Mealybug pests followed cassava from South America to Southeast Asia, and now a parasitic wasp is tagging along to hold the mealybug population in check.

At the start of a carefully crafted emergency campaign to thwart a pest outbreak that is wreaking havoc on Thailand's vital cassava crop, agricultural researchers are releasing parasitic wasps, the pest's most effective natural enemy, in the eastern part of the country.

Mealybugs on cassava crops in northeastern Thailand. Photo: Neil Palmer/CIAT.

Thailand accounts for more than 90% of global exports of this tropical root crop, which is critical for food security and economic growth in many developing countries. About 5 million growers across Southeast Asia supply cassava to domestic and foreign processers, which extract starch from the roots for a wide variety of food and other products and also convert them into animal feed and biofuel. "Cassava is an important crop for small-scale farmers in our country, so there's no time to lose in applying the fastest, most reliable solution available, which is biological control using the parasitic wasp species Anagyrus lopezi," said Amporn Winotai, a senior entomologist in Thailand's Department of Agriculture.

The cassava pest, a mealybug (Phenacoccus manihoti) originating in South America, sucks sap from the plants, causing them to shrivel. Cassava is itself a South American native, carried by Portuguese traders to Africa and Asia, where it thrived in the absence of the insect pests that inhabit its home territory.

But, eventually, the mealybug and other pests caught up with cassava, devastating crops first in sub-Saharan Africa and now in Southeast Asia. The spread of P. manihoti has been confirmed on 160,000 hectares in eastern and northeastern Thailand, where the pest is cutting yields by as much as half.

"Since the country's cassava industry generates about US$1.5 billion of income each year, reductions of that magnitude could translate into hundreds of millions of dollars in economic losses if the pest is allowed to spread further," said Tin Maung Aye, an agronomist with the International Center for Tropical Agriculture (CIAT by its Spanish abbreviation).

In mounting the emergency campaign, Thai scientists consulted with the International Institute of Tropical Agriculture (IITA) and CIAT. These two Centers and various partner organizations curbed mealybug attacks on Africa's cassava crop during the 1980s through a highly successful biocontrol campaign, which staved off a major food security catastrophe, according to IITA entomologist Georg Goergen, who took a colony of live wasps from Benin to Bangkok last year for mass rearing.

Identifying P. manihoti in Thailand was at first complicated, Goergen reported, by its resemblance to another, closely related mealybug species, P. madeirensis, which is probably also from South America but poses no threat to cassava.

Within a year after confirming the presence of P. manihoti, Winotai recounted, her team had arranged to import the parasitic wasps, following strict quarantine procedures. They then carried out mass multiplication and controlled field testing. "The results convinced us that it was time to begin releasing the wasp on a large scale to contain the damage as soon as possible," she added.

Early on, researchers discarded the option of containing the mealybug in Thailand with pesticides. "Applying chemicals on such a large scale would be environmental vandalism," said Tony Bellotti, a CIAT entomologist, who has spent 35 years investigating cassava pests. "Sending in the wasps is a proven way to kill mealybugs quickly and effectively."

Measuring just 2 millimeters in length, A. lopezi has already shown itself to be a formidable natural enemy of the cassava mealybug in South America and sub-Saharan Africa. Even when infestations are low, female wasps are able to detect and home in on their prey, injecting their eggs into the mealybugs. As the wasp larvae grow and as adult females feed on the host insect, the mealybug population starts to decline. The wasps pose no threat to humans or other animals.

A. lopezi proved so effective in sub-Saharan Africa that Hans Herren, the scientist who led the biocontrol effort there, was awarded the World Food Prize in 1995. The collaborative effort also earned IITA and CIAT the CGIAR's 1990 King Baudouin Award, which recognizes outstanding contributions to agriculture in developing countries. The economic benefits from biocontrol of the cassava mealybug exceeded the research investment by a factor of at least 200.

CIAT scientists are investigating reports that that the cassava mealybug has already spread to Cambodia, Laos and Vietnam. Bellotti expects that it will soon reach other parts of Southeast Asia as well, including Myanmar, southern China, and, eventually, Indonesia and the Philippines.