Comparative evaluation of certain aspects of the biology of cotesia flavipes and cotesia sesamiae (hymenoptera: braconidae) for the management of chilo partellus (lepidoptera:pyralidae) in Kenya
Sallam, Mohamed Nader Said
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A comparative study was conducted on two gregarious larval endoparasitoids of lepidopteran stemborers, Cotesia flavipes Cameron and Cotesia sesamiae (Cameron) (Hymenoptea: Braconidae). The first parasitoid was imported from Pakistan and released at the Kenya Coast during the long rains of 1993 in an attempt to increase the natural control of the spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae). The second is indigenous to Africa, and it is the most common parasitoid of gramineous stemborers in Kenya and other parts of East and Southern Africa. The dispersal capacity of the exotic parasitoid was tested in a 100x100 meter maize field at Kilifi, Kenya coast. Results showed that female parasitoids were able to fly at least 64.03 meters downwind before parasitising a host. The highest rate of parasitism was found inside the maize plant where the majority of suitable hosts were located. A study of the funcional response of the two parasitoids on C. partellus and an indigenous stemborer, Sesamia calamistis (Lepidoptera: Noctuidae), was conducted in large field cages at the Kenya coast and in the laboratory. Results showed that C. Flavipes had as stronger functional response than C. Sesamiae when C. Partellus was the host, while differences were not significant when S. Calamistis was the host. Studies on host discrimination indicated that naive as well as experienced females of the two parasitoids oviposited in hosts previously parasitised by the other species. However, experienced female of the two species were significantly more attracted to non-parasitised host larvae than larvae parasitised by conspecific females. Fitness consequences of superparasitism were examined for the two parasitoids on both hosts. Progeny production of C. flavipes increased gradually with increasing numbers of stings, but leveled off at three stings per C. partellus larva,then decreased. Cocoon weight, emergence and sex ratio of progeny were not affected by superparasitism. However, larval development was lengthened, and the longevity and fertility of adult progeny decreased. When S. calamistis was the host, no difference was found in the duration of immature stages, emergence rate, progeny production or sex ratio of the progeny, but cocoon weight, adult longevity and fertility were negatively affected. When C. partellus was the host for C. sesamiae, low progeny production and poor survival of host larvae were recorded over the different number of stings. Moreover, larvae that were stung more than three times died. Multiple stinging led to a male biased sex ratio and low fertility of female progeny. When S. calamistis was the host, progeny production of C.sesamiae increased with the number of stings, but leveled off at at three stings per S. Calamistis larva, then decreased. No significant effect of superparasitism was found on the rate of emergence, longevity or sex ration of adult progeny. However, longer duration of immature stages, lower cocoon weight and lower fertility of adult females were recorded. Multiple parasitism studies showed that C.flavipes was able to eliminate C. sesamiae during the egg stage when C. partellus was the host. However, When S. calamistis was the host, both Cotesia species emerged from host larvae, but with a higher number of C.flavipes. C. flavipes appeared to be competitively superior, both intrinsically and extrinsically, to its African Congener, C.sesamiae, when C. partellus was the host. Both species were equally competitive when the host was S. calamistis. A possibility of local displacement of C.sesamiae by C. flavipes in areas dominated by C. partellus is therefore expected. The implication of this displacement on the impact of biological control of stemborers in Kenya is discussed.