Carnivorous Mite Protects African Cassava

A major breakthrough in the control of Cassava Green Mites (CGM) has been recorded by scientists at the Biological Control Program of the International Institute of Tropical Agriculture (IITA).

Having received the 1991 King Baudouin Award for their highly successful biological control program against the cassava mealybug, a major foodcrop pest in Africa, scientists at IITA's Africa Center in Cotonou, Benin, are now employing the same strategies for the control of another less damaging but economically important cassava insect pest in Africa -- the Cassava Green Mites (CGM).

CGM is a major cassava pest causing about 30 percent yield reduction and is only second to mealybug in terms of devastation and spread. It often causes total failure in heavily infested fields because the plants are usually poorly established without adequate leaves to manufacture nutrients for the plant's survival. CGM was first noticed in Uganda in 1971 and has since invaded almost all of the cassava belt in the humid areas of Africa.

The control of CGM by parasites proved difficult because no known effective parasites of the pest were found. All other natural enemies tested for CGM control have not succeeded because they did not establish well in Africa.

The green mites are tiny insect pests sucking juice from cassava shoots and leaves, gradually removing the entire chlorophyll required by the crop for good leaf formation, better crop establishment and tuberisation. The plants remain stunted with poorly developed leaves. The leaves play an important role in cassava growth. In a healthy plant, the leaves which resemble a human palm must open all the ''fingers'' to receive the sunlight. The greenness of the leaves is an evidence of effective photosynthesis in the plant.

The excess food in the plant is stored as carbohydrates in its roots -- the main staple food of over 200 million people in sub-Saharan Africa, considered a 'poor man's crop' and an invaluable fall-back in hunger years because of its high drought tolerance.

The CGM's life cycle is also affected by weather. They cannot fly but are easily dispersed by wind especially in the dry season. Unlike the mealybug which forms a cocoon on which the parasite lays its eggs, the CGM is restlessly moving and can only be effectively attacked by similarly mobile insects.

Explaining why it has taken scientists so long a time to find an acceptable remedy to the problem of the CGM, Dr. Benoit Megevand, an IITA entomologist at the Biological Control Program, said ''unlike the mealybug which has an effective parasite that established well in the cassava belt of Africa, the CGM has no known parasites that can work effectively in African ecologies because the pest has many different species.'' What controls a particular species of CGM may not necessarily control the other, he added.

After more than ten years of research, the scientists have been able to identify predatory mites--phytoseiids-- that feed on CGM. Imported from Brazil, the phytoseiids have established well after experimental releases in Benin, Burundi and Kenya. They have succeeded in reducing the population of the CGM in all the sites chosen for the release. To that effect, more releases are being undertaken in collaboration with national agricultural research systems in Benin, Cameroon, Ghana and Nigeria.

''The phytoseiid mites have so far proven to be the best way of achieving classical biological control of CGM," Dr. Megevand said. They are not parasites but they are carnivorous mites feeding on the CGM while not attacking cassava at all.

Three species of the phytoseiids have been identified to establish well in different African ecologies. Imported from Brazil, the predators have earlier been well established in laboratories and field experiments. IITA scientists have now embarked on mass rearing of the mite eating mites for further field releases.

(IITA news release)