Diversity and ecological preference of parasitoids associated with lepidopteran stem borers in Kenya
Mailafiya, D. M.
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Field surveys were carried out from 2005 to 2007 to assess parasitoid species diversity and stemborer parasitism in cultivated and natural habitats in four agroecological zones in Kenya. To increase the chances of collecting stemborer parasitoids, random sampling was employed in cultivated habitats, while both random and non-random samplings were applied in natural habitats. During this study, 7443 and 3676 stemborers were collected from cultivated and natural habitats, respectively. A total of 33 parasitoid species were recorded, of which 18 species were recovered from 6 stemborer species feeding on cereal crops, while 27 species were recovered from 21 stemborer species feeding on 19 wild host plant species. The most common parasitoid species in cultivated habitats were Cotesia flavipes (Hymenoptera: Braconidae), C. sesamiae (Hymenoptera: Braconidae), Pediobius furvus (Hymenoptera: Bethylidae) and Siphona sp. (Diptera: Tachinidae). In natural habitats, Siphona sp. was the most common. Parasitoid species diversity was significantly higher in natural than in cultivated habitats. Most of the parasitoid species recovered from cultivated habitats were also found in natural habitats. Natural habitats surrounding cereal crops appeared to serve as refugia for sustaining the diversity of stemborer parasitoids in adjacent cereal fields. Given that parasitoid diversity was higher in natural habitats with more diverse plant communities, but parasitoid abundance was higher in cultivated habitats, this study provided partial support for the `natural enemy' hypothesis. Across agroecological zones, stemborer parasitism was positively dependent on parasitoid species richness and abundance during different seasons in both cultivated and natural habitats. Rainfall had a significant negative effect on parasitoid abundance as well as stemborer parasitism in both cultivated and natural habitats. The negative relationship between parasitism and temperature in natural habitats only, suggested greater sensitivity of parasitoids to extreme temperatures in natural habitats. Across seasons, stemborer densities were generally higher on maize and sorghum in cultivated habitats than on wild host plants in . natural habitats. Stemborer parasitism rates were generally low (<15.00%) in both cultivated and natural habitats. Neither larval nor pupal parasitism differed significantly between seasons in natural habitats. Cotesia sesamiae was found in Kakamega and Suam in the Western and Rift Valley regions, respectively, where Busseola fusca (Lepidoptera: Noctuidae) predominates. By contrast, C. flavipes was found in Mtito Andei and Muhaka in the Eastern and Coastal regions, respectively, where Chilo partellus (Lepidoptera: Pyralidae) predominates. There was no indication of host plant and stemborer species specificity by C. sesamiae, as its host ranges varied with both habitat type and locality. Conversely, C. flavipes exhibited a high level of specificity to its target host C. partellus on maize and sorghum in cultivated habitats and on S. arundinaceum in natural habitats. Of the two Cotesia species, natural habitats served as a reservoir for C. flavipes only, because these habitats sustained some of the individuals on C. partellus during both rainy and dry seasons. This was however not applicable to C. sesamiae, as its cereal stemborer hosts B. fusca and Sesamia calamistis (Lepidoptera: Noctuidae) were scarce in wild host plants.