Across the Northeast, Midwest, South, and Southwest United States, homeowners are watching with horror as their lawns turn from green to brown, sometimes in less than 48 hours, and wondering, “What happened this year—and how did it happen so fast?”
The culprit: the fall armyworm.
Scott D. Stewart, a professor of entomology at the University of Tennessee, Knoxville, and director of West Tennessee AgResearch and Education Center, recently explained for The Conversation what these annual invaders are and how to help control them.
The fall armyworm, Spodoptera frugiperda, isn’t a worm. It’s a striped caterpillar, the larvae of an ordinary and benign brown moth. It’s native to the Americas and is extremely adaptable, thriving everywhere from lush forests to arid regions and in pristine, disturbed and urban landscapes.
This moth survives year-round in warmer locales, from the tip of South America to the southern US. Each year they invade more northern regions until cold weather ends their occupation.
From larva to moth, its entire life cycle is about 30 days during the summer and 60 in spring and fall. Adult moths survive just two weeks. During that time, a female lays up to 2,000 eggs, deposited underneath leaves in clusters of 100 to 200.
The moths aren’t the problem; it’s their larvae. When eggs first hatch, the tiny caterpillars are barely noticeable, about one-sixteenth of an inch long. By the time the caterpillars reach full size—an inch and a half—they’ve become ravenous eaters.
Depending on the season, the armyworms eat and grow for 14 to 30 days. Initially they chew holes in leaves, sometimes reducing them to a lacework skeleton. If they run out of food, they become cannibals, with the larger armyworms preying on the smaller ones.
Then they burrow into the ground, encase themselves in a cocoon, and pupate. When they emerge as moths, the cycle repeats, with the next generation propelling their expansion across the country.
An Invasive Species
Meanwhile, fall armyworms have spread around the globe as an invasive species, reaching the Near East, Asia, Australia, Africa, and India. Without a native complement of parasites, predators, and diseases to control them, these rapacious caterpillars pose a serious agricultural threat to newly invaded countries.
Farming practices have fueled their proliferation. Most of the affected countries do not grow armyworm-resistant GMO crops, and many have limited access to newer insecticides and modern application equipment.
Armyworms have been particularly destructive in sub-Saharan Africa, where they devour maize, the continent’s staple crop. Damage is estimated at US$2 billion per year. It also causes major damage to corn, rice, sorghum, sugar cane, vegetable crops, and cotton.
This Year’s ‘Perfect Storm’
Entomologist David Kerns sounded the alarm in June, warning that armyworms in Texas were bad and heading north and east. They’d gotten off to an early start, aided by good weather in their winter home range.
Once the moths are on the move, they leave their natural enemies behind, taking their new territories by surprise. They can migrate hundreds of miles, riding the winds to reinfest the northern part of their domain. But with an early start this year, they rode farther than normal. By the end of August, much of the southern US east of the Rocky Mountains had suffered serious assault, akin to a plague of locusts.
How Do We Control the Invasion?
There are two ways to deal with an infestation: wait it out, or fight. For those concerned about lawns, waiting may be the answer. Armyworms don’t feast on all grasses, and a well-established lawn will often recover, though it may not look great for a while. However, armyworms particularly love freshly laid sod, which may sustain irreparable damage.
Waiting it out isn’t an option for farmers. Applying insecticides is the only way to save crops, which may prove difficult as pandemic-fueled disruptions have left some insecticides in short supply. Success is a numbers game: killing 80 percent of a group of 100 armyworms controls them, but with larger numbers it still means many crops will be devastated.
Some evidence suggests that fall armyworms may be developing more resistance to certain insecticides. This pest is infamous for developing resistance to the insecticidal proteins from Bacillus thuringiensis produced by genetically modified crops. One of Stewart’s colleagues, Professor of Entomology and Plant Pathology Juan Luis Jurat-Fuentes, is trying to understand how the fall armyworm becomes resistant to Bt toxins in Bt corn and cotton.
Jurat-Fuentes’ work is also revealing how insecticidal protein-resistant armyworms are spreading their genes across the Americas. Stewart and Jurat-Fuentes are currently collaborating on a project using gene silencing to help control outbreaks of fall armyworm. The technique can turn off specific genes, including those that make the fall armyworm resistant to insecticides. The goal is to develop extremely specific and effective insecticides that have minimal impact on the environment and other wildlife species.
The Cost—and the Future
The economic costs of fall armyworm invasions are high. This year alone they have preyed upon millions of acres of crops, hayfields, lawns, and turfgrass. Farmers, homeowners, and businesses have spent tens of millions of dollars on insecticide applications. Some farms have suffered major crop losses.
The battle is not quite over. It will continue for a few more weeks as the fall armyworm continues to spread farther north and east.
Was this “year of the armyworm” a fluke? Will they be back? The answer to both questions is probably yes. It’s unknown why fall armyworms started off en masse in 2021, but the extreme infestations were hopefully a rare anomaly. There is concern, however, that a warming climate will allow these and other subtropical and tropical insects to expand their territories northward.
Stewart confirmed that armyworms will reinvade much of the southern US every year as they always have, and northern states should expect more frequent incursions from insect neighbors to the south.
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Lindsey Owen (865-974-6375, firstname.lastname@example.org)