Agriculture Reference
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control whiteflies at the height of the epidemic. Some other explanation is required
for the increased whitefly populations and attention has been given to two
possibilities that are not mutually exclusive. One is that the original pre-epidemic
population of
B. tabaci
has been supplemented or displaced by a new invader type
that is more fecund and may also be more efficient as a virus vector. The other
possibility is that there is a complex interaction between virus, host and vector such
that whitefly populations on cassava are enhanced as a direct or indirect
consequence of virus infection.
Initial studies on adult whiteflies collected from cassava in different parts of
Uganda used electrophoresis to detect heterogeneity within, but no consistent
differences between populations from epidemic and non-epidemic areas (Legg
et al.,
1994). Maruthi
et al
. (2001) also collected whiteflies from epidemic and pre-
epidemic areas of Uganda and detected no differences in fecundity or development
time between the two groups and no evidence of mating incompatibility. Moreover,
there were no differences between epidemic and pre-epidemic populations in
analyses of DNA extracts using a RAPD-PCR technique. This led to the conclusion
that the epidemic and associated increase in whitefly populations were not
associated with a novel reproductively isolated biotype.
In parallel studies, Legg
et al
. (2002) used molecular techniques to analyse the
DNA sequences of the mitochondrial cytochrome oxidase I gene of whiteflies
collected along transects from pre-epidemic to epidemic areas of southern Uganda in
1997 and from two post-epidemic sites in Uganda in 1999. Two distinct haplotypes
were distinguished in the 1997 collections. The one designated Ug1 occurred
primarily at sites ahead of the epidemic front and was considered to be 'the local
vector'. Ug2 was referred to as 'the invader' and occurred at or behind the epidemic
front. Only Ug1 was detected in each of the ten samples collected in 1999. Ug2 was
regarded as an invader from elsewhere in Africa that is associated with the epidemic
in Uganda, even though it was not detected in 1999 and may have integrated with
the pre-existing Ug1 population. The significance of these findings is unclear in the
absence of any evidence of differences in the fecundity of Ug1 and Ug2 populations,
or in their ability to transmit different cassava mosaic viruses. This indicates the
need for additional studies in Uganda and also in Kenya, Tanzania and other parts of
Africa where the pandemic continues to progress. Meanwhile, there has been an
emphasis on the interaction hypothesis in seeking to explain the big increase in
whitefly populations that has occurred in epidemic areas (Colvin
et al.,
1999, 2004;
Legg and Thresh, 2000).
Initial field observations provided no evidence to support the interaction
hypothesis, as adults and nymphs were fewer on CMD-affected cassava than on
equivalent healthy plants (Gibson
et al.,
1996). It was also observed that nymphs
and eggs were far fewer on the chlorotic areas of CMD-affected leaves than on
adjacent dark green areas. However, in subsequent studies using growth chambers
whitefly fecundity was greater on CMD-infected than on uninfected plants and this
was associated with increased concentrations of asparagine and three other amino
acids that are known to enhance the fecundity of other sap-sucking insects (Colvin
et al.,
1999). Moreover, it was noted that the whiteflies on CMD-infected cassava
