Screening for Development of Resistance in the African Stem Borer (Busseola fusca Fuller) and in the Spotted Stem Borer (Chilo partellus Swinhoe) to Bt-Maize o-endotoxins
The African stem borer, Busseola fusca Fuller (Lepidoptera: Noctuidae), and the spotted stem borer, Chilo partellus Swinhoe (Lepidoptera: Pyraridae), are the major stem borers species of great economic importance in Kenya. Studies were carried out in the biosafety level 2 greenhouse to determine the development of resistance to Bt o-endotoxins by stem borers for five and eight generations of B. fusca and C. partellus, respectively. Treatments included Bt-maize containing either of the two gene constructs (Event223: :cry lAb: :Ubiquitin and event 10::cry I Ba:: Ubiquitini, and the non-transgenic CML216 control. B. fusca colony was developed from collection from Kitale in Highland Tropical maize growing zone, while C. partellus colonies were from the Coast Lowland Tropics and from the Dry Mid-altitude areas of Kenya. A fourth colony (mixed colony) was derived from mating the C. partellus from the Humid Coastal Lowland Tropics and the Dry Mid-altitude areas of Kenya. Sowing of the maize was synchronized to pupae stages of the insects. Three hundred (300) neonates from each colony were infested into maize leaves at the six to eight-leaf stage and allowed to feed for one to three hours, with the exception of B. fusca, which was allowed to feed for 48 hours. The surviving larvae were removed and reared in artificial diet up to completion of their life cycles. The subsequent generations were similarly exposed to the o-endotoxins, and allowed to complete their life cycle in artificial diet. The responses to the o-endotoxins were assessed over time by counting the number of surviving larvae for each generation cycle, and by measuring pupae weight for each cycle. Results showed significantly fewer surviving larvae from both Bt-maize events compared to the non-transgenic CML 216 control. There was a significant difference (P<O.05) between the pupae weight of the control and those fed on Btmaize tissues. There were no significant differences between the C. partellus colonies from the different maize growing zones in Kenya, in their response to Bt-maize 8- endotoxins. There was also no significant difference among the cycle of selection of the two Bt-maize events (P<O.05). There was neither development of resistance to C. partellus for eight generations nor increase in resistance in B. fusca for five generations the Bt-maize events that were studied. This information could be used as baseline data in developing resistance management and deployment strategies to extend the efficacy of Bt-maize.