Evaluation of bioassays for testing Bt sweetpotato events against sweetpotato weevils.
Sweetpotato weevil (Cylas puncticollis) Boheman is a serious pest throughout Sub-Saharan Africa region and is a big threat to sweetpotato cultivation. Ten transgenic sweetpotato events expressing Cry7Aa1, Cry3Ca1, and ET33-34 proteins from Bacillus thuringiensis (Bt) were evaluated for resistance against C. puncticollis. Four bioassays used were: (i) 1st instar larva in an artificial diet using root powder of transgenic events, (ii) whole root of transgenic events infested with female adults, (iii) root chip, and (iv) small root of transgenic events both infested with egg-plugs. DAS-ELISA analysis showed variation in Cry protein concentration among the events that ranged between 0.1 and 0.394 μg g-1 of root fresh weight. The highest protein quantity was observed in the event carrying the ET33-34 transcript. In general, transgenic events had no significant effect on larval survival (P = 0.28) and pupal (P = 0.86) development, though maximum pupation of 96% was observed on event CIP410009.12 that had very low expression levels of cry3Ca1 gene. Root chips were prone to damage by fungi and desiccation especially during the first larval instar. The whole root bioassay had less handling injuries to weevils compared with other methods. However, it requires a large number of adult females for oviposition and roots per event to be tested to obtain efficacy results with statistical rigour. The small root egg plug bioassay required much fewer roots and experimental insects to assess larval mortality and development. The root chip method was the least desirable because of susceptibility to fungal and bacterial contamination. In addition, this method was the most labour intensive in terms of frequent replacement of root chips for weevil development. Hence, the most appropriate method for testing Bt efficacy in sweetpotato is the small root egg-plug bioassay. Nonetheless, none of the transgenic events tested provided weevil control probably because of low Cry protein expression in storage roots.
Keywords: Cry genes, Cylas puncticollis, Ipomoea batatas