THE race to get to grips with one of the most destructive pests on the planet is gathering pace. The fall armyworm has ruined billions of dollars of crops in Africa over the past few years and is spreading quickly. Last week, Japan agreed to buy a huge consignment of maize from the US, largely because of fears its own crop will be eaten by the pest. With the prospect of this marauding insect reaching Australia and even Europe before long, scientists are now working frantically to stop it.
The first thing you need to know about the fall armyworm is that it is actually a caterpillar, the offspring of an innocuous-looking brown moth. It was until recently found just in South America and southern areas of the US. The pest is a big problem there, but natural predators stop it from causing a total disaster.
Its march further afield was first noted in January 2016, when . Over the next six months, they were found in three other west African countries: São Tomé and Príncipe, Benin and Togo. Three years on, we still don’t know how they got to Africa. We do know that the caterpillars eat many vital food crops, including rice, sugar cane, sorghum and especially maize. They can destroy a farmer’s entire crop in a single night.
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The alarm was soon sounded in Africa. “We got involved quite early on,” says Roger Day at the Centre for Agriculture and Bioscience International (CABI) office in Nairobi, Kenya. CABI runs to farmers. Fall armyworm quickly became one of the main concerns.
By August 2017, the pests had spread to 28 countries in Africa. In that time, Day’s team had established that, if left unchecked, infestations could in Africa’s maize-growing countries. That would result in an economic loss of up to $6.2 billion.
“The caterpillars eat rice, sugar cane and maize, and can destroy a farmer’s entire crop in a night”
Governments and NGOs began drawing up action plans. There are four key strategies, says Toby Bruce at Keele University, UK. The first is to identify crops that are more resistant to the armyworms, and grow those in place of vulnerable ones.
The next two strategies are linked. One is to grow flowering plants like tick clovers alongside the maize. “They release a smell that repels the moths that lay the eggs that develop into these caterpillars,” says Bruce. The farmer also grows “trap crops” elsewhere that lure the moths away from maize. Bruce’s team has been trying this push-pull strategy in western Kenya. Early tests suggest it is , reducing infestations by an average of 83 per cent.
Finally, it should be possible to unleash predators that will control the fall armyworms. The main candidates are parasitoid wasps. These lay their eggs inside other animals, including caterpillars, which are then eaten from the inside when the eggs hatch. It is wasps like this that control the armyworm in the Americas.
Could those wasps be imported to quash the pest in Africa? “That requires a lot of safety testing to make sure you’re not introducing something that’s going to cause more problems,” says Day. His team is planning to safety test one or two species in Africa as soon as possible.
This approach has worked before. In the 1970s, another South American pest, the cassava mealybug, was accidentally introduced to Africa, causing havoc. It was successfully controlled by introducing a parasitoid wasp from its home range.
A less risky tactic would be to use a parasitoid that is native to Africa. Bruce’s colleagues have been trying to identify such an insect by rearing armyworms and watching to see what emerges from within them.
However, the most promising wasp appears to be Telenomus remus. This species is known to target armyworms in South America and was being considered for import when, in March, researchers reported that it was already established in at least five African countries. This means it could be reared and released in large numbers without such stringent safety testing.
“Unchecked armyworm infestations could destroy more than half of the maize grown in Africa”
There are other strategies. One would be to use GM crops that contain a gene that allows the plant to make an insecticide. But these are banned in many African countries and may be too expensive for smallholders to use anyway. “The economic model is going to be very different and I don’t know how you make it compatible with the smallholder situation,” says Melanie Bateman at CABI’s office in Delémont, Switzerland.
Another possible solution would be to use a pesticide. A study from researchers in Ethiopia this year looked at a range of commercial pesticides and . But the UN’s Food and Agriculture Organization says pesticides should only be . If used inappropriately, they can have negative side effects, such as killing parasitoid wasps. Bateman says there are better alternatives such as the pesticidal oil of the south Asian neem tree and viruses or nematode worms that specifically attack armyworms.
Meanwhile, the caterpillars are still marching. In 2018, they were spotted in India, then Sri Lanka, Yemen and Myanmar. This year, they have been confirmed in China and, as of July, they are in Japan too. Day and his colleagues say Australia could be next.
There is one saving grace: the pest can’t survive freezing temperatures, which should limit its spread north. But there are a few places in Europe where it could overwinter and many others where it could live during warmer months. The one thing stopping it reaching Europe at the moment is the natural barrier of the Sahara desert. “But with time it’ll spread across northern Africa,” says Day. “Then it’s an easy hop into Europe.”
