Busseola fusca and Chilo partellus survival on Bt and stacked-gene maize varieties
Genetically modified Bt maize expressing Cry proteins are used to control lepidopteran pests of maize. Damage caused by these stemborer species Busseola fusca (Lepidoptera: Noctuidae) and Chilo partellus (Lepidoptera: Crambidae) may result in economically important yield losses. Effective control of insect pests on maize is therefore of utmost importance. However, B. fusca has been reported to be resistant to Bt maize in South Africa. The main reason for resistance evolution was thought to be poor compliance to the high dose/refuge strategy requirements, but later studies showed that other factors could also have played a role. Therefore the possible effect of stacking of insecticidal (Bt) and glyphosate resistance (GR) traits as well as application of glyphosate to plants of stacked events (Bt/GR) on the efficacy of Bt needed to be investigated. The aim of the study was to determine if the combination of Bt and GR traits as well as the application of glyphosate have an effect on the efficacy of Bt maize against stem borers and whether the Bt expression levels in plants may be effected when stacked with the GR trait or if glyphosate is applied. The survival and larval development of two stem borer species were evaluated on single gene (Bt and GR) and stacked gene (Bt/GR) maize cultivars. Laboratory bioassays were conducted over three consecutive seasons and in greenhouse trials (2014/15). These six treatments were: 1) non-Bt control, 2) Bt-maize, 3) GR maize, 4)Bt/GR stacked maize, 5) GR maize sprayed with glyphosate and 6) Bt/GR stacked maize sprayed with glyphosate. All Bt gene varieties expressing the same Bt gene (MON810) were used. Plants were inoculated either with B. fusca or C. partellus in a greenhouse trial whereas plant tissue was used in the laboratory trials. Although not significantly different, larval survival recorded from plant tissue of the GR treatment and GR maize onto which glyphosate was applied tended to be the highest in six out of the seven trials for B. fusca and five out of seven for C. partellus. Results indicated that the GR trait together with the application of glyphosate does not have an adverse effect on larval development, and that it may have a positive effect on larval survival and growth. Larval development as the only effect indicators of stacked traits and glyphosate application on Bt expression levels cannot be used solely to make a final conclusion. Results from ELISA’s that were conducted using leaf tissue of plants of all treatments showed large variation in the Bt expression levels in these plants. Indications were however that glyphosate application or the presence of the GR trait in a stacked event with Bt trait, did not affect Cry protein expression in plants.