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Fall Armyworms: Eating their way around the world

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Foreign invaders have conquered China. The attackers are mud-colored grubs from the Americas, whose deadliest weapon is their appetite. Which is, to be clear, titanic.

Armyworms, and more specifically Spodoptera frugiperda or fall armyworms as they are commonly called, are more than your average very hungry caterpillar. They’re larval eating machines—feared by farmers from Argentina to Florida for turning hundreds of acres of cropland into vegetal stubble in a single night.

In 2016 fall armyworms appeared in Africa for the first time and have since destroyed billions of dollars worth of crops. Since then they’ve marched steadily east, to India, Nepal, China, Taiwan, and the Philippines, turning untold acres of crops into worm food. They don’t bite, sting, or ooze slime, but their appetites alone qualify them as a global scourge.

What will the armyworm invasion do to food prices in China?

The timing of the armyworm incursion is very bad indeed. Thanks to African swine fever, which has wiped out about a fifth of the country’s pork supply, food prices are already climbing, leaping 7.7% in May, versus the same month a year ago. The armyworm outbreak may exacerbate that trend. Though a grain and vegetable shortage in China might benefit US farmers, the tariffs China imposed on American agricultural products certainly won’t help curb prices.

Despite the alarming swiftness of the infestation, grain prices have so far stayed flat. But the armyworm swarms now hover on the edge of the North China Plain, the country’s top grain-producing region, which is scary news for a nation with pre-existing food supply woes and a slowing economy.

Life cycle

There are four distinct stages in an armyworm’s life, and during the summer months the entire cycle lasts about 30 days, 60 in the fall, and closer to 90 in winter months.

Eggs: Adult moths lay eggs in clusters of 50-200 on young plants. They hatch in waves.

Larvae: The most distinctive phase—these are the worms that just keep eating and eating, and eating.

Pupae: After around two weeks of all-you-can-eat corn, peanuts, rice, or Bermuda grass, the larvae burrow into the ground and pupate for a week to a month.

Moths: They emerge as dun-colored moths that will spend the next 10 days laying up to 2,000 eggs, while covering as much ground as possible. And it starts all over again.

Betting on bacteria

Bacillus thuringiensis (Bt) is a naturally occurring bacterium found in soil. It’s also one of the most widely used microbial pesticides in the world. When sprayed on crops, it’s deadly to a wide range of insects, safe for humans, accepted by organic farmers, and often used against armyworms. In the US, the vast majority of commodity corn is a genetically modified variety that has been approved by the US Department of Agriculture since 1996, and contains a Bt gene that staves off infestation by the likes of the fall armyworm. African and Chinese farmers are also looking to Bt corn as an armyworm solution, but suspicion of GM crops and the companies that sell GM seed has made planting Bt corn as political as it is agricultural.

Armyworms on the march

In the Americas, fall armyworms devour corn, rice, sorghum, and even grass, and they’re a familiar, if perennially dreaded, foe of farmers, landscapers, and lawncare fanatics alike. In January of 2016, they showed up in Nigeria, the species’ first known trip to Africa, and their spread across the continent and beyond has been rapid and devastating (pdf). In May of 2017, Ghana declared a state of emergency in response to armyworm crop damage, and by September of 2018 (pdf), their presence was confirmed in Gujarat, India.

In January, farmers near China’s border with Myanmar noticed fall armyworms for the first time. Now, less than six months later, the infestation spans the entire southern half of China, and has jumped to Taiwan and to the Philippines. In colder regions, like most of the US, armyworms are kept in check by the seasons—they can’t survive the winter. The pests are far more challenging to control in warmer regions where they can’t be frozen out.

Fun fact!

Fall armyworms aren’t just voracious consumers of crops—they also eat each other, even when there’s plenty of other food available.

Fighting bugs with bugs

“Biological control” involves introducing a predator to control the population of some undesirable critter—kind of like adopting a cat to keep out mice. Farmers in southern China started using citrus ants to control pests at least 1,700 years ago, and ladybugs and other beetles are still often used to combat crop-destroying insects. In the ongoing battle against armyworms, farmers in the Americas have used native insect predators known as parasitoid wasps. These critters lay their eggs inside other insects. A hatchling bides its time by feeding on its host’s insides until it finally bursts forth, Alien-style—an event that usually results in the host’s death.

With the armyworm barbarians now at the gates of China’s corn belt, the government is exploring biological control with an unlikely ally—stinkbugs. China’s top research institute is raising these predatory insects by the millions, with a mind to deploy them against the caterpillar invaders (link in Chinese). Stinkbugs kill by paralyzing their armyworm prey, then sucking the moisture out of the boneless bodies. One of the stinkbug species being tested can dispatch more than 40 armyworms a day, according to the research institute, and the plan is to breed 10 million stinkbugs annually, says Bloomberg.

By digits

14: Lifespan in days for an armyworm larva during the prime eating season of summer

1,500: Eggs a female armyworm lays on average during the 10-day moth stage of the life cycle

60 miles–300 miles (100 km–500 km): Distance an armyworm moth can travel in a single night

$600 million: Amount Brazil spends protecting crops from armyworms each year

20 per cent–50 per cent: Yield loss in corn crops in Africa, when infested

$2 billion–$6 billion: Estimated value of crop losses in Africa in 2018 due to armyworms

1 inch (2.5 cm): Average length of an armyworm

82 per cent: Share of the US commodity corn crop grown that is genetically modified to stave off pests, including armyworms

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