Armyworm

Fall armyworms like these have been damaging lawns this year as the pest marches across central Illinois.

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In recent weeks, University of Illinois Extension has received reports of fall armyworm (Spodoptera frugiperda) spottings across central Illinois.

The Department of Crop Science notes reports are on the rise across western and southern Illinois as well, making this pest a widespread problem this year.

Homeowners have reported this pest primarily damaging turfgrass, although it is noted to consume a wide array of plants, including trees and shrubs as well as some vegetable crops. They can be a pest in field crops as well, feeding on wheat, corn, rice, soybeans and other crops.

Damage to lawns is typically quite noticeable, as these caterpillars hatch in large numbers and “march” together across an area consuming blades of grass and leaving dead-looking spots in their wake.

Due to their army-like populations, damage often occurs quickly, often limiting our ability to seek preventative control.

This non-native pest is a migratory moth that moves north from Mexico and Central America to the southern U.S., typically reaching the Midwest in varying numbers each year.

It is a tropical insect that is cold sensitive, so it cannot overwinter in Illinois, which usually works to limit its numbers in our area.

However, it can overwinter in southern Texas and Florida, making it a problem in adjacent southern states most years.

Larger outbreaks across the South typically spell increased infestation in our area as adults migrate north this time of year.

Entomologists across the South have noted epic populations this summer that have greatly impacted a variety agricultural crops.

In late July, the southern outbreak prompted the Environmental Protection Agency to approve special use of specific herbicide to control fall armyworm in rice fields, illustrating the magnitude of this pest’s impact in 2021.

UI Extension educator Kelly Allsup has some recommendations if you suspect fall armyworm damage in your turfgrass.

“A quick drench in areas of dead grass in your lawn with soapy water will force fall armyworm to the surface,” she said.

Caterpillars more actively feed in the early morning and evening, which are good times to look for the pest.

Allsup recommends mixing 1 tablespoon of dish soap per gallon of water and pouring the mixture over about a 1-square-foot area of infected lawn. The soapy water should send armyworms wiggling to the surface.

Mature caterpillars are typically a little larger than 1 inch in length and can vary in color from yellow or green to grayish-black.

They usually have a lighter-colored line down their back with a yellow or white strip on each side. However, a lighter-colored Y-shaped mark on their head is the main feature that distinguishes this insect from other caterpillars.

“Strong turfgrass can endure low to moderate infestations if properly irrigated and fertilized,” Allsup noted, “although severe cases may need to be treated with insecticides to control caterpillars.”

As caterpillars feed, they only consume above-ground vegetation, so healthy stands of turf can typically recover when provided adequate water, since root systems are left intact. Once all the grass blades in an area are consumed, caterpillars will move as a group to new areas of turf to feed, so dead areas are normally delineated by the actively feed front of caterpillars moving toward healthy turf.

Pesticide treatments are effective in heavy infestations and can help turf recover quicker, while milder infestations can typically be managed with additional irrigation for the rest for the remaining growing season. Although infected lawns may look terrible now, have faith that the healthy root system below can produce a new stand of leaves, especially as cooler fall weather and adequate moisture create ideal growing conditions for cool-season turfgrass.

For more information on fall armyworm identification and control recommendations, consult the UI Extension Home, Yard & Garden Newsletter, which has an excellent past article by Phil Nixon, at go.illinois.edu/fallarmyworm.

Ryan Pankau is a horticulture educator with UI Extension, serving Champaign, Ford, Iroquois and Vermilion counties. This column also appears on his ‘Garden Scoop’ blog at go.illinois.edu/GardenScoopBlog.

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