Investigations into the incidence and ecology of bilobata subsecivella (zeller) (lepidoptera: gelechidae) : a new pest of groundnut in South Africa.
Buthelezi, Nokubekezela Makhosi.
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The leaf-mining moth, Bilobata subsecivella (Zeller) (Lepidoptera: Gelechiidae), thought to be an invasion from Indo-Asia (where it is known as Aproaerema modicella (Deventer); but hereafter referred to as B. subsecivella) has become a major pest of groundnut (Arachis hypogaea L.) and soya bean (Glycine maxi (L.) Merr.) in South Africa and Africa as a whole. Following the sudden outbreaks of B. subsecivella as a new pest of groundnut in a number of African countries, the continent has been confronted with the problem of having no information on the biology and ecology of the pest that can be used for its management/control. In this context, the main aim of the research for this thesis was to study the biology and ecology of B. subsecivella in South Africa with the main objective of obtaining information that will assist in its management as a novel pest of groundnut. To achieve this objective, several studies were carried out. First, a detection survey of B. subsecivella infestation was conducted on groundnut, soya bean and lucerne (Medicago sativa L.), the common host crops for B. subsecivella in India, at six widely separated sites in South Africa during the 2009/2010 growing season. The sites included the Agricultural Research Council research stations at Potchefstroom and Brits as well as the farms surrounding the Brits research farm in the North West province, Vaalharts Research Station in the Northern Cape province, the Department of Agriculture Lowveld Agricultural Research Station near Nelspruit in Mpumalanga province, and Bhekabantu and Manguzi in the northern part of the KwaZulu-Natal province. The study had three objectives. The first was to build a complete host crop/plant list and record damage symptoms caused by B. subsecivella in South Africa. The second was to identify the pest to species level. The third was to determine its inter- and intra-population genetic diversity by analysing in, both cases, the mitochondrial DNA (mtDNA) COI gene of specimens collected from these sites. Sixty specimens comprising 24 larvae, 24 pupae and 12 moths were collected from the six survey sites, and their mtDNA COI were sequenced and compared with those from the Barcode of Life Data System (BOLD) gene bank. Infestation by B. subsecivella was observed on groundnut and soya bean, but not on lucerne. The mtDNA COI from all specimens of the pest, irrespective of whether they were from groundnut or soya bean, matched 100% with the sequences in BOLD belonging to a B. subsecivella population occurring in Australia (referred to as Aproaerema simplexella (Walker)) and known as the soya bean moth in that country). There was very little genetic diversity between and within the populations from the six sites, which suggested that the populations were maternally of the same origin. Further molecular and phylogenetic studies were also completed to determine the evolutionary relationships between B. subsecivella populations collected from Australia, Africa and India. These studies involved sequencing and analysing five gene regions of mitochondrial and nuclear DNA, including COI, cytochrome oxidase II (COII), cytochrome b (cytb), 28 ribosomal DNA (28S rDNA), and intergenic spacer elongation factor-1 alpha (EF-1 ALPHA). The mtDNA COI analysis also included B. subsecivella (but called A. simplexella) sequences downloaded from the National Center for Biotechnology Information (NCBI) GeneBank collected from different areas in Australia. In four phylogenetic trees (COI, COII, cytb and EF-1 ALPHA), sequences of B. subsecivella personally sampled from Australia were grouped separately from the others, whereas sequences of B. subsecivella from South Africa, India and Mozambique were clustered in one group in most cases. Furthermore, in the mtDNA COI phylogenetic tree, one Australian sequence of B. subsecivella that was downloaded from the NCBI GeneBank was grouped with other sequences from South Africa, India and Mozambique. Moreover, one sequence of B. subsecivella personally sampled from Australia was grouped with the other two sequences of B. subsecivella from Australia that were downloaded from the NCBI GeneBank. Based on these results, it could be hypothesized that there is genetic diversity within B. subsecivella populations in Australia. The mtDNA COI gene analysis in the current study revealed that there are B. subsecivella populations in Australia that are similar to the B. subsecivella populations in South Africa, Mozambique and India. Phylogenetic