Responses Of Spotted Stemborer Chilo Par Tell Us (swinhoe) (lepidoptera: Pyralidae) To Ba Cillus Thuringiensis Berliner Formulations In Kenya
Ngei, Christine N.
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This study sought to quantify the effects of geographic isolation (allotropy) and altitude on three widely distributed populations of the spotted stalkborer Chilo partellus, a pest of maize, by determining their response to various strains of the bioinsecticide Bacillus thuringiensis (Bt). The findings would serve to illustrate the perception since the Darwinian ages that geographic isolation and temperature variations due to altitude are some of the important factors that cause speciation and thereby promoting the evolutionary process. These factors are said to bring about genetic changes in individual organisms, which are then spread through its population or are eliminated by natural selection. Three populations of C. partellus were selected from widely separated localities approximately 500 km apart, namely, Muhaka in the Coast province, Katumani in Eastern province, and Mbita in Nyanza province. Although the localities differ ecologically, they all are suitable for maize crop farming and hence harbour the stalkborer insect pests. The bioassay technique which depend on the quantal or all-or- nothing response to quantify the biological activity of serially diluted samples, was performed using various concentrations of Dipel (a commercial Bt formulation from Abbot Laboratories) and a local Bt isolate (lCIPE 023). For these Bt serial dilutions, the quantal response sought from the test insect C. partellus was genetic variations between the three populations as revealed by percentage mortality and determined by random amplified polymorphic DNA (RAPD) analysis. Results with Dipel 0.250g/1 showed that there were significant differences at a 0.05, with a calculated F-value of 6.11 against a critical F-value of 3.47. According to Tukeys groupings analysis, the source of genetic variation between the populations was from the Mbita sample. Similarly, at dose of 0.375 g/l Dipel, there were significant differences in the genetic composition of the three C. partellus at a lower calculated F-value of 4.77 against the same critical F-value of 3.47 and the source of variation was again the Mbita population. But the variations in the genetic make up of the populations were not significant at the highest dose of 0.500 g/l With regard to the local Bt strain, significant differences in genetic composition in allopatric C. partellus populations occurred at dose dilutions 2 ug/rn 1 and 10 ug/rnl, with the respective calculated and critical F- values ratios of 3.78 : 3.47 and 3.58: 3.47 for both doses. In this case, the Tukeys grouping analysis test identified the source of genetic variation as being due to the Muhaka C. partellus population. Due to these significant genetic variations, it was inferred that the geographically isolated C. partellus populations contained enough variation to warrant distinctive races or even subspecies. This observation was supported by Nei's diversity estimate test for the genetic material of each population , which also revealed significant genetic diversity in Muhaka population (H=0.3724), followed by Katumani (fj =0.3069), while the least diversity was found in Mbita with a mean diversity estimate of (H=O.2716). However, the notion of distinctive races was not supported by the analysis of the same data using molecular variance (AMOYA). This method gave the percentage of variation within populations as much as 91.67 % and that between populations as little as 8.33%. Clearly, this showed that there was very little difference between the genetic composition of the three allopatric populations suggesting some gene flow between them. Nei's genetic distances grouped together into the same cluster the Mbita and Katumani populations while the Muhaka one formed a separate cluster on its own. This indicated that Mbita and Katumani are more closely related. Overall, these investigations revealed that although there was varied genetic diversity between the C. partellus populations in Kenya, allopatry was not responsible. Likewise this diversity may not be enough indicators for the formation of distinctive new species. However, there is a strong indication that genetic factors are responsible for C. partellus tolerance to Bt as evidenced by high percentage mortality probably due to the genetic diversity narrowed down by possible environmental bottlenecks which were not accounted for in this study.