A Comparative Study Of The Development Of Cowpea Weevil Callosobruchus Maculatus (F) (Coleoptera: Bruchidae) In Some Kenyan Cowpea Varieties As A Measure Of Their Resistance To The Pest
Experiments were undertaken to compare the performance of the bruchid Callosobrnchus maculatus (V) on four different cowpea varieties commercially cultivated in Kitui district, Eastern province. Kenya. The local varieties tested were Ndamba (Kitui red), Kathoka (Kitui cream), Kanga'u (Kitui white) and Ndune (Kitui brown). One exotic variety Tvu 2027 which is known to be resistant to C. maculatus was used as a standard resistance check for the local varieties. The • » parameters tested were namely: oviposition preference and the associated adult bruchid emergence; egg hatching patterns, adult emergence patterns over one generation of bruchid development inside the seed; population increase over three generations and the associated seed weight loss. Initial field infestation was found to be very low (below 3%). On the variety Kitui white only.2.63% infestation was observed while infest fitions of 2.56, 2.53% and 2.50% were recorded on Kitui brown, Kitui red and K;tui cream, respectively. Oviposition preference tests showed significant (.P = 0.05) differences in the average number of eggs 4 r laid on samples of 20 seeds of each local variety. Based ort the number of eggs laid, the variety Kitui brown which had the highest number (4.14) of eggs deposited per seed was apparently the most susceptible of these local varieties. On the other hand the variety Kitui red with 2.84 eggs/seed deposited was considered the least susceptible variety while Xitui white (3.76) and Kitui cream (3.38) were regarded as being intermediates. Observations on the associated bruchid emergence also followed a similar trend, with Kitui brown (2.88 adults/ seed) being considered the most susceptible variety and Kitui red (2.24 adults/seed) as the least susceptible variety. Again the varieties Kitui white (2.67 adults/ seed) and Kitui cream (2.54 aciults/seed) emerged as intermediates between the two in their levels of susceptibility. The results of egg-adult survival however indicated that Kitui red with 78.87% was a potentially highly susceptible variety. This was confirmed by experiments carried out over four bruchid generations. In these experiments. Kitui red yielded the highest number of adults at the three temperatures (20°C , 27°C and 34°C) tested and lost the greatest weight. Kitui white on the other hand yielded the least number of adults over each of the three temperature regimes used and lost the least amount of weight. Cased on these results, Kitui white finally emerged as the least susceptible among the local varieties tested while Kitui red was the most susceptible to the pest. When the local varieties were tested against the standard resistant check Tvu 2027, it was found that all the local varieties were susceptible. There were significant differences (P = 0.05) between Tvu 2027 and all the local varieties together in the preference lor oviposition and also in the adult bruchids that emerged from the eggs. On Tvu 2027, a 21.Hl% egg-adult survival was recorded. Local varieties had much higher egg-adult survival percentages (Kituj. red = 77.34; Kitui brown = 70.19%; Kitui white - 75.00%; and, Kitui cream = 74.19%). It was therefore concluded that these local varieties did not have appreciable levels of resistance to the pest. In addition it was observed that the variety Kituj white received fewer eggs when the local varieties were tested together with Tvu 2027. This was in comparison with the situation when Kitui white was tested together with other local varieties in the absence of Tvu 2027. This alteration in the oviposition behaviour of the bruchids was attributed to the similarity of the colour of seeds of the twp varieties. It was considered that the non-preference effect on oviposition of the bruchids of the seeds of Tvu 2027 which are white in colour and have rough seed coats was transferred to the white and smooth seeds of the local variety, Kitui white. In addition, the experiments revealed that at a lower temperature (20°C) the pest population increase was lower than at higher temperatures (27°C and 34°C). "Similarly seed weight loss for all the varieties was the lowest at 20°C. In conclusion, the studies reported here revealed the following: a) that all the local varieties tested did not possess any appreciable levels cl resistance co the pest, but that their levels cl susceptibility were variable with Kitui white on the overall being least susceptible; b) that infestation of the seeds started in the field but the level of infestation was very low (below 3%); c) that Tvu 2027 (resistant check) when presented together with the local varieties altered the oviposition preference of the bruchid, with fewer eggs being deposited on seeds of Kitui white whose colour was similar to that of Tvu 2027 than when the variety was presented together with only the local varieties; and d) that at 20°C the damage to cowpea seeds by the pest was relatively less than at 27°C and 34°C.
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