The role of gut microbes on the efficacy of Bt maize against lepidopteran stem borers

Abstract

The evolution of pest resistance to Cry proteins threatens the long-term use of Bt crops. Busseola fusca developed resistance to Bt maize in South Africa but the mechanism of resistance is not well understood. According to the gut microbiota theory, extensive cell lysis caused by Cry proteins provide gut microbes access to the more favourable environment of the hemocoel where they germinate and reproduce, causing septicemia and death of the host. This theory brought on questions about the role of gut microbes in the efficacy of Bt maize against target lepidopteran pests. The aim of this study was to determine whether microbes present in the mid-gut of B. fusca influence the efficacy of Cry 1Ab proteins. Larvae were collected from 30 different geographical locations, dissected to excise the mid-gut and mid-gut content which was separated according to morphological types. The morphological types were used to test the antibiotic susceptibility of the bacteria and proved that ciprofloxacin, ampicillin and doxycycline were the most effective bacteriostatic and bactericidal antibiotics. These three antibiotics were exposed to the morphological types at different concentrations to visualise the possible deleterious effects of the antibiotics on the bacteria. This visualisation was performed by observing the growth curve of the bacteria in the presence of the combination of antibiotics. The antibiotics concentration of 500 μg/ml showed the highest efficacy compared to the other concentrations tested. An antibiotic concentration of 500 μg/ml of ciprofloxacin, ampicillin and doxycycline was incorporated into an artificial diet for the larvae to feed on for 7 days. This method was used to rid the larvae of gut microbes before allowing them to feed on Bt maize (MON810) plant material expressing Cry proteins. The results suggests that by placing antibiotic reared larvae on a Bt plant, the absence of the mid-gut microbes contributed to larvae survival on Bt maize. This observation will contribute to understanding the role of gut microbes on the efficacy of Cry proteins.