Incidence, Distribution and Characteristics of Major Tomato Leaf ...

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
2.1.3.1.1.1 Whitefly Vector Population in the Agro-ecosystem
In many parts of Africa, seasonal contrasts in rainfall influence pest populations
(Elkinton, 1993). Rainfall provides adequate moisture for growth of plant hosts, but also
affects the success or failure of oviposition and egg development of the vector (Duffus,
1992).
Nono Womdim et al. (1996) reported that in Tanzania, whitefly population size changed
with season, the major variables being rainfall and temperature. Similar results were
reported for Egypt (Moustafa, 1991). Riley and Wolfenbarger (1993) recorded a similar
pattern in Bemisia tabaci populations, which change from year to year and season to
season. Natural enemies and pesticides were also found to affect whitefly populations.
Riley et al. (1995) reported that natural enemies and agricultural pesticides, in addition to
climatic factors influence whitefly population dynamics. Furthermore, Baumgartner and
Yano (1990) stressed the role of natural enemies. It was established that the use of
pesticides reduces populations of natural enemies, and subsequently enhances whitefly
population growth, while the possibility for whitefly resistance to pesticides was not
ruled out (Duffus, 1995). Another factor influencing the development of whitefly vector
populations is intensified agricultural practices.
In the Americas, international transport of plant materials introduced new B. tabaci
biotypes into the ecosystem (Duffus, 1992; Polston and Anderson, 1997). On the other
hand, plant hosts already in the ecosystem do also affect whitefly population dynamics.
Legg (1996), while working with whiteflies on cassava, found that B. tabaci population
growth on cassava, sweet potato and cotton host plants differed from each other. He
observed that there were variable establishment success levels on individual crops. B.
tabaci survived better on cotton than on any other host (Legg, 1996; Byrne et al., 1990;
Von Arx et al., 1983). Lettuce (Lactuca sativa) was also reported to be a very good host
for B. tabaci (Byrne et al., 1990). It was further observed that there were variable
establishment success levels on individual crops. It was further observed that as
whiteflies migrate from one host to another, their population increases on the new host as
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