For cassava pests, biological control on Africa-wide basis

Tropical crop specialists are convinced that a $30 million investment in a biological pest control system over the next five years could rapidly and significantly reduce the numbers of the two major insect pests currently causing an estimated $2 billion damage annually to Africa's cassava harvest. Governments of a score of countries in western, central, and eastern Africa have signaled their confidence in the system by requesting trial releases of natural enemies of the two pests - the cassava mealybug and the cassava green spider mite.

The proposed project, a collaborative effort of a number of national and international agencies, would involve the establishment of a centre for mass production of the pests' natural enemies and their release, by millions, from both ground-level and low flying aircraft. Preliminary trial releases over the past four years have already established the effectiveness of certain parasitic species in reducing mealybug populations - and damage.

Although much of the funding for the project remains to be committed, its organizers have decided to step up the pace of research and development in order to be prepared for a continent-wide campaign. To be established under the auspices of the Scientific, Technical and Research Commission of the Organization for African Unity (OAU/STRC), the project would have two components: a research programme that would be the responsibility of the International Institute of Tropical Agriculture (IITA), which has played a leading role in the project's development thus far, and an operational programme that will be managed by a board representing FAO, IITA, OAU/STRC, and both donor and participating governments.

Research already carried out has yielded promising results. After investigating other potential methods for control, including pesticides, cultural practices, and the breeding of resistant cassava varieties, IITA opted for biological control as the safest, simplest, speediest, and most economical means, and the one most likely to be acceptable to farmers. Research then centred on the original South American habitats of the two pests (they have been found in Africa only during the past dozen years) in order to identify effective natural enemies. With the collaboration of a number of other research institutions including the International Centre for Tropical Agriculture (CIAT) at Cali, Colombia, and the Brazilian Government's research institute, EMBRAPA, a total of 17 species of the mealybug's natural enemies and one predator of the green spider mite have been located. Six of these species have already been brought to IITA at Ibadan, Nigeria, having first cleared rigorous quarantine procedures at the Commonwealth Institute of Biological Control in London to ensure that they attack only the insect pests and that they will not themselves become pests.

The most promising of these introductions is a parasitic wasp, Epidimocarsis lopezi, which has been field tested over the past three years in Nigeria and Zaire, and which is credited with reducing mealybug populations and the damage caused to cassava crops in the release area. A later survey indicated that the wasp had spread outward over more than a 100-km radius from the farm where it was originally released.

With 20 countries in Africa's cassava belt now asking for trial releases of these beneficial insects, the question of an adequate supply of parasites becomes critical. IITA has investigated a number of options for mass production systems and has drawn up preliminary specifications or a unit that could produce 15 million beneficial insects per day. As insects emerge from the production unit, they are transferred to a packaging room where parasites-as many as 1 500, depending on the species - are placed in small plastic capsules which are put in turn into cassettes at are kept in specially designed containers with their own cooling system. These are loaded into aircraft which fly over cassava fields at speeds of 250 to 330 km/hr releasing about one capsule per second. Specially designed wind tunnel tests carried out in Austria have shown that the resulting accelerations and air currents do not harm the released insects.

If the proposed project can be successfully mounted on a continent wide basis, it would offer some prospect of helping to overcome the chronic problem of low yields in Africa's cassava fields. Although the African area planted to cassava, considerably more than half of the global total, output is only about 40 per cent of world production, reflecting yields that averaged about six metric tons per hectare, compared with an average of more than 11 tons per hectare attained in Asia and South America. Researchers at IITA had made significant progress during the 1970s in developing improved cassava ones that were resistant to such diseases as cassava mosaic and bacterial blight that often combined to cut yields by as much as 90 per cent, only to have the sudden invasion of the mealybug and green spider mite pose a new menace.