Following the introduction in Congo of the cassava mealybug, Phenacoccus manihoti in 1973, a French Congolese team was set up to tackle the problem. The main results of the team's studies that aimed to the effectiveness of Epidinocarsis lopezi a parasitoid introduced in Africa from South America by IITA, are reported here.

The studies were simultaneously developed in the laboratory and in the field, In the areas of biology, physiology, behaviour and ecology of the Encyrtidae. These studies allowed the drawing up of a balance of its qualities and limits observed, its effectiveness in natural conditions and provide the opportunity to share some ideas on the use in Africa of the South American's auxiliary. The different areas of the studies are successively examined.

Role of trichome density in cassava resistance to mealybug, Phenacoccus manihoti Matile-Ferrero was evaluated on the apex, the first expanded leaf and the stem in pubescent and glabrous cassava varieties. Pubescence significantly retarded the rate of settling and establishment of P. manihoti. The weight of the developing larvae was also affected becoming more pronounced at the pre-ovipositing adult stage. However, the relationship between pubescence and P. manihoti weight was not significant. Moreover, the significant differences in weight at the preovipositing adult stage was not reflected in the corresponding fecundity on the glabrous and pubescent varieties. Pubescence accounted for only 18% of the differences in observed fecundity. There was also no significant difference in the development period of the mealybug on glabrous and pubescent varieties. Pubescence appeared to be a mechanical irritant only.

Three severe outbreaks of cassava mealybug (CM), Phenacoccus manihoti, have occurred in Zaire. Two were associated with droughts. The third occurred in the driest part of the country and ended only when nearly all cassava plants had been killed. Native coccinellid predators sometimes greatly reduced CM populations, but only after the cassava crop had been heavily damaged. The exotic coccinellid Diomus sp. may be an effective biological control agent in Kinshasa, where CM on a hybrid form of cassava remained common during the rainy season. CM populations in Kinshasa declined greatly in the second year after the release of the exotic parasitoid Epidinocarsis lopezi. In one study the number of mummies with E. lopezi increased with host densities up to 20 CM per shoot tip. Since CM populations remained low after the releases of E. lopezi it is tentatively concluded that E. lopezi, despite considerable rates of hyperparasitism, prevents CM outbreaks over the whole area of its distribution (130,000 km2) in western Zaire.

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.

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.

Over 130 species of insects were found in association with the cassava mealybug (CM), Phenacoccus manihoti, and its newly introduced parasitoid Epidinocarsis lopezi in Africa. As the CM in SW Nigeria declined under the influence of E. lopezi, this introduced wasp maintained densities of 2.8–3 per CM-infested tip early in the dry season and 1.3–1.6 per infested tip late in the dry season, but declined overall from 1.3 to 0.2 per randomly collected tip within 1 year. The decline of CM led to a sharp reduction in numbers of indigenous coccinellids, particularly of Hyperaspis spp. and to a lesser extent of Exochomus sp., due to competition with E. lopezi for the common food source. Numerous species of other polyphagous predators and parasitoids of coccinellids were found in low numbers. Indigenous primary parasitoids, Anagyrus spp., were rare, but their 10 spp. of hyperparasitoids were often reared from E. lopezi, with Prochiloneurus spp. and Chartocerus spp. being the most common. Hyperparasitism declined from 41.3% in March 1983 to 16.9% in Dec 1984 and proved to be positively density-dependent. The effects of hyperparasitoids on this biological control programme are discussed.