FEEDING HABITS OF THE COTTON WHITEFLY, BEMISIA TAB AC I GENN. (HOMOPTERA: ALEYRODIDAE)

The cotton whitefly, Bemisia tabaci Genn., is important in the Sudan first as the vector of leaf-curl virus, and secondly because it, principally when immature, directly damages plants. It damages both leaf-curl-resistant and susceptible varieties, and during recent years the increase in DDT spraying against the cotton jassid has increased the numbers and importance of the cotton whitefly.
The macroscopic effects of the nymphs on the leaves include: (I) 'asal' (honeydew), often produced in quantities sufficient to cover the leaves and contaminate the lint in open bolls; (2) chlorotic spots caused by the saliva of feeding nymphs removing chlorophyll and starch; (3) development of anthocyanin; (4) leaf shedding and a reduced growth rate.
The stylets of nymphs on the lower leaf surface can reach the phloem, except when they follow a convoluted path. The stylets usually penetrate between the epidermal cells. Penetration through the parenchyma is predominantly intercellular, and the objective is the phloem. Stylets terminated in the phloem in 82% of examples examined, with the remainder ending in the parenchyma. Only one case of partial penetration of the xylem occurred. A stylet, or salivary, sheath is rare; it is a delicate structure with an annular construction. Stylet tracks occur rarely and indicate probing in the region of the phloem.
Damage to the tissues is limited to slow chloroplast destruction and occasional plasmolysis; the phloem is neither blocked nor obviously damaged.