An investigation into the development and status of resistance of Busseola fusca (Lepidoptera: Noctuidae) to Bt maize
Based on surface area, South Africa is currently ranked 8th in planting genetically modified (GM) crops in the world. The stem borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is of economic importance throughout sub–Saharan Africa. Bt maize (MON810) has been grown to control lepidopterous stem borers in South Africa since its first release in 1998. The first report of resistance to Bt maize was made in the Christiana area of South Africa in 2007. The objectives of this study were to evaluate the status of resistance of populations of B. fusca to Bt maize; to evaluate farmers' perceptions of the regulatory aspects guiding the planting of Bt maize and refugia and how the field situation developed between 1998 and 2010; to compare the fitness of the fertility, fecundity and longevity of Bt–resistant and susceptible B. fusca populations and to determine if there are fitness costs associated with resistance of B. fusca to Bt maize. Questionnaire surveys were conducted amongst 185 farmers in seven districts throughout the maize production region. The questionnaire addressed signing of contracts upon purchasing GM seed, refuge compliance, pest management practices, perceived benefits and risks relating to Bt maize. In order to study fitness and fitness costs that may be associated with resistance development, the life history parameters of known Bt–resistant and susceptible populations were compared in the laboratory using a diapauses–as well as second–generation populations collected in maize fields. The following parameters were compared between different stem borers populations and treatments: pupal mass, moth longevity, fecundity, fertility, larval mass and survival, and sex ratio. This study confirmed resistance of B. fusca to the Cry1Ab toxin (MON810) and that larvae collected from refugia at Vaalharts were resistant and survived on Bt maize. Compliance to refugia requirements was low especially during the initial 5 – 7 years after release. An alarmingly high number of farmers applied insecticides as preventative sprays on Bt maize and refugia. Except for moth longevity and LT50– values, no other fitness costs were observed to be associated with the resistance trait in the highly resistant B. fusca population used in this study. The LT50 may indicate some degree of fitness cost but does not translate into observable costs in terms of fecundity, larval mass and survival. The absence of fitness costs may promote the use of alternative Bt–resistance management strategies, such as the introduction of a multigene strategy. The introduction of a stacked event such as MON89034 which produces more than one protein with activity active against the resistant target pest, together with compliance to the refuge strategy, is most likely the only solution to managing Btresistant stem borer populations in South Africa.