Fortuitous antixenosis in transgenic sugarcane: Antibiosis-expressing cultivar is refractory to ovipositing herbivore pests
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The ovipositional preferences of two stemboring pests, Eoreuma loftini Dyar and Diatraea saccharalis (F.) (Lepidoptera: Pyralidae), were evaluated in choice tests on transgenic sugarcane expressing Galanthus nivalis agglutinin (GNA) and the corresponding conventional near- isogenic sugarcane. Differences in various host-selection indices indicated that both D. saccharalis and E. loftini preferentially laid eggs on conventional sugarcane versus the transgenic cultivar. A significantly higher proportion of leaves per plant were allocated eggs on conventional relative to transgenic sugarcane plants by both D. saccharalis and E. loftini. In addition, larger proportions of both total eggs and egg masses laid by females were allocated to conventional (66-71%) relative to transgenic (29-34%) sugarcane, but the size of egg masses of both stemborer species did not vary with plant type. An oviposition preference index, computed as the ratio of the difference in egg loads (eggs or egg masses) allocated to transgenic or conventional plants and the total number of eggs laid, indicated that transgenic sugarcane plants were refractory to ovipositing E. loftini and D. saccharalis. However, differences in host-selection indices for transgenic and conventional sugarcane plants in D. saccharalis were associated largely with the presence of the transgene (i.e., plant type), while differences in E. loftini were associated largely with plant morphological characteristics related with plant type. The unintended antixenotic effects of transgenic sugarcane observed in this study are important because they highlight the validity of concerns over unanticipated consequences of releasing transgenic plants before intensive evaluation.