Base-line susceptibility of Busseola fusca for Bt maize in South Africa
Genetically modified maize that express insecticidal proteins (Bt proteins) have been commercialized in South Africa for the control of Busseola fusca (Lepidoptera: Noctuidae). Busseola fusca has been reported to be resistant to Bt maize (Cry1Ab protein) at several localities in South Africa. Reports of pest infestation in Cry1Ab Bt maize (MON810) are regularly made in several regions, however resistance has only been confirmed in few controlled laboratory experiments. There is an urgent need to evaluate B. fusca populations in South Africa for their susceptibility to Bt maize. The aim of the study was to screen different populations of B. fusca for resistance to Bt maize and to generate baseline data regarding pest susceptibility for South Africa. Results provided an indication of the resistance status of B. fusca populations across the maize production area. Stem borer larvae were collected from 11 different field sites in and around the main maize production area of South Africa. Laboratory feeding studies with maize events expressing Cry1Ab (MON810) and Cry1A.105+Cry2Ab2 (MON89034), were conducted to compare pest fitness to that on non-Bt iso-hybrids as control. Different life-history parameters were monitored during the laboratory feeding bioassays. These were: larval survival and mass, LT50, mortality, larval duration, pupation percentage, male and female pupal mass, male and female pupal duration, sex ratio and male and female moth longevity. Large differences in susceptibility were observed between populations. Larval survival of up to 54.8% on MON810 was observed in two populations and no survival was recorded on the MON89034 event. Larval mass for some populations was significantly higher on the non-Bt iso-hybrid compared to the single-gene event. The LT50 for larvae feeding on the non-Bt maize control treatments ranged between 16-33 days compared to those on MON810 treatments with 6-25 days, and MON89034 with 4-8 days. The corrected percentage mortality for a Venda population (susceptible) was 94.16% compared to the known resistant population from Vaalharts at 0%. Larval development period on non-Bt maize was shorter compared to that on the MON810 treatment. No significant difference was observed between the non-Bt and Bt treatment in terms of the pupal mass, sex ratio or moth longevity. This study documented the levels of resistance of B. fusca and will allow us to be able to give earlywarning if this pest also evolves resistance to the pyramid events which have been launched in South Africa from 2013 onwards.