Centers Declare Victory Over the Cassava Green Mite

The Cassava Green Mite (CGM) is a pest responsible for between 30 and 50 percent yield loss of cassava, a starchy root crop in tropical Africa. Cassava is a staple food for more than 200 million people in sub-Saharan Africa. Rich in calories, highly drought tolerant, thriving in poor soils and easy to store in the ground, cassava is popularly called "the staff of life" for the poorest of the poor in Africa.

A breakthrough in CGM control was made possible through identification and mass release of an effective natural enemy of the CGM in Africas cassava regions. Peter Neuenschwander, Director of Plant Health Management at the International Institute of Tropical Agriculture (IITA) based in Ibadan (Nigeria), says: "The Institute had chosen the option of classical biological control strategy to fight crop pests and diseases because it constitutes the most environment-friendly technology."

In the early 1990s, IITA had already achieved a substantial breakthrough by introducing a biological control method which virtually freed sub-Saharan Africa of another important pest, the cassava mealybug. "The new breakthrough in GCM control through the use of predatory mites constitutes another milestone in the classical biological control of crop pests and diseases," says Neuenschwander. "However, the green mite has been much more difficult to control than the mealybug -- using the same biological control methods -- because so little is known about mites in general, and especially mites on cassava."

When both the cassava mealybug and the CGM appeared in the African continent in the 1970s, causing widespread damage and loss, the lives and livelihoods of millions of people were threatened. The first outbreak of the CGM, Mononychellus tanajoa, (in French: acarien vert) was noticed in Uganda in 1970. The pest now covers virtually all cassava producing areas of sub-Saharan Africa. Its attack on cassava shoots is most damaging during the dry season when its population explodes; it travels and multiplies unimpeded by rainfall mortality. "In the absence of natural controls, the mite swept through the 27 countries of Africas cassava belt," says IITA Acarologist Steve Yaninek who is leading a team of scientists at the IITA Biological Control Center in Cotonou, Benin.

Both pests originated in South America, the genetic ancestral home of cassava, where they do not cause much damage because natural enemies help suppress their populations make their presence largely insignificant. After the mealybug had been brought under control in Africa by introducing the parasitoid Apoanagyrus lopezi from South America, the battleground shifted to the CGM.

Control of the pest through application of toxic chemicals was ruled out because of possible adverse effects of chemicals on illiterate farmers and the environment. Also, disease pathogens and pests tend to gradually develop over time resistance to chemicals pesticides. Moreover, chemicals most pesticides are not selective and might destroy the natural enemies pests together with the peststheir natural enemies.

To effectively control cassava pests and ensure better living standards for cassava farmers in sub-Saharan Africa, an inter-continental integrated pest management research project was launched several years ago. The project involves international agricultural research centers in Africa and Latin America, national agricultural research systems, and farmers in a joint effort to develop an ecologically sustainable cassava plant protection strategy. Under the arrangement, IITA is collaborating with the International Center for Tropical Agriculture (CIAT), in Cali, Colombia, the Brazilian Agricultural Research Corporation (EMBRAPA) in Brazil, and several African national research systems.

The United Nations Development Programme provided four-year funding for this project to help scientists at the centers and in national institutions in four West African countries (Benin, Cameroon, Ghana and Nigeria) to work directly with farmers and extension agents. The project also enjoys support from the International Fund for Agricultural Development (IFAD), Denmark and Germany. The goal was to develop, test and implement ecologically sound cassava plant protection technologies.

Great emphasis was placed on farmer participatory research, a method in which farmers and scientists work as partners in identifying local problems and finding possible solutions. The Ecologically Sustainable Cassava Plant Protection (ESCaPP) project, known as PROFISMA in Brazil, brought together a large variety of scientists from the two continents, working directly with farmers and extensionists in addressing the daily problems faced in cassava production.

After evaluating many species of predatory mites -- natural enemies of CGM -- over a period of ten years, EMBRAPA identified several species from Brazil that can survive in Africa. One of them, Typhlodromalus aripo, was found to reduce pest populations by as much as 90 percent in the dry season when pest populations are usually high; in the wet season, pest attacks are not as severe and therefore the reduction in GCM populations is less dramatic.

After its introduction to the Africa's cassava belt in 1993, T. aripo found a conducive environment to prey on CGM. Impact assessment studies carried out by IITA at the sites where the CGM natural enemies had been released revealed that cassava yields increased by 35 percent within one season. Farmers are gaining about 70 US-dollars per hectare of cassava planted. In West Africa alone this adds up to a total profit per planting season of about $60 million for the cassava farmers.

According to Steve Yaninek, T.aripo was first released on cassava farms in 1993 in Benin after it had been transported from Brazil. It has subsequently been released in 11 countries and is now confirmed as established in all of them, except Zambia. T. aripo has also spread into Togo and Côte d'Ivoire from neighboring countries. It spreads about 12 km in the first year, and as much as 200 km in the second year. Today, the CGM predator has been established on more than 400,000 square kilometers of Africas cassava growing areas.

"T. aripo is able to spread quite easily," says Steve Yaninek, "because it has many food sources -- for instance red mites, whiteflies, maize pollens, honey dew, and plant exudates -- on which it can survive. However, in order to reproduce it requires mite prey." Tests have shown that in the absence of the CGM, T. aripo either disperses to find CGM or goes extinct locally, thus not becoming an ecological nuisance.

(International Institute of Tropical Agriculture (IITA)/Centro Internacional de Agricultura Tropical (CIAT)