Before chemical pesticides were invented, farmers relied upon local predators to control crop-devastating pests for millennia, but now the practice is getting a modern revival.
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Scattered among the highly biodiverse forests of South East Asia, millions of farmers eke out their livelihoods by growing cassava. This cash crop — grown by both small-scale farmers who own just one or two hectares and industrial farms spread across thousands of hectares — is sold mainly to manufacturers who use its starch in plastics and glues.

When cassava was first imported to South East Asia from South America (as it was to Africa a few decades earlier), it was able to grow without the help of pesticides. Then in 2008, the cassava mealybug followed the root vegetable to the region and began devastating the crops. To compensate for the losses, farmers began encroaching into the forests around their plots to try to get a little bit more produce from their land.

“Some of those areas are under significant pressure from deforestation,” says Kris Wyckhuys, an expert in biological controls at China Academy of Agricultural Sciences’ Institute of Plant Protection in Beijing. “Cambodia has some of the highest rates of tropical deforestation.”

The arrival of the cassava mealybug not only had major impacts on the livelihoods of those who grow cassava, it affected the national economies of the countries in the region and might have had rippling effects elsewhere.

Substitute products in the starch market like corn and potato rose in price. There was a threefold increase in the price of cassava starch in Thailand — the world’s number one exporter of cassava starch.

“When an insect reduces crop yields by 60–80%, you have a major shock,” says Wyckhuys. The solution was to find the mealybug’s natural enemy, a 1mm-long parasitic wasp (Anagyrus lopezi), in its native South America. This wasp is extremely selective about using the cassava mealybug as a host for its larvae. By late 2009 it had been introduced to the cassava cropland in Thailand and had started working its way through the mealybugs.

There’s no detailed information on how quickly the wasp drove mealybug populations down in the country. But by mid-2010, “parasitic wasps were being reared by the millions and mass-released throughout Thailand, including by airplane, and we can assume that their impacts on mealybug populations could be felt fairly quickly,” says Wyckhuys.
The cassava crop is incredibly important to the economies of South East Asia (Credit: Getty Images)

The cassava crop is incredibly important to the economies of South East Asia (Credit: Getty Images)

When the same wasp was used to control mealybugs in West Africa in the early 1980s, it promptly suppressed the pest population levels from more than 100 individuals on each cassava tip to fewer than 10–20. Less than three years later, the wasp had effectively dispersed over 200,000 sq km (77,220 sq miles) in southwestern Nigeria and could be found on the vast majority of cassava fields in the area.

This type of intervention is called classical biological control. You find a natural predator and introduce it to a crop to curb the spread of a pest. Wyckhuys calculated the economic benefit to the farmers across 26 countries in Asia-Pacific at $14.6bn to $19.5bn (£11.4bn to £15.2bn) per year. “The action of a 1mm wasp helped to resolve a major financial shock in the global starch market,” he says.

Biological control was the default for thousands of years, so it’s funny to think of it as new — Rose Buitenhuis

Our understanding of the benefits of the right predator in cropland stretches back millennia, though biocontrol has largely fallen out of fashion in modern farming practices. “Biological control was the default for thousands of years, so it’s funny to think of it as new,” says Rose Buitenhuis, a scientist at the independent horticulture science organisation, Vineland Research and Innovation Centre, in Ontario, Canada.

If biocontrol can be so successful, why is it now an uncommon method of fighting pests? What happens when it goes wrong? And why are researchers pushing to change that?