The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a cosmopolitan insect pest of Brassica crops. In South Africa, there are no action thresholds for its chemical control which makes it difficult for growers to make informed decisions on when to apply insecticides and how frequently to apply them in order to achieve optimal crop yield. To contribute towards optimum application of insecticides against P. xylostella, this study compared the impact of weekly and bi-weekly applications of a selective insecticide Dipel® (Bacillus thuringiensis Berliner var. kurstaki) applied at 250 g/ha, and a broad-spectrum insecticide Dichlorvos (an organophosphate) applied at 1 ml/L against biological control (Control) on the pest population density on cabbage during October– December 2011 and March–May 2012. The use of both selective and broad-spectrum insecticides for experiments enables us to understand if efforts to optimise cabbage yield depend mainly on effective suppression of P. xylostella densities. Furthermore, investigations were carried out to determine the impact of these chemicals on parasitism rates of P. xylostella and species richness of its primary parasitoids. During the October–December 2011 growing season, the lowest infestation of P. xylostella occurred on cabbage plots that received weekly application of Dipel and the highest on untreated control plots. Cabbage weights were negatively related to infestation levels, implying that weekly application of Dipel yielded bigger cabbage heads. During March– May 2012, P. xylostella infestations were again higher on the control followed by weekly and bi-weekly treatments of Dichlorvos, then weekly and bi-weekly applications of Dipel. Despite the significant differences observed, infestation levels were much lower (< 1 P. xylostella per plant on average) in all treatments during this season. Consequently no significant differences in cabbage weights were observed among the treatments. The lower infestation levels were attributed to higher parasitism levels (≥50 %), especially during the early stages of crop development. A total of four parasitic Hymenoptera species were recorded from P. xylostella larvae and pupae during October–December 2011, while three species were recorded during March– May 2012. However, Cotesia vestalis (Haliday) (Braconidae) accounted for >80 % of total parasitism levels in all treatments. Parasitism levels were not significantly different among the treatments in both seasons. Parasitoid species richness was highest on the control. Although two parasitoid species were recorded in all Dipel and Dichlorvos treatments during October–December 2011, only one parasitoid species was recorded in the Dipel treatments during March–May 2012 compared to two species in Dichlorvos treatments. Although weekly applications of Dipel ensured good yield and crop quality during October–December, weekly applications of the chemical did not lead to better quality crop during March–May crop growing season. Thus, it is not necessary to apply insecticides during periods in which natural mortality of P. xylostella is high due to parasitoids. Since P. xylostella abundance was a determining factor of crop quality, these results imply that insect pest management should focus mainly on suppressing its numbers. Furthermore, there was no evidence that application of either insecticide type had a negative impact on parasitism rates of P. xylostella. The lower parasitoid species richness on Dipel treated plots was the consequence of its higher efficiency in suppressing the pest population which substantially reduced availability of potential hosts for parasitoids, hence only the efficient C. vestalis was recorded at low host densities / Agriculture and  Animal Health / M.Sc. (Agriculture)

Title page, contents and abstract only. The complete thesis in print form is available from the University of Adelaide Library. / Brassica vegetables and oilseeds are economically important crops worldwide. These crops are associated with several destructive and widespread insect pests. In Australia these pests include six species, Plutella xylostella (Linnaeus), Pieris rapae (Linnaeus), Hellula hydralis Guenée, Helicoverpa punctigera (Wallengren), Brevicoryne brassicae (Linnaeus) and Myzus persicae (Sulzer), which are the focus of this research. Among them P. xylostella (diamondback moth or DBM) is the most serious and destructive insect pest. Little is known about the potential of predators, which may be able to contribute to control diamondback moth, although some field studies have shown the overall importance of predators in controlling this key pest. The aim of this study was to develop a method that allows study of predator-prey trophic interactions in the field. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1294814 / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2007

Title page, contents and abstract only. The complete thesis in print form is available from the University of Adelaide Library. / Brassica vegetables and oilseeds are economically important crops worldwide. These crops are associated with several destructive and widespread insect pests. In Australia these pests include six species, Plutella xylostella (Linnaeus), Pieris rapae (Linnaeus), Hellula hydralis Guenée, Helicoverpa punctigera (Wallengren), Brevicoryne brassicae (Linnaeus) and Myzus persicae (Sulzer), which are the focus of this research. Among them P. xylostella (diamondback moth or DBM) is the most serious and destructive insect pest. Little is known about the potential of predators, which may be able to contribute to control diamondback moth, although some field studies have shown the overall importance of predators in controlling this key pest. The aim of this study was to develop a method that allows study of predator-prey trophic interactions in the field. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1294814 / Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2007

