Fall armyworm invasion is fierce this year

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Scott D. Stewart, University of Tennessee

(THE CONVERSATION) Across the Northeast, Midwest, South, and Southwestern United States, homeowners watch in horror as their lawns turn from green to brown, sometimes in less than 48 hours, and wonder: get there so fast ?

The culprit: the fall armyworm.

As an entomologist, I can attest that their emergence is not new: it is an annual problem, but the scale of this year’s invasion is unprecedented. These voracious eaters destroy lawns and grasses, attack golf courses, pastures, football and soccer fields – and they can completely defoliate rice, soybeans, alfalfa and other cultivated fields within days. They are called legionaries because of their habit of walking across the landscape.

The invader


The fall armyworm, Spodoptera frugiperda, is not a worm. It is a striped caterpillar, the larvae of an ordinary, benign brown butterfly. It is native to the Americas and is extremely adaptable, thriving everywhere from lush forests to arid regions and in pristine, disturbed and urban landscapes.

This butterfly survives year round in warmer regions from the tip of South America to the southern United States. Each year, it invades regions further north until the cold puts an end to its occupation.

From larva to moth, its complete life cycle is about 30 days in summer and 60 in spring and fall. Adult butterflies only survive for two weeks. During this time, a female lays up to 2,000 eggs, deposited under the leaves in clusters of 100 to 200.

Moths are not the problem; these are their larvae. When the eggs first hatch, the tiny caterpillars are barely noticeable, measuring about a sixteenth of an inch long. By the time the caterpillars reach their maximum size – an inch and a half – they have become voracious eaters.

Depending on the season, armyworms eat and grow for 14 to 30 days. Initially, they chew holes in the leaves, sometimes reducing them to a lacy skeleton. If they run out of food, they become cannibals, with the larger legionaries preying on the smaller ones.

Then, they burrow into the ground, lock themselves in a cocoon and pupate. When they emerge as moths, the cycle repeats itself, with the next generation propelling their expansion across the country.

An invasive species

During this time, fall armyworms have spread across the world as an invasive species, reaching the Near East, Asia, Australia, Africa and India. Without its native complement of pests, predators and diseases to control it, these raptor caterpillars pose a serious agricultural threat to these newly invaded countries.

Agricultural practices have fueled their proliferation. Most of these countries do not cultivate armyworm resistant GMOs and many have limited access to new insecticides and modern application equipment.

Armyworms have been particularly destructive in sub-Saharan Africa, where they devour maize, the continent’s staple crop. The damage is estimated at $ 2 billion per year. It also causes significant damage to maize, rice, sorghum, sugar cane, vegetable crops and cotton.

This year’s “perfect storm”

Entomologist David Kerns sounded the alarm in June, warning the Texas Legionaries were evil and headed north and east. They had started early, helped by the good weather in their winter home range.

Once butterflies are on the move, they leave their natural enemies behind, taking their new territories by surprise. They can migrate hundreds of kilometers, riding the winds to re-infest the northern part of their domain. But with an early start this year, they pushed the winds further than normal. By the end of August, much of the southern United States east of the Rocky Mountains had suffered severe assaults, similar to a locust invasion.

How to control the invasion?

There are two ways to deal with an infestation: Wait or fight. For those worried about lawns, waiting may be the answer. Army caterpillars don’t feast on all grasses, and a well-established lawn often recovers, although it may not look great for a while. However, Legionnaires especially like freshly laid sod, which can suffer irreparable damage.

Waiting is not an option for farmers. Applying insecticides is the only way to save crops, which can be difficult as the disruption caused by the pandemic has left some insecticides in short supply. Success is a numbers game: killing 80% of a group of 100 Legionnaires controls them, but with more Legionnaires, killing 80% still means many cultures will be devastated.

There is also some evidence to suggest that fall armyworms may develop greater resistance to certain insecticides, and this wouldn’t be the first time. This pest is infamous for having developed resistance to the insecticidal proteins of Bacillus thuringiensis produced by genetically modified crops. My colleague Juan Luis Jurat-Fuentes is trying to understand how the fall armyworm becomes resistant to Bt toxins in corn and Bt cotton.

His work also reveals how armyworms resistant to insecticidal proteins spread their genes across the Americas. We are currently collaborating on a project using gene silencing to help control outbreaks of FAW. The technique can turn off specific genes, including those that make the fall armyworm resistant to insecticides. The aim is to develop highly specific and effective insecticides that have minimal impact on the environment and other wildlife.

The cost – and the future

The economic costs of FAW infestations are high. This year alone, they tackled millions of acres of crops, hay fields, lawns and sod. Farmers, homeowners and businesses have spent tens of millions of dollars on insecticide applications. Some farms have suffered significant crop losses.

The battle is not quite over. It will continue for a few more weeks as the Fall Armyworm continues to spread further north and east.

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Was this “year of the armyworm” a fluke? Will they be back? The answer to both questions is probably yes. We don’t know why Fall armyworms started en masse in 2021, but extreme infestations were, hopefully, a rare anomaly. However, there are fears that global warming will allow these and other subtropical and tropical insects to expand their territories north.

We know that armyworms will re-invade much of the southern United States each year, as they always have, and northern states should expect more frequent incursions of southern neighboring insects.

This article is republished from The Conversation under a Creative Commons license. Read the original article here: https://theconversation.com/the-fall-armyworm-invasion-is-fierce-this-year-and-scientists-are-researching-how-to-stop-its-destruction-of-lawns-football-fields- and-cultures-167098.


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