analysis of the 28S gene region revealed a lack of genetic diversity between sequences of B. subsecivella from India, South Africa, Mozambique and Australia. Genetic pairwise distances between the experimental sequences ranged from 0.97 to 3.60% (COI), 0.19% to 2.32% (COII), 0.25 to 9.77% (cytb) and 0.48 to 6.99% (EF-1 ALPHA). Field experiments were then conducted at Vaalharts, Brits, Nelspruit, Manguzi and Bhekabantu during the 2010/2011 and 2011/2012 growing seasons. These experiments pursued three objectives. The first one was to determine B. subsecivella infestation levels on groundnut, soya bean, lucerne, pigeon pea (Cajanus cajan L.) and lablab bean (Lablab purpureus L.) under field conditions. The second was to develop a host plant list for B. subsecivella and the third was to determine the effect of cypermethrin application on damage by B. subsecivella to groundnut and soya bean plants. In the 2010/2011 season, larval infestation was monitored on groundnut crops planted in November 2010 and January 2011. In the 2011/2012 season, larval infestation was monitored on groundnut, soya bean, lucerne, pigeon pea and lablab bean planted in November 2011 and January 2012. Wild host plants were inspected for damage symptoms and the presence of larvae. An experiment which examined the effect of cypermethrin application on B. subsecivella damage to groundnut and soya bean plants was completed in the 2011/2012 season at Vaalharts and Nelspruit. A survey for wild plant hosts of B. subsecivella was conducted in the proximity of the field experiments during the 2011/2012 growing season, as well as in winter. Amongst the host crops tested, soya bean was highly infested by B. subsecivella followed by groundnut, at all sites. The pest was also observed on pigeon pea at all sites, but the infestation was very low, while lucerne had very low larval infestation. No infestation was observed on lablab bean across these sites. Groundnut and soya bean crops planted in January were severely infested by B. subsecivella, compared to the crops planted in November; however, B. subsecivella infestation on crops was observed 5-6 weeks after crop emergence. Sprays of cypermethrin on groundnut and soya bean reduced larval infestation in both crops to very low levels. Wild plant hosts identified were from five families which included three species in the Leguminosae, two species in the Convolvulaceae, two species in the Malvaceae and one species each in the Lamiaceae and Asteraceae. Seasonal monitoring of the flight activity of B. subsecivella moths was completed at Manguzi, Bhekabantu, Nelspruit, Brits and Vaalharts over a two-year period (from November 2010 to December 2012). The objective of this study was to monitor the flight activity of B. subsecivella in order to understand its dispersal and off-season survival tactics and to predict its initial occurrence. Pheromone traps were used to monitor the moths’ flight activity. Information collected included climatic data (rainfall, temperature and humidity) that were obtained from ARC weather stations placed at four planting sites. Pearson’s test for correlation was performed to assess the relationship between B. subsecivella moth catches and environmental factors (rainfall, temperature and humidity). Results from this study showed variation in B. subsecivella populations throughout the monitoring period. The highest peak in B. subsecivella catches was between January and April/May for both seasons. Though low in numbers, B. subsecivella moths were caught in winter at Manguzi, Nelspruit, Vaalharts and Bhekabantu. No B. subsecivella moths were trapped during the winter months at Brits. Pearson’s test for correlation indicated that there was a significant negative association between temperature and B. subsecivella catches in pheromone traps at Nelspruit, whereas at Vaalharts there was a significant positive association between humidity and B. subsecivella catches. There was no correlation between environmental factors and B. subsecivella catches at Manguzi and Brits. Furthermore, it was found that B. subsecivella in Australia (moths collected for DNA analysis in the current study) responded to the species-specific lure that was developed from the sex pheromone of B. subsecivella, referred to as A. modicella in India. Overall, the study revealed important ecological and genetic information on B. subsecivella populations occurring in southern Africa. More importantly, this study established the genetic connection between B. subsecivella populations from Australia, India and Africa. Hence, the species conforming to these populations were tentatively synonymized as B. subsecivella in this thesis.
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