Assesment of Sustainability of Different Populations of Stemborer Species for the Development of Cotesia Flavipes (Hymenoptera: Braconidae) and the Establishment of the Latter in Zimbabwe.
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Maize and sorghum are very important crops in Zimbabwe. They are, however, attacked by several pests, among them stemborers. Host suitability studies using Cotesia flavipes Cameron (Hymenoptera: Braconidae) against Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae) populations from Bushu, Mamina, Musikavanhu, Muzarabani and Sanyati as well as Busseola fusca Fuller (Lepidoptera: Noctuidae) from Harare were conducted. The studies showed that C. flavipes successfully developed on all the C. partellus populations investigated. The highest parasitism was 77.3 %, which was recorded for Sanyati, and the lowest was 61.9 %, which was recorded for Mamina. Muzarabani had the shortest parasitoid developmental period of 17.4 days while Mamina had the longest of 18.2 days. The progeny was generally female - biased and the highest percentage was 78.1 recorded for Bushu. However, on Muzarabani C. partellus, adult female parasitoids comprised 13.3 % of each brood. Muzarabani C. partellus also recorded the lowest number of progeny (11.1 adults/brood) while the other sites recorded between 26.5 and 27.2. No development took place on B. fusca due to egg encapsulation. Releases of C. flavipes were conducted for the first time at Mamina, Coburn Estates (Chegutu). Three different release methods were used. These included the release of mated adults, release as C. flavipes cocoons and the release of pre - stung C. partellus larvae. A recovery rate of 4 % was recorded at Sanyati in the first season of release. Continued monitoring is needed to confirm establishment. Prospects for the establishment at both sites are high considering the results of host suitability studies and the presence of large numbers of the co - evolved host, C. partellus. The composition of stemborer species and their associated natural enemies as well as the establishment of C. flavipes Cameron were studied during the cropping seasons of 2003/4 and 2004/5 at various lowveld (< 600 m), middleveld (600 - 1200 m) and highveld (> 1200 m) sites. This involved sampling infested maize and sorghum plants, dissecting them and recording the stemborer species and their natural enemies. Three species were recorded, namely, C. partellus, B. fusca and Sesamia calamistis Hampson (Lepidoptera: Noctuidae. Chilo partellus dominated in the lowveld (Muzarabani and Musikavanhu) and middleveld (Bushu and Sanyati) while B. fusca dominated the highveld (Mamina). Sesamia calamistis occurred in all the agroecological zones under consideration but in very low proportions. It was also noted that C. partellus is colonizing the highveld as revealed by its abundance at Mamina. Cotesia flavipes was released for the first time in the country between July 1999 and March 2001 before recent releases which were conducted between July 2004 and April 2005. The surveys to determine the establishment of C. flavipes have been going on since July 1999 depending on the location. However, establishment was only confirmed at Musikavanhu, Bushu and Muzarabani where it parasitised 22.5 %, 23.2 % and 3 % of C. partellus larvae respectively sampled in the 2004/5 season. During the same period, the indigenous Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) was associated with all the three stemborer species. Other indigenous natural enemies recorded included the nematode, Hexamermis sp. (Mermithidae) which parasitised 0.7 % C. partellus larvae at Bushu and the pupal parasitoid, Dentichasmiasis busseolae Heinrich (Hymenoptera: Ichneumonidae) recovered at Musikavanhu and Muzarabani which parasitised 20 % of the field collected pupae from each of the two sites.