| Transgenic corn expressing delta-endotoxins from Bacillus thuringiensis Berliner is highly effective in controlling European corn borer—the major target pest. However, this technology has potential disadvantages, particularly insect resistance. High toxin expression in conjunction with a refuge of susceptible insects is the accepted resistance management strategy for Bt corn. However, this approach may be problematic for more tolerant target pests, such as the corn earworm (Helicoverpa zea (Boddie)). Thus, this project determined the effects of MON810-based Bt corn on corn earworm and whether the high-dose condition is satisfied, and also identified potential factors that may contribute to resistance development by this pest. Field studies showed that Bt corn reduced corn earworm populations by an average 75%, thus indicating a moderate dose expression and the need for a different resistance management plan than the one currently recommended. The variable expression of toxin among kernels and the pattern of kernel feeding by larvae in Bt ears suggested the potential for behavioral adaptation by larvae to avoid toxin-expressing kernels. Surviving larvae of sublethal Bt exposure exhibited retarded development and reduced fecundity as adults. This effect caused a delay in moth emergence and triggered a greater portion of pupae to enter diapause. Such changes in insect phenology may result in asynchronous mating between refuge and Bt-produced populations, thus potentially increasing the rate of resistance development. Non-functional ears, which degraded as the corn plant senesced, had the same effect on larval survival and weight gain, as did functional ears. Thus, these ears did not serve as a means of refuge expansion or escape from toxin exposure. Larvae reared on Bt corn exhibited less aggressive behavior when paired with unequally-sized larvae. This suggests that susceptible larvae may be at a disadvantage during cannibalistic encounters with resistant individuals, thus favoring resistance genes. Despite the potential for resistance development by corn earworm, MON810-based corn has many benefits for corn earworm management including reduced kernel injury, less adult recruitment, and fitness disadvantages. Since moths recruited in corn move to other host crops later in the season, Bt corn may provide localized management of corn earworm populations. |
