurate, but based on the discussion presented above, I expect that armyworm feeding may become abundant by the end of April or first week of May. Thus, scouting for armyworm larvae should be a priority in the following days to avoid damage from this pest. Armyworm larvae are highly active at night and on cloudy days. Larvae can damage plants by eating leaf margins of upper leaves, defoliating entire plants, and clipping heads of maturing plants.

Scouting and Management

I am aware that preventive sprays is the standard practice for aphids and armyworms in western Kentucky; however, preventive treatments for armyworms are not justified in terms of farm economy or environmental sustainability. Insecticide usage should be the last resource. I hope that insecticide sprays this year are based on tallies of armyworm larvae.

Follow these guidelines to scout for armyworms in small grain fields:

  • Examine a 4-square-foot area in 5 random locations
  • Count numbers of larvae between 1/2 and 3/4 inches long

Follow these guidelines for making insecticide application decisions:

  • Control in small grains is recommended when average of larval tallies is greater than or equal to 16 worms per 4 square feet
  • If armyworms are larger than 11/4 inches long, control is not profitable
  • If the average number of larvae is equal or above 16 caterpillars, an insecticide application might be required.

Table 1. Accumulated degree days for different stages of true armyworm (Mythimna unipuncta). (Modified from J.C. Guppy (1969) Can. Entomol. 101:1320-1327).

More Information

  • Armyworms in Small Grains (EntFact-111)
  • Historical Kentucky IPM Insect Trap Data (UK IPM Programs)
  • Insecticide Recommendations for Small Grains (ENT-47)
  • Beware of True Armyworms –Mild Winter Provides Conditions for Potential Injuries in Small Grains (KPN 03-28-17)



By Raul T Villanueva, Extension Entomologist

Posted in Grains