Cassava green mite (CGM), Mononychellus tanajoa s.l., originating from South America, has become a serious pest in Africa, where it often occurs in mixed infestations with indigenous spider mites like Tetranychus lombardinii. The neotropical phytoseiid Neoseiulus idaeus is a candidate predator for classical biological control of CGM. Its prey preference and reproductive success on CGM and T. lombardinii were studied in the laboratory. Foraging individually on leaf discs, N. idaeus consumed CGM and T. lombardinii adult females equally in single-species populations but showed a distinct preference for CGM when both species were presented together at the same density. The predators developed and reproduced equally well on CGM and T. lombardinii.

A regional programme, involving seven national programmes and three international agencies, has been launched in East Africa to promote research on the biological control of cassava green mite, Mononychellus tanajoa (Bondar) sensu lato. The organization and objectives of the programme are described and outline results of the first 2 years of work are reported. Trials to assess losses due to mites were carried out at 10 sites, using acaricides to protect split plots of two to five varieties in each trial; at 6 sites, significant losses of tuber yield were recorded on one or more varieties. Surveys of indigenous natural enemies indicated that the predator complex is similar throughout the region and does not provide adequate green mite control. Two Neotropical phytoseiid predators have so far been introduced for local evaluation. One of these, Neoseiulus idaeus, has been released at 12 sites in three countries; recoveries were made at several sites, but no impact on the pest population was detectable. Plans for further collaborative work are discussed.

Systems of different technological levels have been developed for the mass-rearing of cassava mealybugs, Phenacoccus manihoti, cassava green mites, Mononychellus tanajoa and their natural enemies. Prototypes have been built and are being evaluated for host plant growth, host insect and mite production and output of natural enemies. One of the prototypes uses a paternoster system in which trays each holding 66 cassava plants rotate around a light source. After each rotation the trays receive and empty a given amount of nutrient solution. The other uses a perforated vertical cylinder holding up to 250 cassava cuttings. The plants are irrigated through a pump or gravity system. Six to eight weeks are required to complete the cycle from planting to harvest of the natural enemies in either system. These systems reduce significantly the labour input required for conventional rearing systems and at the same time improve the reliability and quality of the end product.

In Africa, cassava green mite (CGM) is an exotic tetranychid, recently introduced from the Neotropics. In the absence of effective natural enemies, seasonal changes in the weather, either directly through several climatic factors or indirectly through the host plant, dominate the phenology and limit the growth rates of CGM populations. The intrinsically high rate of increase of CGM results in large populations when enhanced by high temperatures and suitable host plants during periods when evaporation exceeds precipitation. Mite numbers decline as precipitation increases and when suitable host plants are not available. The suitability of a host plant, as measured by CGM's intrinsic rate of increase, varies with drought stress, plant age, leaf age and cultivar. The introduction of effective natural enemies from CGM's area of origin is expected to dampen the seasonal abundance of this mite without changing its general phenology.

A programme to control the cassava green mite (CGM), Mononychellus tanajoa, using classical biological control was initiated in 1983 as part of the Africa-wide Biological Control Programme (ABCP). Basic studies on the field ecology and biotic potential of CGM, and surveys of CGM and associated indigenous natural enemies found on cassava throughout Africa are being used to develop ecologically sound release and follow-up strategies. Applied work includes colony maintenance of exotic phytoseiids, development of methodologies for mass producing, transporting and releasing natural enemies, plus experimental releases and post-release surveys. Important parts of the work are being done in collaboration with several institutes and agencies, including foreign exploration and preliminary screening of exotic natural enemies, international quarantine services, regional release and follow-up surveys, taxonomy of CGM and phytoseiids, simulation modelling of CGM in the cassava ecosystem, artificial diets for transporting CGM natural enemies, survey of entomopathogens of CGM and predatory behaviour studies of exotic phytoseiids.