The incidence of Bemisia tabaci (Homoptera: Aleyrodidae) and its parasitoids on cassava and associated plants in Uganda

Biotic and abiotic factors can influence the population dynamics of Bemisia ta­baci (Gennadius) (Homoptera: Aleyrodidae), a key pest of cassava and a vec­tor of several viral diseases, and that of its parasitoids. To gain a better under­stan­ding of possible roles these factors, different crops/cropping systems and weeds play in determining B. tabaci population dynamics, cassava fields were surveyed monthly from November 2003 to December 2004 in Buliisa (Buliisa district), Bu­sukuma (Wakiso district) and Lyantonde (Lyantonde district), Uganda. Bemi­sia tabaci and its parasitoids were more abundant on cassava than on both inter­crops and weeds. Eretmocerus mundus Mercet (Hymenoptera: Aphelinidae) was the most abundant parasitoid species followed by Encarsia sophia Girault & Dodd (Hymenoptera: Aphelinidae). An undescribed Encarsia sp. was relatively rare. The level of parasitism on cassava was highest at Busukuma (41.7 %), followed by Buliisa (37.2 %) and Lyantonde (32.2 %). The only companion crop that har­boured B. tabaci nymphs was sweet potato with 22 nymphs from 20 plants and 68 % parasitism. Similarly very low numbers of nymphs were ob­served on the following weeds: Commelina benghalensis (4 parasitized nymphs from 1340 plants), Melhania sp. (over 76 nymphs from 60 plants with about 60 % parasitism), Bidens pilosa (2 parasitized nymphs from 1080 plants) and Eu­phorbia sp. (8 nymphs from 580 plants with 75 % parasitism). This de­mon­strates that cassava companion crops and weeds in the surveyed loca­tions are not major hosts of B. tabaci. For these two reasons, cassava is the only signi­fi­cant source of cassava-colonizing B. tabaci. Whitefly and disease con­trol on cas­sava should therefore focus on an integrated management strategy, in­cluding breeding for resistance to both constraints, and enhancing natural ene­mies, in a manner that addresses specific agro-ecosystems