Late-summer insect update: Fall armyworms

Fall armyworms, Spodoptera frugiperda, can damage several important Kansas crops as well as pasture, turf, and home landscaping, and are currently being reported across the state in various crops and pasture scenarios.  The first fall armyworm flight into Kansas occurred during the first week of July in south central parts of the state, and trap counts of adult moths continued to increase in those areas through July, while moths also started showing up in the central and northeast parts of the state in notable numbers.  There were scattered and treatable infestations of caterpillars in these areas during that timeframe. During this first week of August, trap counts of moths are increasing dramatically in south central areas, and reports of significant infestations in the central and eastern parts of the state have been reported.

Given these recent trap observations and reports of caterpillar outbreaks, scouting should continue until the first frost of the season for at-risk crops and pasture.  It is critical to scout often and treat, if needed, when the caterpillars are less than an inch long.  Larger caterpillars are harder to control and do the most damage.  Recommended thresholds and products labeled for control of fall armyworm caterpillars (Table 1) can be found below.

Fall Armyworm Thresholds

Alfalfa: 1-2 caterpillars per square foot can destroy seedling alfalfa.  10-15 per square foot can destroy 12” tall plants.

Corn: Damage to the whorl stage in early summer is typically not a concern and will not impact yield.  Bt corn may prevent ear damage.

Sorghum: Damage to the whorl stage in early summer is typically not a concern. 1-2 larvae/head during flowering to soft dough reduces yield by 5-10%.

Wheat: Larval “window-paning” in early planted wheat can be a concern. Very late second-generation fall armyworms will be the biggest concern for wheat.  If 25-30% of plants show damage, examine the field frequently.  Treat at 2-3 active larvae/ft.

Pasture/Brome: If damage is notable or there is window-paning, treatment may be warranted with 4 to 5 caterpillars per square foot.

Table 1. Registered products for the control of fall armyworm in Kansas crops.  For more specific information relative to any insecticide, always refer to the actual label on the product.

*For use only in bermudagrass pastures.

For additional information on the life history of fall armyworm in Kansas, please refer to the following early season eUpdate:  https://eupdate.agronomy.ksu.edu/article/fall-armyworm-season-approaches-get-ready-to-scout-fields-647-7

Anthony Zukoff, Entomologist – Garden City
[email protected]

PHOTO: Fall armyworm, notice the thickest stripe is black. (Photo courtesy K-State Entomology Department.)