Inter-plant movement of first-, third- and fifth-instar tobacco budworm, Heliothis virescens, (F.) were observed in field plots containing mixtures of transgenic (Bt) cotton plants, Gossypium hirsutum, expressing the D-endotoxin from the bacterium, Bacillus thuringiensis Berliner spp. kurstaki, and non-transgenic cotton. The ratios of Bt:non-Bt plants used in the mixture stands were: (1) 100: 0, (2) 90:10, (3) 80:20, (4) 70:30, and (5) 0:100. We found that 37-38% of third- and fifth-instar tobacco budworms moved at least one plant in 24- and 48-hour periods. There was a trend for fifth-instar tobacco budworms to move greater distances and a greater number of plants as the proportion of Bt plants increased in the mixed stands compared with pure stands of non-Bt cotton. Fifth-instar tobacco budworms moved a significantly greater distance in the pure stand of Bt than in the pure stand of non-Bt at 24 hours after infestation. Third-instar tobacco budworms moved a shorter distance and less often than fifth-instars.

A field study was also conducted in 1994 and 1995 to determine the efficacy of different Bt and non-Bt plant mixtures on suppression of tobacco budworm and bollworm, Helicoverpa zea (Boddie). The mixture ratios were: (1) 100:0, (2) 90:10, (3) 85:15, (4) 80:20, (5) 75:25, and (6) 0:100. One other treatment (7) 0:100 was sprayed as needed for lepidopteran control. The other six treatments were not sprayed to suppress tobacco budworm or bollworm. As the proportion of non-Bt plants increased in a mixture, the number of larvae and amount of flower bud and boll injury increased for both years. Terminal damage also increased as Bt plants were decreased in mixtures in 1995.

Lint yield increased as the density of Bt plants in a mixture increased. The number of eggs in terminals was not significantly different between mixtures and pure stands of Bt or non-Bt. The number of injured flower buds, bolls, terminals and number of larvae in the pure stand of non-Bt that was sprayed for lepidopteran control was not significantly different from the pure stand of non-Bt that was not sprayed.

These data suggest that tobacco budworms and bollworms can develop on non-Bt plants in mixtures, move to Bt plants, feed, cause injury and be selected for resistance. Moreover, all adult tobacco budworms and boll-worms produced from mixed stands containing Bt cotton plants will have fed on Bt plants and have been selected for resistance to the Bt toxin.