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Thailand brings in wasps to save cassava industry

Mealybugs are a major cassava pest (Neil Palmer (CIAT))
Mealybugs are a major cassava pest
Neil Palmer (CIAT)

The tiny wasp that once saved Africa's cassava farmers was brought to Thailand last year, launching a new campaign against a globe-trotting pest. Responding to the rapid destruction of a key agricultural industry by the cassava mealybug, the Thai Department of Agriculture procured the wasps, a natural South American mealybug predator, from the engineers of the successful biocontrol progamme in Africa in the 1980s. The Department is now raising and releasing a quarter of a million wasps in Thailand.

World travellers

Portuguese traders carried cassava from South America across the Atlantic and Indian Oceans centuries ago. The crop survived long sea voyages better than its pests, so it proved a runaway success in the new environments of Africa and Southeast Asia. It took the age of air travel for South American pests, like the cassava mealybug (Phenacoccus manihoti), to catch up, but when they did - first in Zaire in the 1970s - the epidemic threatened to wipe out cassava production across Africa.

Fortunately, an intercontinental research effort and a pioneering experiment in biocontrol turned the tide in the 1980s. International Institute of Tropical Agriculture (IITA) scientists in West Africa worked with the International Center for Tropical Agriculture (CIAT) in Colombia and dozens of other partners, universities and governments to bring a wasp (Anagyrus lopezi) to Africa, preventing a looming agricultural emergency. When a replay of the same disaster began last year in Thailand, researchers were prepared.

Cassava production is highly industrialised in Southeast Asia, with about 5 million farmers growing the starch crop for domestic and foreign processors, who convert it to everything from tapioca to glue and ethanol. Thailand's growers alone produce more than 60 per cent of global exports of the crop. With the appearance of the mealybug, the Department of Agriculture estimated losses of 40-50 per cent, adding up to more than US$150-200 million in crop damage in the first year alone.

Saviour in a suitcase

It is hoped the tiny parasitic wasp will help tackle the mealybug outbreaks in the country (Rod Lefroy (CIAT))
It is hoped the tiny parasitic wasp will help tackle the mealybug outbreaks in the country
Rod Lefroy (CIAT)

Acting fast, the Department contacted IITA Scientist Emeritus Peter Neuenschwander to get help from the original experts in bringing the tiny parasitic wasp to Asia. With the Thai Government's clearance, IITA entomologist Georg Goergen personally hand-carried a colony of 500 wasps on a flight from Benin to Bangkok and oversaw the technology transfer. Following a large-scale mass rearing effort by the Department and the Thai Tapioca Development Institute (TTDI), the first official release began in July in the country's northeast.

Tony Bellotti, a CIAT entomologist who was part of the original team who found the wasp, can attest to the benefits of four decades of experience with the mealybug. "The result was that this time, the scientists could identify the pest much more quickly and we know the key to controlling it. Instead of ten years, it's taking only a year or so to respond." And none too soon: with the Thai release under way, scientists are already investigating new reports of mealybugs in Cambodia, Burma, Laos and Vietnam. Cassava growing areas of southern China, Indonesia and the Philippines are considered vulnerable.

Dr. Bellotti is confident of the wasps' abilities in Southeast Asia. The parasite has proven as adaptable as the mealybug itself: in Africa, there is no single environment or climate invaded by the mealybug where the wasp was unable to follow. The insect begins each lifecycle by injecting its eggs into mealybugs; as the eggs hatch and feed on the mealybugs, the pest population falls drastically. A. lopezi can function with low host populations - and keep them low - thanks to what Dr. Bellotti calls a high searching capacity. "The wasp keys in on plant volatiles that the cassava plant releases into the air when it's being injured. These are called synomones, and the plant uses them to alert the wasp that the pest is there."

Five million farmers grow cassava for domestic and foreign processors (Neil Palmer (CIAT))
Five million farmers grow cassava for domestic and foreign processors
Neil Palmer (CIAT)

Restoring balance

A. lopezi is clearly at the front line of this global fight, but other pest management measures will be required to maintain the wasp's winning edge, predicts IITA's Dr. Neuenschwander. "There are no mealybug-resistant cassava varieties, but some are clearly less susceptible and are preferred by African farmers. We have also shown that naked, sandy soils boost mealybug reproduction so much that the wasps can't keep up, but this can be reversed by mulching. We can conclude that biological control by A. lopezi cannot replace good farming."

Most importantly, Thai farmers will need to be more careful about pesticides. "Pesticides have been tried out on mealybugs in almost all countries where they've appeared, and have never worked under local conditions," Dr. Neuenschwander warns. "Pesticides are more likely to destroy the wasps, which actively hunt their prey, than the sedentary and hidden mealybugs. In our trials, insecticide-treated fields have mealybug populations at least ten times higher than unsprayed fields." Now that A. lopezi is free in Southeast Asia, restraint in pesticide use will be the key to restoring a natural balance in the cassava fields.

Written by: T. Paul Cox

Date published: September 2010


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