The distribution, biology and behaviour of major natural enemies of Bemisia tabaci (Gennadius) (homoptera: aleyrodidae) on cassava (Manihot esculenta crantz) in Uganda
Otim, Michael Hilary
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The whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) is an important pest of cassava (Manihot esculenta Crantz) causing substantial yield losses due to transmission of cassava mosaic geminiviruses that cause cassava mosaic disease (CMD) and direct damage to CMD-resistant varieties. In this study, the population levels of B. tabaci and its natural enemies on cassava, cassava component crops and weeds, the life history of the principal parasitoids of B. tabaci and, the influence of cassava leaf pubescence on the searching and oviposition behaviour of the principal parasitoids of B. tabaci were investigated. Bemisia tabaci and its natural enemies were most abundant on cassava than on cassava component crops and weeds. The parasitoid, Eretmocerus mundus Mercet was the most abundant followed by Encarsia sophia Girault and Dodd and an undescribed black-headed Encarsia (=Encarsia sp.). The incidence of parasitoids and parasitism differed significantly between locations and time of the year, being highest at Busukuma, followed by Bulisa and Lyantonde. Predators of B. tabaci recorded on cassava included: ants, predatory beetles (Serangium sp.), spiders, lacewings, hoverfly larvae and predatory bugs. Ants, predatory beetles and spiders were the most abundant at all the surveyed locations. No component crops or weeds harboured high populations of whitefly or its natural enemies. Nevertheless, fewer parasitoids were recorded from Ipomea batatas, Commelina sp., Melhania sp., Bidens sp., and Euphorbia sp. Eretmocerus mundus emerged with more mature eggs and produced more offspring than E. sophia. Encarsia sophia on the other hand developed faster, lived longer, and had a higher intrinsic rate of increase, net reproductive rate and generation time compared to E. mundus. The females of E. sophia developed ca. two days earlier than their male counterparts. The parasitoids exhibited a similar search pathway on the two varieties, but often stopped and groomed for a while upon encountering leaf hairs before resuming host searching. The observed frequencies of some behaviours deviated from what was expected. The duration of host assessment (antennation) was higher for both parasitoid species on the glabrous variety than on the hirsute clone. Eretmocerus mundus females probed, groomed and rested on the leaf for a longer duration on the glabrous variety than on the hirsute clone, whilst E. sophia host-fed and fed on liquids on the leaf for a longer duration on the glabrous variety. Combining all the data together, it is evident that, although parasitoids cause up to 42% parasitism of B. tabaci, parasitoids by themselves were not effectively controlling B. tabaci on cassava. This is partly because no crops or weeds host high populations of whitefly and their natural enemies to enable earlier colonization of the crop by parasitoids and, the limitations posed by low reproductive rates of the parasitoids. Nevertheless, this study has demonstrated the importance of parasitoids in regulating B. tabaci populations and generated important information for comparison in case of importation or the need for augmentative biological control.