In DepthFood Security

New crop pest takes Africa at lightning speed

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Science  05 May 2017:
Vol. 356, Issue 6337, pp. 473-474
DOI: 10.1126/science.356.6337.473

In Rwanda, the drab caterpillars were first spotted last February. By April, they were turning up across the country, attacking a quarter of all maize fields. As farmers panicked, soldiers delivered pesticides by helicopter and helped pick off caterpillars by hand.

Unknown in Africa until last year, the fall armyworm (Spodoptera frugiperda) is now marching across the continent with an astonishing speed. At least 21 countries have reported the pest in the past 16 months. “This has come like a storm,” says Sunday Ekesi, director of research at the International Centre of Insect Physiology and Ecology (ICIPE) in Nairobi.

The larval stage of a small gray moth, the fall armyworm can devastate maize, a staple, and could well attack almost every major African crop. One research group estimates that the damage to maize could total $3 billion in the next 12 months. “It's really a worrying scenario,” says B. M. Prasanna of the International Maize and Wheat Improvement Center (CIMMYT) in Nairobi, which co-hosted a meeting last week at which experts emphasized the need for research on cheap, safe tools for farmers fighting the worm.

Armyworms get their name because when the caterpillars have defoliated a field, they march by the millions to find more food. The adult moths can travel hundreds of kilometers per night on high-altitude winds. The endemic African armyworm (S. exempta) already causes major crop losses every few years. But the fall armyworm, a native of the Americas, causes more damage because females lay their eggs directly on maize plants rather than on wild grasses, and the caterpillars have stronger, sharper jaws. In Brazil, controlling it costs farmers an estimated $600 million a year.

The worm probably came to Africa as a stowaway with plants or produce. Caterpillars were first found on Nigerian research fields in January 2016. Since then, the pest has reached South Africa and raced across eastern Africa (see map, below). “The speed has shocked me,” says insect ecologist Ken Wilson of Lancaster University in the United Kingdom. Zambia apparently has suffered the worst infestation, affecting about 223,000 hectares of maize. Some 90,000 hectares, 8% of the total crop, were a total loss.

In many other countries, damage reports are still preliminary. “We don't yet know if this is going to cause a food security crisis,” Wilson says. In the Americas, the armyworm feeds on more than 80 plants, seriously damaging maize, sorghum, and pasture grass.

The response in Africa has often been haphazard and ineffective, agriculture experts say. Worried governments bought pesticides and donated them to farmers, but in some places, the chemicals arrived after the caterpillars had grown large enough to survive spraying. Older caterpillars are also protected from many chemicals because they hide during the day in the whorls where maize leaves join the stalks. “The fall armyworm is like an insurgent,” says Sina Luchen, an agronomist with the United Nations Food and Agriculture Organization (FAO) in Johannesburg, South Africa. “It comes out at dawn and dusk to feed, then it hides again.”

In the Americas, the worm has evolved resistance both to several pesticides and to some kinds of transgenic maize. A team led by entomologist Johnnie Van den Berg of North-West University in Potchefstroom, South Africa, is testing whether it has brought any resistance with it to Africa. Regardless, most African farmers should not rely on pesticides, experts say. “If you encourage growers to apply pesticides heavily, this is a recipe for disaster,” Ekesi says. Poor farmers cannot afford the compounds or the protective equipment to apply them safely; some may not use them properly. In Zambia, farmers have sprayed so much pesticide since December 2016—the government alone gave out more than 100,000 liters—that the Ministry of Agriculture issued a warning against misuse.

CREDITS: (GRAPHIC) A. KITTERMAN, SCIENCE; (DATA) KEN WILSON, CIMMYT
PHOTO: RDF

Nonchemical control is an appealing option, and several such strategies are in use in the Americas. Some farmers fight the armyworm with neem oil and other natural products; Ekesi says neem extract mixed with grit has helped deter fall armyworm in Zambia and Malawi. Brazil has released parasitoid wasps to devour caterpillars; they could be imported to Africa, if regulators approve. Another approach could be to rear and release African predators of the fall armyworm, but they would first need to be identified. There's also a biopesticide, sold in the Americas, that is based on a virus. Wilson is now testing whether a virus endemic to Africa and lethal to the African armyworm also kills its newly arrived cousin; if so, it might win faster approval than a foreign virus. Those viruses can be mass-produced and sprayed on crops. Crop management may help, too. ICIPE has long trained farmers in Zambia and Malawi to deal with pests by planting an insect-repellent legume alongside the maize. That seems to help deter fall armyworms as well, Ekesi says.

CIMMYT researchers in Kenya will test nontransgenic maize with partial resistance against fall armyworm in the coming season, but Prasanna expects it could take 4 to 5 years before new varieties are ready for farmers. Eventually, a variety of pest-management techniques could help prevent major economic damage. “It's a big problem,” says Joyce Mulila-Mitti, a plant protection officer with FAO in Harare. “But it's not insurmountable.”

That's fortunate, because the pest appears likely to spread beyond Africa. The moths will probably arrive in Yemen within a few months, Wilson says. Migration or trade also could bring the pest to Europe, he adds, making it important to inspect imported plant material and conduct field surveys with pheromone traps. If the species reaches Asia, says entomologist Ramasamy Srinivasan of the World Vegetable Center in Taiwan, “its introduction might have a huge economic impact.”

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