Neozygites floridana (Weiser & Muma) Remaudière & Keller and Hirsutella thompsonii Fisher are important fungal pathogens of mites on cassava in Benin. Bioassays were conducted to evaluate the influence of four concentrations (50, 100, 200 and 500 ppm) of a water-dispersible formulation (Gaucho 70 WS) of imidacloprid on the conidial germination of these two fungi. Imidacloprid significantly reduced the germination of primary conidia and formation of infective capilliconidia in N. floridana. In contrast, imidacloprid concentrations of 100 ppm and above increased conidial germination in H. thompsonii. Another bioassay was conducted to evaluate possible synergism between imidacloprid and H. thompsonii in the mortality of the cassava green mite, Mononychellus tanajoa (Bondar). Hirsutella thompsonii alone and in combination with imidacloprid both killed around 95% of the mites after five days. In contrast, the mortality of mites treated with imidacloprid alone was close to that of untreated mites, which was around 30% on the fifth day after treatment.

Cassava varieties resistant to the cassava green mite, Mononychellus tanajoa (Bondar) ( Acari:Tetranychidae) have been reported in many parts of Africa where the pest has been introduced. However, the resistance mechanisms have not been identified. Laboratory studies on biology and feeding behaviour of cassava green mite were carried out at the International Institute of Tropical Agriculture (IITA), in order to determine whether this resistance is mainly due to pubescence.

Cassava pubescence was found to have no antibiotic effect; both pubescent and non-pubescent varieties were preferred by the mite for oviposition, food and shelter. In addition, pubescence does not afford a mechanical barrier to cassava against green mite infestation.

The various pests of the cassava plant, Manihot esculenta Crantz, were reviewed with particular reference to the two exotic pests viz: cassava mealybug,Phenacoccus manihoti, Mat-Ferr, and cassava green mite, Mononychellus tanajoa Bondar which were accidentally introduced into Africa without their natural enemies in the early seventies.

Classical biological control programme for these two pests initiated and championed by the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria was also reviewed and both successes and limitations of the programme highlighted. Suggestions on how to improve the efficiency of the programme are made.

The damage threshold of the green spider mite, Mononychellus tanajoa (Bondar) on cassava was determined in the greenhouse using four levels of mite infestation namely: 5, 10, 15 and 20 adult mites per cassava plant. Cassava cuttings were planted in plastic buckets in the greenhouse. The plants were artificially infested at 10 weeks old after planting.

Visible symptoms of mite damage appeared on cassava leaves at 4 weeks after infestation and at this period, the average number of mites was found to be eight mites per leaf. The mean number of active stages of M. tanajoa (larvae, nymphs and adults) was highest at the infestation level of 20 mites per plant, followed by that of 15, 10 and 5, in that order. The mean number of eggs was also highest at the infestation level of 20 mites per plant.

A study of several populations of the cassava green mites from eleven African countries representing wide ranging ecological zones were studied using a Principal Component Analysis. This study revealed that there is only one species of the Mononychellus present on cassava in Africa and is here designated M. tanajoa.

The green cassava mite, Mononychellus tanajoa (Bondar) and the red spider mite, Tetranychus urticae Koch infest all varieties of cassava in Sierra Leone. Both tetranychid species are widely distributed in the country and are causing severe damage to the crop mainly in the major cassava producing areas in the north-western regions. M. tanajoa caused chlorosis and 91° loss in photosynthetic area of the attacked leaves and defoliation of the plant either by acting alone or in concert with T. urticae. Late planted crop was particularly vulnerable to the infestation. M. tanajoa and T. urticae appeared to avoid severe interspecific competition on the plant by habitat stratification.

The Africa-wide Biological Control Programme of Cassava Pests (ABCP) was established in 1980, following requests for assistance by an increasing number of African countries. Its objectives in the short term are to: (1) achieve permanent, ecologically safe and economically sustainable control of the cassava mealybug (CM), Phenacoccus manihoti, and cassava green mite (CGM), Mononychellus tanajoa, throughout the African cassava belt, (2) provide specialized training in biological control techniques and (3) initiate National Biological Control Programmes (NBCPs). In the long term, the objective is to establish full-fledged NBCPs in countries requesting assistance by using the ABCP as a model. ABCP research thus far includes: exploration for natural enemies in South America, bionomic studies of the pests and their natural enemies, field ecology, modelling plant growth, and experimental ground releases and large-scale aerial releases of CM and CGM natural enemies. Epidinocarsis lopezi, a CM parasitoid, has been established in 13 countries within 2 years of its release. CM populations are being maintained mostly below the injury level in all areas of establishment where detailed surveys have been made.

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.

A systems model is described for cassava, Manihot esculenta Crantz, two of its introduced herbivores, the cassava green mite (CGM), Mononychellus tanajoa (Bondar), sensu lato, and the cassava mealybug (CM), Phenacoccus manihoti Mat.-Ferr., the introduced CM parasitoid, Epidinocarsis lopezi (DeSantis) and coccinellid predator of the genus Hyperaspis. The systems model includes the effects of weather, soil nitrogen and water levels on the interactions of the system's components.

The model simulates the distribution of developmental times of cohorts initated at the same time, as well as the number and biomass (energy) dynamics of all populations over time. Biomass acquisition and allocation at the population and organism subunit levels (e.g. leaves, fruit, ova) were also simulated. A common acquisition (i.e. functional response) submodel was used to estimate daily photosynthetic as well as nitrogen and water uptake rates in cassava, in addition to herbivory, parasitism and predation rates for the arthropod species.

This paper presents an overview of the systems model. Simulation results for the plant under pest free conditions were compared to field data. In addition, the model was used to estimate tuber yield losses due to CM and CGM feeding, and to examine the beneficial effects of introduced CM natural enemies as measured by reductions in tuber yield losses.