Organic insecticides play a big role in reducing the usage of chemical insecticides
and their negative impact on the environment. Bacillus thuringiensis (Bt) spays are
the only tool that organic farmers are allowed to use for the control of pests. Genetic
engineering and modification of crops have been made possible with scientific
advances in cell and molecular biology. These advances are used to transfer some
of the Bt Cry toxins into crops for control of target species to reduce yield loss. Bt
maize were commercialised for the first time in South Africa in 1998 and the
economic important stem borers, Busseola fusca (Fuller) (Lepidoptera: Noctuidae),
Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and Sesamia calamistis
(Hampson) (Lepidoptera: Noctuidae) were exposed to the Cry1Ab toxin that is found
in Bt maize. Busseola fusca developed resistance to Cry1Ab under field conditions
within eight years after it had been released. Eldana saccharina (Walker)
(Lepidoptera: Pyralidae) is a major pest on sugarcane in South Africa and although it
has not been recorded on maize in this country, is it known as a major pest of maize
in other African countries. African armyworm, Spodoptera exempta (Walker)
(Lepidoptera: Noctuidae) has a very wide distribution in Africa and is known to be an
occasional pest on maize. The aims of this dissertation were to determine the
efficacy of Bt spray applications for control of four lepidopteran pests and whether
development of Cry1Ab resistance by B. fusca caused a loss in susceptibility to other
Bt toxins (i.e. cross-resistance). Susceptibility bioassays with 10 day old larvae were
conducted under laboratory conditions. Treatments included application of various
dosage rates of Dipel® and deltamethrin as well as exposure to MON810 (maize
leaves). Stemborer populations of C. partellus, E. saccharina, and B. fusca (Venda)
as well as the S. exempta were effectively controlled by the Bt spray, Dipel®. Care
should be taken not be interpret the percentage C. partellus, E. saccharina and S.
exempta larvae that survived after exposure to MON810 and Bt spray treatments as
development of resistance without verification of these experiments with earlier
instars that are known to be more susceptible. Spodoptera exempta is active
throughout a year in temperate zones of Africa. If S. exempta develop resistance to
Cry toxins and Bt maize events would be released for commercial planting in these
areas, S. exempta pose a threat added to their injuriousness. Busseola fusca larvae
were sampled from Venda (susceptible population), Ventersdorp and the Vaalharts Irrigation Scheme (resistant population). The Ventersdorp B. fusca population was
controlled by MON810 and MON89034 and Bt sprays, but the percentage larvae that
survived showed reduced susceptibility within the population. Dipel® treatments,
MON810 and MON89034 did not provide effective control of the Vaalharts B. fusca
population reported to be resistant to Cry1Ab, in two experiments. The high survival
rates indicate a reduction in susceptibility to Cry toxins other than Cry1Ab and
therefore development of cross resistance in the Vaalharts B. fusca population. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2014
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/11744 |
| Date | January 2014 |
| Creators | Leyden, Patrick |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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