The demographic statistics of the ladybird Exochomus flaviventris Mader (Coleoptera: Coccinellidae) were investigated under laboratory conditions on the cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae), reared on four host plants with different levels of antibiotic resistance to the mealybug: two cassava (Manihot esculenta Crantz, Euphorbiaceae) varieties, Incoza and Zanaga, Faux-Caoutchouc (FC) (hybrid of M. esculenta × M. glaziovii Muel. Arg.) and water weed (Talinum triangulare Jack, Portulacacae). Preimaginal mortality, mean oviposition time and total fecundity of female ladybirds were strongly affected by the host plant even though there was no linkage with antibiotic resistance. The net reproduction rates R0 of the predator were 1.5,1.7 and 2.4 times higher on Zanaga than on water weed, Incoza and FC respectively, and the generation time (T) was significantly longer on Zanaga than on the other three plants. The intrinsic rate of increase rm was not modified by the antibiotic resistance of the host plants. With the two cassava varieties, Incoza and Zanaga, it was found that antibiosis significantly affected the life history parameters of the ladybird without modifying its fitness, and provided better cassava mealybug control.

The number of eggs and mobiles (immatures and adults) of Amblyseius manihoti on entire plants was determined during a 10-month period in a cassava field located in the Bahia region of Brazil. Observed percentages of eggs, immatures and adults were compared to theoretical stable age distributions (58 % for eggs, 24 % for immatures and 18 % for adults). These comparisons served as a basis to discuss the influence of fecundity, mortality and migration on the age structure of populations.

The development and morphometrics of the whitefly Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) were studied on cassava and a description was made of the different nymphal instars and oter biological characteristics. The nymphal instars were distinguished using easily recognisable morphological characteristics. Abnormally developing individuals were also easily recognised. Regression analysis was highly significant between the length and width of nymphal instars modelled collectively indicating two-dimensional growth at least across three nymphal stadia. Males were smaller than females on the basis of body length, wing width and width of the abdomen. The insects showed better development on a Colombian cassava variety as compared to an African variety, Ebwanateraka from Uganda, and consistently showed a higher proportion of males than females (2.33: 1) for early-emerging individuals. Mean developmental period on both varieties was 22 days and the first instars accounted for the major part of the developmental period which ranged from 19 to 29 days. The mean number of eggs/female/day was 4.89. The actual events of first instar eclosion, which can last up to approximately five minutes, were described. The intervals of honeydew flow to the vasiform orifice lasted a minimum mean time of 20.8 seconds and a maximum mean time of 123 seconds.

The aim of the present study was to determine, by a field experiment, whether the seasonal variations in the levels of cyanide and rutin in the phloem, secondary compounds potentially implicated in cassava resistance to the cassava mealybug (Phenaccocus manihoti), could be influenced by different kinds of soil fertilisation (manure, NPK, KC1, lime, mulch). It was found that mean cyanide and rutin contents varied significantly with season, soil treatments and infestation. For each soil treatment, the highest contents of these compounds on uninfested plants was registered in the main dry season (Jul-Aug) for cyanide and at the end of the dry season (Jul-Aug) and beginning of the rainy season (Oct-Nov) for rutin. The infestation factor induced an increase in cyanide and rutin contents for 43 combinations of period and soil treatment among 48 in total. For cyanide content (phagostimulant function in mealybug), this increase appeared significant at the end of the rainy season (Apr-May) and in the main dry season (Jul-Aug) for control and KC1 fertilisation. For rutin content (an unfavourable substance for the insect) significant increase was induced by infestation in each period for manure and mulch and especially in the main dry season (Jul-Aug) and at the beginning of the rainy season (Oct-Nov) for other soil treatments. Concerning the seasonal variations in the level of rutin, these results showed that mulch and manure are the best fertilisers to increase cassava resistance against mealybugs.

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.

During surveys conducted in 40 maize-cassava intercrops in the humid forest zone of southwestern Nigeria, the following termite species were identified from soil samples: Microtermes spp., Macrotermes nigeriensis (Sjöstedt), Macrotermes subhyalinus (Rambur), Amitermes evuncifer (Silvestri) and Anoplotermes quietus (Fr. Muller). High incidence of Microtermes and Macrotermes spp. was observed. The population density of predominant species was generally higher in the rainy seasons than in the dry seasons, indicating the role of moisture in their spread. The soil texture of surveyed farms did not influence the presence of the subterranean species Microtermes. The level of weeds in the surveyed farms also did not affect the population density of termites. However, Microtermes population increased significantly (P < 0.05) with increase in crop residues and its presence in farms partly depended on the presence of maize in such farms. Microtermes and Macrotermes species inflicted most of the damage observed on maize crops, while that caused by Amitermes evuncifer was low. No termite attack was observed on growing cassava during the surveys. The presence of live cassava in the intercrop did not influence damage by termites. Nasutitermes and Microcerotermes species which attack economic trees such as cocoa, citrus, oil palm, and forest trees did not attack cassava or maize.

The temperature, pH, moisture, carbon and nitrogen contents of soils at oviposition sites of Zonocerus variegatus (L.) (Orthoptera: Pyrgomorphidae) located around cassava farms at Aiyepe, southwestern Nigeria, were compared with those of soils at non-oviposition sites (control) during different months. Although oviposition site soils did not differ in particle composition from control soils, their temperatures were, generally, significantly lower. Soils at oviposition sites lost significantly less organic carbon to oxidation and nitrogen to leaching. They also contained significantly more moisture than soils at control sites during February-April when oviposition occurred and newly deposited eggs, lacking the serosal cuticle, would be prone to desiccation in dry soils. Conditions at oviposition sites are a key factor in the selection of egg deposition by gravid Z. variegatus.

The effect of a 2-month infestation by the cassava mealybug (Phenacoccus manihoti Mat. Ferr.) on the metabolism of nitrogen (amino acids) and carbon (carbohydrates), leaf area and total dry weight of five cassava varieties (Manihot esculenta Crantz), faux-caoutchouc (a hybrid of M. esculenta and M. glaziovii Muell, Arg.), poinsettia (Euphorbia pulcherrima Wild) and talinum (Talinum triangulare Jack) was studied. Free amino acid and free sugar contents as well as relative free amino acid composition in the leaf extracts, although found to be very different from one plant to another, were not significantly modified by P. manihoti infestation, except for the total amino acid contents of the cassava variety 30M7. Variations in one particular amino acid induced by mealybug infestation were not linked to the antibiotic resistance of these plants. Infestation by the cassava mealybug did not modify the total dry weight but reduced the total leaf area although this reduction was only significant in cassava varieties 59M2, 30M7 and M'pembe, and in faux-caoutchouc. Within the genus Manihot, the reduction in leaf area was strongly correlated (r= -0.878, P≤0.05) to the degree of antibiotic resistance and was coupled to an increase in the ratio of sugars to amino acids, suggesting a similarity between the effects of water stress and those of mealybug infestation.

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.

Preliminary investigations have shown the presence of pathogenic fungi in natural populations of the cassava green spider mites. Among others, Entomopthora sp. and Hirsutella sp. were isolated from dead mites sampled from the field. Laboratory bioassays have given mite mortality of up to 60%, indicating the possibility of utilizing some of these pathogens in mite control. Tests with Beauveria bassiana, isolated from lepidopteran pests, have also shown that the mite is susceptible to this fungus.