Abundance and Host Preference of the Whitefly Bemisia tabaci in north-western Tanzania

Cassava mosaic disease (CMD), caused by cassava mosaic geminiviruses (CMGs) and transmitted by the whitefly Bemisia tabaci, is the most important constraint of cassava production in Africa. To assess the abundance and host preference of the whitefly, a survey was conducted in May 2010 in north-western Tanzania where the pandemic of severe CMD is expanding. Whitefly adults and nymphs were counted on cassava plants, intercrops and weeds within and around 80 farmers’ cassava fields. Abundance levels of whitefly adults and nymphs on cassava ranged from 6.9 to 233.5 and from 15.3 to 599.9, respectively. Higher abundance of adults and nymphs coincided with greater CMD incidence. Variations were also noted in whitefly abundance in surrounding non-cassava crops and weed species. No-choice experiments were then conducted to evaluate host preference of cassava-derived whiteflies on pumpkin (Cucurbita pepo), sunflower (Helianthus annuus), cowpea (Vigna unguiculata), sweet potato (Ipomoea batatas), Sida cordifolia, Euphorbia heterophylla, Benghal dayflower (Commelina benghalensis) and cassava. Highest fecundity levels were recorded on cowpea although nymphs were unable to develop beyond the first instar. Sida cordifolia supported the highest survival rate followed by pumpkin, E. heterophylla and cassava. No eggs hatched on sweet potato or Benghal dayflower. Major mortality factors identified included inviability of eggs, dislodgement and predation of nymphs, as well as pest and disease attack of supporting leaves. The co-occurrence of large populations of whitefly and high CMD incidences, coupled with the apparent ability of cassava-derived B. tabaci to utilize other crops and weeds as hosts highlights the acute threat of further expansion of the severe CMD pandemic in Tanzania and elsewhere in eastern, central and southern Africa.