AFRICA has been invaded on quiet wings. First they landed by ship in the west. Then they spread across the continent, wreaking havoc as they went. Now, two years later, the invaders are worrying officials in almost every sub-Saharan country. It’s not the French, British or even the Chinese. This time it’s a simple American moth, the voracious fall armyworm, that has marched through Africa’s fields and is threatening to cause a food crisis.

When just a hungry caterpillar, the fall armyworm will happily munch on more than 80 plant species. But its favourite is maize—the staple for more than 200m sub-Saharan Africans. The UN’s Food and Agriculture Organisation (FAO) estimates that sub-Saharan Africa has about 35m hectares of maize grown by smallholders, and that almost all of it is now infested or at risk of infestation.

If the pest is not controlled, it could gobble up as much as 20% of the region’s total maize crop. Some countries may be particularly hard hit. The Centre for Agriculture and Biosciences International (CABI), an association of agricultural research centres in 12 countries, thinks that big producers such as Nigeria or Tanzania could lose more than half their maize harvest.

Originally from the Americas, these worms were a plague there for hundreds of years. Yet American farmers have beaten them back with the help of genetically modified plants and advanced pesticides. By contrast, the worms are meeting little resistance in Africa. They were first officially detected in Nigeria in January 2016. Now they can be found in 43 other African countries (see map).

Two factors explain their rapid spread. The first is biology. Africa already has its own variety of the worm, which farmers can control. But the foreign species migrates and reproduces much faster. After it turns into a moth, it can fly as far as 100km (60 miles) a night. During her ten days of adulthood, a female moth can lay up to 1,000 eggs.

The second is that most of Africa’s farming is done by smallholders who use outdated techniques and whose yields are already low. The worm “is coming on top of other constant threats faced by farmers, including drought, new crop diseases, and low soil fertility,” says Joe DeVries of the Alliance for a Green Revolution in Africa.

Yet labour-intensive farming also offers opportunity. Experts fret that if farmers use too much cheap pesticide to kill the worms, they may end up poisoning their crops. Allan Hruska of the FAO hopes instead to teach farmers to use some of the techniques that smallholders in the Americas have long used. These include mixing crops, encouraging natural predators and patrolling fields to crush the eggs by hand.

Better still would be to copy America’s commercial farmers, who plant GM crops that are largely resistant to the worm. Almost all African countries apart from South Africa have formally or informally banned GM crops, following iffy advice from ecowarriors. Lifting these restrictions would lead to fewer hungry caterpillars and fewer hungry people.