Sowing seed mixtures of transgenic and nontransgenic plants is one approach to establishing refuges for resistance management of crops transformed with Bacillus thuringiensis (Bt) toxins. We studied larval dispersal and survival of two rice stem borers, Scirpophaga incertulas (Walker) and Chilo suppressalis (Walker), to evaluate the potential effectiveness of seed mixtures for resistance management of Bt rice, Oryza sativa L. Experiments were conducted with two cry1Ab-transformed rice varieties, ‘IR58’ and ‘Tarom Molaii’, and corresponding nontransgenic controls. During 0–24 and 24–48 h after egg hatch, the proportion of C. suppressalis larvae dispersing from the natal plant did not differ between transgenic and control plants for either rice variety. Dispersal by S. incertulas did not differ between transgenic and control plants of IR58 during either 0–24 or 24–48 h, but a greater proportion of S. incertulas larvae dispersed from transgenic Tarom Molaii than from control plants 24–48 h after eclosion. Larvae that dispersed 0–24 h after eclosion on IR58 and 0–24 and 24–48 h after eclosion on Tarom Molaii were collected and transferred to stem pieces of control plants. No differences in survival were detected between larvae that dispersed from transgenic or control plants 24 (IR58) or 72 h (Tarom Molaii) after dispersal. Because both stem borer species move among plants during larval development, seed mixtures may not be the most effective approach to maintaining refuges for resistance management. However, additional experiments to compare the dispersal and fitness of Bt-resistant and -susceptible stem borer larvae are needed.
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