Relative abundance of Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) and its parasitoids,and the impact of augmentative release of Eretmocerus spp. (Hymenoptera: Aphelinidae) on the population dynamics of the pest in Burkina Faso (West Africa)

Whitefly infestations and parasitism were monitored year-round in overlapping cotton crops sown on three dates in Burkina Faso. The relative abundance of B. tabaci (Gennadius) and its parasitoids, Eretmocerus spp. and Encarsia spp., was recorded in control and insecticide-sprayed plots. Low B. tabaci populations developed during the first half of the rainy season. Pest populations increased when rainfall was ending, and the levels reached were higher in insecticide-treated plots (48 nymphs/leaf) than in control plots (25 nymphs/leaf). Parasitism reached 88.7% in control plots, and 53.7% in insecticide-treated plots. Eretmocerus spp. nymphs were more abundant than Encarsia spp. in both treated and control plots. A positive and significant curvilinear relationship was observed where % parasitism, on a linear scale, rose to a plateau with logarithmic increase in host density. In general % parasitism was correlated with the abundance of pest populations except in March and April where parasitism increased while B. tabaci populations decreased. In a separate experiment, adult Eretmocerus spp. were released into caged cotton plants to study the impact of augmentative releases of the parasites on the population dynamics of the pest. Pest densities increased from 1.47 nymphs/leaf to 39.4 nymphs/leaf in the control, but were reduced to 0.8 and 0.6 nymphs/leaf in the cages where, respectively, 4 and 8 parasitoids were released per plant. It appears that parasitism is an important factor reducing B. tabaci populations during and after the cotton-growing season, and that Eretmocerus spp. are promising biological control candidates against the pest in cotton.