Host-associated differences within populations of the whitefly Bemzsiu tabucz from cassava, okra and other host plants in C6te d'Ivoire, West Africa, were investigated by iso-enzyme electrophoresis and experimental host range studies. Two biotypes were identified. One was found only on cassava and eggplant; the other was polyphagous, but did not infest cassava. Differences in esterase patterns matched these host range restrictions exactly. The implications of these finding are discussed in relation to the role of B. tabuci as a virus vector.

SUMMARY The major disease affecting Dioscorea cayenensis in the Ivory Coast is caused by a virus which was transmitted by mechanical inoculation to some Dioscorea spp. and Nicotiana benthamiana. In extracts of D. cayenensis leaves infectivity was lost after 10 min at 60 d̀C but not 55 d̀C and after dilution to 10‐3 but not 10‐2. A purification procedure is described. The virus particles are flexuous filaments c. 785 nm long. The virus was transmitted by four aphid species in the non‐persistent manner, and is serologically related to four African potyviruses. The name yam mosaic virus is proposed; the present cryptogram is: */*:*/6:E/E:S/Ve/Ap, potyvirus group.

The spread of African cassava mosaic disease (ACMV) into healthy cassava fields was recorded at weekly intervals. In addition, 21 yellow water traps were placed in one field and the number of whiteflies caught was recorded twice a week. The number of Bemisia spp. feeding on cassava was also estimated. The results indicate that the pattern of disease spread is related to the pattern of infestation with Bemisia.Airborne whiteflies carried by the south-west prevailing wind alighted preferentially on cassava plants along the upwind edges (south and west borders) of the plantings. The pattern of incidence of mosaic disease resembled that of whiteflies. Along the SW-NE diagonal, there was a gradient of disease incidence with a maximum at the SW corner block. Similar gradients occurred in three different fields and they were maintained throughout the 6-month study, although gradually flattening with time. There were indications that the reservoirs both of the virus and of the vectors were located some distance upwind from the experimental fields.

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Population phenologies of the whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), in young cassava crops in Cote d'lvoire, West Africa, are described for three field seasons. Populations of all stages were consistently greatest 6 -1 2 weeks after the crop was planted. The number of adults on plants as well as on attractive and non-attractive sticky traps displayed cycles of buildup and decline each year, the periodicity of these cycles corresponding to the generation time of B. tabaci under field conditions. Adult population declines were probably caused by emigration from the crop. Rainfall was negatively correlated with both nymph and adult populations, possibly due to reduced oviposition after rain. B. tabaci is the vector of African cassava mosaic geminiviruses (ACMV) and the observed B. tabaci population trends fit well with the pattern of ACMV buildup in the crop.