The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a cosmopolitan insect pest of Brassica crops. In South Africa, there are no action thresholds for its chemical control which makes it difficult for growers to make informed decisions on when to apply insecticides and how frequently to apply them in order to achieve optimal crop yield. To contribute towards optimum application of insecticides against P. xylostella, this study compared the impact of weekly and bi-weekly applications of a selective insecticide Dipel® (Bacillus thuringiensis Berliner var. kurstaki) applied at 250 g/ha, and a broad-spectrum insecticide Dichlorvos (an organophosphate) applied at 1 ml/L against biological control (Control) on the pest population density on cabbage during October– December 2011 and March–May 2012. The use of both selective and broad-spectrum insecticides for experiments enables us to understand if efforts to optimise cabbage yield depend mainly on effective suppression of P. xylostella densities. Furthermore, investigations were carried out to determine the impact of these chemicals on parasitism rates of P. xylostella and species richness of its primary parasitoids. During the October–December 2011 growing season, the lowest infestation of P. xylostella occurred on cabbage plots that received weekly application of Dipel and the highest on untreated control plots. Cabbage weights were negatively related to infestation levels, implying that weekly application of Dipel yielded bigger cabbage heads. During March– May 2012, P. xylostella infestations were again higher on the control followed by weekly and bi-weekly treatments of Dichlorvos, then weekly and bi-weekly applications of Dipel. Despite the significant differences observed, infestation levels were much lower (< 1 P. xylostella per plant on average) in all treatments during this season. Consequently no significant differences in cabbage weights were observed among the treatments. The lower infestation levels were attributed to higher parasitism levels (≥50 %), especially during the early stages of crop development. A total of four parasitic Hymenoptera species were recorded from P. xylostella larvae and pupae during October–December 2011, while three species were recorded during March– May 2012. However, Cotesia vestalis (Haliday) (Braconidae) accounted for >80 % of total parasitism levels in all treatments. Parasitism levels were not significantly different among the treatments in both seasons. Parasitoid species richness was highest on the control. Although two parasitoid species were recorded in all Dipel and Dichlorvos treatments during October–December 2011, only one parasitoid species was recorded in the Dipel treatments during March–May 2012 compared to two species in Dichlorvos treatments. Although weekly applications of Dipel ensured good yield and crop quality during October–December, weekly applications of the chemical did not lead to better quality crop during March–May crop growing season. Thus, it is not necessary to apply insecticides during periods in which natural mortality of P. xylostella is high due to parasitoids. Since P. xylostella abundance was a determining factor of crop quality, these results imply that insect pest management should focus mainly on suppressing its numbers. Furthermore, there was no evidence that application of either insecticide type had a negative impact on parasitism rates of P. xylostella. The lower parasitoid species richness on Dipel treated plots was the consequence of its higher efficiency in suppressing the pest population which substantially reduced availability of potential hosts for parasitoids, hence only the efficient C. vestalis was recorded at low host densities / Agriculture and  Animal Health / M.Sc. (Agriculture)

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is an important pest of cruciferous crops worldwide. The prolonged use of synthetic chemical insecticides as a primary means of control has resulted in the development of resistance in pest populations. In addition, the pest has also evolved resistance to the bacterial insecticidal protein of Bacillus thuringiensis which is also widely used as a method of control. Baculoviruses are considered as effective alternatives to conventional methods of control when incorporated into integrated pest management (IPM) programmes. These viruses target the larval stages of insects, are generally host-specific and are safe for use in the environment. This study aimed to isolate a baculovirus from a laboratory-reared P. xylostella colony, characterise it genetically and then evaluate its virulence against neonate and fourth instar larvae. A laboratory colony of P. xylostella was established using pupae and asymptomatic larvae collected from a cabbage plantation outside Grahamstown in the Eastern Cape province of South Africa. The colony flourished in the laboratory due to prime conditions and availability of food. The duration of development from egg to adult was determined by observation and imaging of the various life stages. The mean developmental time from egg to adult was observed to be 14.59 ± 0.21 days. The population of the insects increased rapidly in number leading to overcrowding of the insect colony, and hence appearance of larvae with viral symptoms. Occlusion bodies (OBs) were extracted from symptomatic larval cadavers and purified by glycerol gradient centrifugation. Analysis of the purified OBs by transmission electron microscopy revealed the presence of a granulovirus which was named PlxyGV-SA. The virus isolate was genetically characterised by restriction endonuclease analysis of the genomic DNA, and PCR amplification and sequencing of selected viral genes. The complete genome sequence of a Japanese P. xylostella granulovirus isolate, PlxyGV-Japan, has been deposited on the GenBank database providing a reference strain for comparison with DNA profiles and selected gene sequences of PlxyGV-SA. BLAST analysis of the granulin gene confirmed the isolation of a novel South African PlxyGV isolate. Comparison of the restriction profiles of PlxyGV-SA with profiles of PlxyGV-Japan and other documented PlxyGV profiles obtained by agarose gel electrophoresis revealed that PlxyGV-SA is a genetically distinct isolate. The data obtained from the sequencing and alignment of ecdysteroid UDP-glucosyltransferase (egt), late expression factor 8 (lef-8) and late expression factor 9 (lef-9) genes with those of PlxyGV-Japan also showed that PlxyGV-SA is a genetically different isolate. In order to determine the biological activity of PlxyGV-SA against neonate and fourth instar P. xylostella larvae, surface dose bioassays were conducted. The median lethal concentration of the virus required to kill 50% (LC₅₀) and 90% (LC₉₀) of the larvae was estimated by feeding insects with a range of doses. In addition, the time to kill 50% of the larvae (LT₅₀) was determined by feeding insects with the LC₉₀ concentration. Larval mortality was monitored daily until pupation. The data obtained from the dose response assays were subjected to probit analysis using Proban statistical software. The time response was determined using GraphPad Prism software (version 6.0). The LC₅₀ and LC₉₀ values for the neonate larvae were 3.56 × 10⁵ and 1.14 × 10⁷ OBs/ml respectively. The LT₅₀ was determined to be 104 hours. The neonate larvae were found to be more susceptible to infection than the fourth instar larvae with the same virus concentration. The concentrations used for the neonate larvae assay did not have a significant effect on the fourth instar as no mortality was recorded. This is the first study to describe a novel South African PlxyGV isolate and the results suggest that PlxyGV-SA has significant potential for development as an effective biopesticide for the control of P. xylostella in the field.