The environmental impacts of genetically modified (GM) crop plants such as Bt (Bacillus
thuringiensis) maize have not yet been fully assessed in South Africa. Bt maize designed
to express Bt endotoxin for control of Busseola fusca (Fuller) (Lepidoptera: Noctuidae)
and Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is planted on approximately
1.103 million hectares in South Africa. The monitoring of GM crops after release is
important in order to assess and evaluate possible environmental effects. No risk
assessment for Bt maize was done in South Africa before its release in 1998 and no
targeted post-release monitoring of possible resistance development or impact on non-target
species have been done. Awareness has risen in South Africa through research
highlighting the possible effects GM crops may have. The aim of this study was to
determine, through feeding experiments, the effects of Bt maize on selected non-target
Lepidoptera, Coleoptera and Diptera species that occur in maize agro-ecosystems in
South Africa. Results provide information for use in future risk assessment studies on Bt
maize and indicate which species could possibly be of importance in post-release
monitoring of Bt maize. Priority insect species were identified and laboratory- and semifield
experiments were conducted to evaluate the effect of Bt maize on these species. In
the light of the reportedly lower toxicity of Bt maize to certain noctuid borers, the effect
of Bt maize was evaluated on Sesamia calamistis (Hampson), Agrotis segetum (Denis &
Schiffermüller), and Helicoverpa armigera (Hubner). Feeding studies were also
conducted to determine the effect of Bt maize on non-target Coleoptera, i.e.
Heteronychus arator Fabricius (Coleoptera: Scarabaeidae) and Somaticus angulatus
(Fahraeus) (Coleoptera: Tenebrionidae). The effect of indirect exposure of the stem borer
parasitoid Sturmiopsis parasitica (Curran) (Diptera: Tachinidae) to Bt toxin was
evaluated to determine if there is any effect when it parasitizes Bt-resistant B. fusca
larvae that have fed on Bt maize. Results from the study conducted with S. calamistis
indicated that Bt maize of both events (Bt11 and MON810) were highly toxic to S.
calamistis. The behavioural characteristic of S. calamistis to feed behind leaf sheaths and
to enter stems directly did not result in escape of exposure to the toxin. Larval feeding on
leaf sheaths therefore resulted in the ingestion of sufficient toxin to kill larvae before they entered maize stems. Results showed that the effect of Cry1Ab toxin on the biology of A.
segetum larvae and moths were largely insignificant. Whorl leaves were observed to be
an unsuitable food source for H. armigera larvae and larval growth was poor. No larvae
survived to the pupal stage on any of the Bt maize treatments. When feeding on maize
ears H. armigera larval mass increased on non-Bt maize whereas no increase occurred on
Bt maize. The feeding study conducted with Coleoptera showed that the effect of Bt
maize on H. arator and S. angulatus was insignificant and no differences were observed
in any of the parameters measured for the two species. Although not always significant,
the percentage parasitism of Bt-consuming host larvae by S. parasitica was always higher
compared to host larvae that fed on non-Bt maize. It could be that Bt toxin affects B.
fusca fitness to such an extent that the immune systems of host larvae were less effective.
The different parameters tested for S. parasitica indicated only one case where fly
maggots originating from diapause host larvae feeding on non-Bt maize had a greater
mass compared to host larvae that fed on Bt maize. The same applied to S. parasitica
pupal length. For other parameters tested there were no significant differences. Sesamia
calamistis is stenophagous and occurs in mixed populations with other borer species. It
was therefore concluded that the ecological impact of local extinctions of S. calamistis
caused by Bt maize is not expected to be great. Bt maize will most likely not have any
significant effect on the control of A. segetum under field conditions. The feeding study
conducted with H. armigera quantified the effects of Bt maize on this species and
provided important information on the potential of Bt maize as protection against this
polyphagous pest. However, the likelihood of H. armigera becoming an important
secondary pest is high. It can be concluded that the Cry1Ab toxin targeting lepidopteran
pests will not have adverse effects on H. arator or S. angulatus. Although some adverse
effects were observed on S. parasitica mass and pupal length it is most likely that this
will not contribute to adverse effects in the field, but that there rather be synergism
between Bt maize and S. parasitica. An ecological approach was followed in which the
potential effects of exposure of priority species to Bt toxin in maize was investigated. A
series of selection matrixes were developed in which each of the above mentioned
species was ranked for its maximum potential exposure to Bt toxin by assessing it
occurrence, abundance, presence and linkage in the maize ecosystem. Through the use of these selection matrixes, knowledge gaps were identified for future research and to guide
the design of ecologically realistic experiments. This study contributes to knowledge
regarding the possible effects of Bt maize on the most economically important non-target
pests in South Africa. There is, however, a need to evaluate other non-target species in
feeding studies, as well as in field studies. From this study it can be concluded that some
species can be eliminated from further testing since Bt maize had no adverse effect while
more research have to be conducted on other species. / Thesis (Ph.D. (Environmental Science)--North-West University, Potchefstroom Campus, 2010.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4105 |
| Date | January 2010 |
| Creators | Erasmus, Annemie |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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