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Increased resistance of emerging cotton leafworm Spodoptera litura in Pakistan and elsewhere to chemical insecticides calls for an alternative method of control. Isolates of nucleopolyhedrovirus (NPVs) of S. litura (SpltNPV) were collected from infected larvae in different ecological and geographic regions of Pakistan. The genotypic diversity of these SpltNPV isolates, their relation to geographical (spatial) distribution and cropping system, and their biological activity, in particular dose response, speed of kill and effects on feeding, were explored. The first Pakistani isolate, SpltNPV-Pak-BNG, was studied and compared to a presumable SpltNPV reference isolate, SpltNPV-G1. SpltNPV-Pak-BNG killed S. litura larvae significantly faster than SpltNPV-G1. Genetic analysis revealed that SpltNPV-Pak-BNG and SpltNPV-G1 are variants of different virus species, SpltNPV and SpliNPV, respectively. The biological activity of SpltNPV-Pak-BNG was determined in different instars larvae of S. litura showing that S. litura second or third instar larvae are preferred targets for S. litura control with SpltNPV-Pak-BNG in field crops. SpltNPV-Pak-BNG infected S. litura larvae with final polyhedrosis showed reduced food intake and weight gain. Interestingly, the mock-infected- larvae and the larvae that survived viral exposure, exhibited the same food consumption and weight gain.
Twenty-two SpltNPV isolates were finally collected from S. litura from different agro-ecological regions and cropping systems in Pakistan to explore the genetic diversity of the virus on a spatial scale and explore its possible adaptation to region and crop systems. Among the SpltNPV-Pak isolates tested, isolates TAX1, SFD1, SFD2 and GRW1 were faster killing than other Pakistani isolates. All isolates were genotypic variants of a single SpltNPV ‘regiotype’, suggesting common recent ancestry, and distinct from the virus species type SpltNPV-G2. There was a strong correlation between geographic location and a SpltNPV genogroup, and less so between the latter and the cropping system. Sequence analysis of SpltNPV-Pak isolates BNG (slow virus) and TAX1 (fast virus) showed major differences, notably the absence of homologous repeat region 17 in TAX1 and gene 125 in BNG. There is strong purifying selection on gene 122, encoding a putative viral fibroblast growth factor known to be involved in baculovirus virulence.
The results presented in this thesis considerably enhanced our understanding of the genetic and biological diversity of SpltNPVs in conjunction with their spatial distribution and the crop system involved. In addition, the results may be used to the develop SpltNPV as an eco-rational control agent of the leafworm S. litura in Pakistan.
|Qualification||Doctor of Philosophy|
|Award date||6 Mar 2018|
|Place of Publication||Wageningen|
|Publication status||Published - 2018|