Diversity and ecological preference of parasitoids Associated with lepidopteran stem borers in kenya

Abstract

Field surveys were carried out from 2005 to 2007 to assess parasitoid species diversity and stem borer parasitism in cultivated and natural habitats in four agroecological zones in Kenya. To increase the chances of collecting stem borer parasitoids, random sampling was employed in cultivated habitats, while both random and non-random samplings were applied in natural habitats. A total of 7443 and 3676 stem borers were collected from cultivated and natural habitats, respectively. Thirty-three species of parasitoid were recovered. Eighteen of them from six stem borer species feeding on cereal crops, and twenty seven from twenty-one stem borer species feeding on nineteen 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. Majority of the parasitoids were stenophagous species and only the following five species were monophagous: Cotesia sp., Enicospilus ruscus (Hymenoptera: Ichneumonidae), Pristomerus nr. bullis (Hymenoptera: Ichneumonidae), Sturmiopsis parasitica (Diptera: Tachinidae) and Syzeuctus ruberrimus (Hymenoptera: Ichneumonidae). In both cultivated and natural habitats, parasitoid species diversity was highest on the most dominant stem borer genera, Busse ola spp. (33.0%) and Chilo spp. (30.0%). On cereal crops, parasitoid diversity was highest on maize (53.0%), and among wild host plants, it was highest on Setaria spp. (20.0%). Parasitoid species diversity was significantly higher in natural than in cultivated habitats. Additionally, most of the parasitoid species were found in both cultivated and natural habitats. It was concluded that natural habitats surrounding cereal crops provide refugia for sustaining the diversity of stem borer parasitoids in adjacent cereal fields. Based on habitat types, results obtained in this study provided partial support for the ‘natural enemy’ hypothesis. On the one hand, parasitoid diversity was higher in natural habitats which had more diverse plant communities than in cultivated habitats. On the other hand, parasitoid abundance was higher in cultivated than in natural habitats. This was probably due to higher fitness of parasitoids from stem borers feeding on cultivated cereals than those from wild host plants. Although stem borer and host plant richness or abundance had significant positive effects on parasitoid richness and abundance, these effects varied with, habitat type and season within a given agroecological zone. In all agroecological zones, stem borer 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 stem borer parasitism in both cultivated and natural habitats. As rainfall increases with altitudes, its detrimental effects are expected to be more severe at high elevations, especially in areas with bimodal rainfall distribution. The relationship of parasitism to temperature, being positive in cultivated habitats, but negative in natural habitats suggests greater sensitivity of parasitoids to extreme temperatures in natural habitats. The current findings also showed that for each habitat type, the incidence of plant infestation by stem borers, stem borer densities, larval/total parasitism and parasitoid abundance/the number of off-springs per borer host either varied significantly among localities during different seasons or between seasons within a given locality.Maize in cultivated habitats had the highest values for the following parameters:percentage plant infestation (23.22%), stem borer densities (mean±SD: 2.14±0.22),percentage larval (15.00%), pupal (4.72%) or total (15.18%) parasitism and mean(±SD)parasitoid abundance (5.8±0.21) or adult emergence per host (24.0±0.13). In natural habitats, Sorghum arundinaceum was leading in percentage plant infestation (15.48%),mean(±SD) stem borer density (0.22±0.07), percentage larval (9.50%) or total (9.53%) parasitism and mean(±SD) adult emergence per host (6.53±0.09), while Panicum spp.and Cyperus spp., had highest values of percentage pupal parasitism (2.50%) and mean(±SD) parasitoid abundance (0.8±0.02), respectively. Overall, across seasons, stem borer densities were generally higher on maize and sorghum in cultivated habitats than on wild host plants in natural habitats. Stem borer parasitism was generally low (<15.00%) in both cultivated and natural habitats. Neither larval nor pupal parasitism differed significantly between seasons in natural habitats. The distribution of C. sesamiae and C. flavipes in different ecological regions in Kenya was most influenced by the suitability of the local stem borer species community. Cotesia sesamiae was found in Kakamega and Suam in the Western and Rift Valley Regions, respectively,where Busseola fusca (Lepidoptera: Noctuidae) predominates. While C. flavipes was found in Mtito Andei and Muhaka in the Eastern and Coastal regions, respectively,where Chilo partellus (Lepidoptera: Pyralidae) predominates. The non-existence of the two Cotesia species in all localities was very likely due to lack of mixed populations of hosts suitable for either parasitoid species. There was no indication of host plant and stem borer 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. Perennation by both Cotesia species occurred mainly in cultivated habitats, with relatively high stem borer parasitism and cocoon mass during both rainy and dry seasons. Stem borer parasitism by either Cotesia species, their cocoon mass and brood size were generally low across seasons in natural habitats. Of the two Cotesia species, natural habitats served as reserviour for C. flavipes only, as it sustained some of its individuals on C. partellus during both rainy and dry seasons. This was however not applicable to C. sesamiae, as its cereal stem borer hosts B. fusca and Sesamia calamistis (Lepidoptera: Noctuidae) were scarce in wild host plants.