SEASONAL OCCURRENCE OF RICE YELLOW MOTTLE VIRUS IN LOWLAND RICE IN COTE-DIVOIRE

Monthly plantings of the rice variety Bouake 189 were made under lowla nd irrigated conditions, to obtain information on the phenological and seasonal occurrence of pests and diseases on the West African Rice De velopment Association (WARDA) research farm near Bouake, Cote d'lvoire . Regular sampling of insect pests and observations on rice yellow mot tle virus (RYMV) disease infection throughout the year provided inform ation on the occurrence of RYMV and potential insect vectors. RYMV inc idence and grain yields varied depending on planting date, and for a g iven planting date, varied from one year to another. There was no evid ence that RYMV incidence increases in successive seasons under continu ous cropping. There was no significant correlation between RYMV incide nce and either rainfall or wind speed. Leaf feeding damage by the beet le vector of RYMV, Trichispa sericea Guerin-Meneville (Coleoptera: Chr ysomelidae), and percentage RYMV infected plants were severe in the Ju ly and August plantings in 1993, but whereas T. sericea was not observ ed thereafter, RYMV spread continued. The white leafhoppers Cofana spe ctra (Distant) and C. unimaculata (Signoret) (Hemiptera: Cicadellidae) , the green leafhoppers Nephotettix spp. (Hemiptera: Cicadellidae), th e spittle bug Locris rubra F. (Hemiptera: Cercopidae), the diopsids Di opsis longicomis Macquart and D. apicalis Dalman (Diptera: Diopsidae), and the grasshopper Oxy hyla Stal (Orthoptera: Acridadae) were the mo st abundant of the insect pests and had distinct population peaks with in a year. However, population abundances were not correlated with RYM V incidence. The variability of RYMV in time and space and the potenti al role of weeds as alternative hosts for RYMV are discussed.