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.

Cassava is one of the most important staple crops in Ghana. Losses from the cassava mealybug (CM), Phenacoccus manihoti, and cassava green mite (CGM), Mononychellus tanajoa, infestations in 1983 were estimated at 0.8 m metric tons of tubers. An integrated control programme with biological control playing a key role has been established within the Plant Protection and Quarantine Unit of the Ministry of Agriculture. Of the five beneficial insect species released, only the parasitoid, Epidinocarsis lopezi has so far been established against the CM over the greater part of Ghana. Damage caused by the CM has been reduced quite remarkably, largely due to E. lopezi, with local beneficial insects playing some contributory role. The activity of hyperparasitoids is high and reduces the overall efficiency of E. lopezi.

The effect of CGM is prominent now, perhaps because of the reduction in CM. Exotic phytoseiid mite species were released but no establishment could be documented.