Managing and monitoring genetic isolation and local adaptation of endemic and introduced Cotesia sesamiae for the biological control of the cereal stemborer Busseola fusca in Cameroon

Abstract

The success of biological bontrol (BC) introductions can be enhanced by considering theory and knowledge of biological systems. The gregarious braconid parasitoid Cotesia sesamiae (Cameroon) is one of the best studied biological control agent from the perspective of molecular ecology. Its evolutionary adaptation to the target host involves symbiotic partners. Polydnaviruses are responsible for immune and developmental adaptations whereas Wolbachia bacteria may reinforce this local adaptation though genetic isolation mechanisms. The noctuid Busseola fusca is a major stemborer pest of maize in sub-Saharan Africa. In contrast to eastern Africa, C. sesamiae is rarely found on B. fusca in western Africa. It is however often obtained from other stemborer species feeding on wild grasses. A biological control project was launched in 2006–2007 by introducing to Cameroon seven crosses of Kenyan populations of C. sesamiae collected in different ecozones. They included populations adapted to B. fusca that develop on maize as well as populations adapted to other hosts feeding on wild plants to allow carryover between cropping seasons. Wolbachia strains responsible for cytoplasmic reproductive incompatibilities with endemic strains were included in the crosses to limit genetic exchanges between introduced and endemic C. sesamiae and preserve genetic adaptation to B. fusca of the introduced populations, while at the same time preserving their ability to survive on wild plants. Six post release surveys were carried out on maize from 2007 to 2013, and on wild grasses in 2013. A total of 393 C. sesamiae individuals, each from one cocoon mass, were genotyped for 11 microsatellite loci. Multidimensional scaling analysis, STRUCTURE and GENECLASS analyses assigned almost all the parasitoids recovered from maize to those introduced from Kenya. The introduced strains were also recovered from wild host plants with little genetic exchanges with endemics. Each population remained strongly associated with its original Wolbachia component, suggesting that Wolbachia may contribute to genetic isolation between endemics and introduced populations in wild host plants when maize is absent, thereby conciliating biological control success and safety.