Where wheat has emerged, fields need to be checked for fall armyworms. This is especially important because this summer and fall were characterized by very active fall armyworm activity. Reports of fall armyworms infesting recently planted wheat fields in Oklahoma should put southern Kansas farmers on alert as this moth migrates northward from southern states to lay eggs on emerged wheat. Freezing temperatures will kill the larvae, but current forecasts do not call for a freeze in southern Kansas any time soon.
When scouting fields for fall armyworm damage, look for “windowpane” injury caused by tiny larvae chewing on seedling leaves (Figure 1). Each individual field should be scouted in several locations, including the field margins and the interior. The larvae themselves are usually too small to be easily observed after they first hatch, and hide in or around the base of seedlings. Within a few days of hatching, the larvae become large enough to destroy entire leaves (Figure 2).
Figure 1. “Windowpane” feeding damage on wheat leaf. Source: http://entomology.k-state.edu/extension/insect-information/crop-pests/wheat/fallarmyworm.html
The suggested treatment threshold is 2-3 actively feeding larvae per linear foot of row in wheat. Fields with 25 to 30 percent of plants with windowpane injury should be re-examined daily and treated immediately if stand establishment appears threatened. Larvae increase in size at an exponential rate, and so do their food requirements. Later instars do the most damage, sometimes destroying entire stands, and are the least susceptible to insecticides. Without treatment, problems can continue until larvae reach maturity or until a killing frost. Thin stands of wheat are especially at risk.
Figure 2. Fall armyworm larva. Source: http://entomology.k-state.edu/extension/insect-information/crop-pests/wheat/fallarmyworm.html
If a killing frost does not occur soon, fields may require chemical treatment. For treatment options, please refer to the latest K-State Wheat Insect Management guide at: http://www.bookstore.ksre.ksu.edu/pubs/mf745.pdf/
Leaf damage by early stage army cutworm larvae will look very similar to that of fall armyworms. Army cutworm larvae are even more difficult to see than fall armyworm larvae because they feed mostly at night and hide in the soil. Although army cutworm larvae will grow more slowly than fall armyworm larvae, they will feed throughout the winter (unlike fall armyworm larvae), burrowing in the soil to escape frost and emerging again to feed during spells of warmer weather.
Army cutworm moths emerge in May and then migrate westward to Colorado where they escape hot summer weather in the mountains.
Mostly the same insecticides are registered for control of both species, but higher rates are recommended for fall armyworm. Any fields with mixed populations should be treated with the fall armyworm rate.
If you are seeing worm-feeding damage, you need to know what kind of worms you have. The upside down 'Y' on the head capsule of fall armyworms is a distinguishing feature (Figure 3) not shared by army cutworms (Figure 4).
Figure 3. Head capsule of fall armyworm, with the upside down “Y” marking. Source: http://www2.ca.uky.edu/entomology/entfacts/ef302.asp
Figure 4. Army cutworm larvae. Source: http://cropwatch.unl.edu/archive/-/asset_publisher/VHeSpfv0Agju/content/it-s-time-to-scout-wheat-alfalfa-for-cutworms
The threshold for treating army cutworms is 5 or more larvae per row foot, provided worms are still small (a certainty at this time of year). However, most damage from this species will occur in the spring and they are difficult to find in the soil while small.
Romulo Lollato, Wheat and Forage Specialist
lollato@ksu.edu
J.P. Michaud, Entomologist, Agricultural Research Center-Hays
jpmi@ksu.edu
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