Fig. 2

Effects of Bt broccoli and OX4319L releases on caged Plutella xylostella populations over multiple generations. Caged populations were established with hybrid Bt-resistant/wild-type insects – the founder strain – with a low estimated frequency of homozygous-resistant individuals. a Graph shows the mean peak population size per plant, per generation, in four experimental treatments over the experimental period: Treatment 1, Bt broccoli, no OX4319L releases; Treatment 2, Bt broccoli, low-rate weekly OX4319L releases (release rate of 3:1 in Generation 1, increased to 5:1 in subsequent generations); Treatment 3, non-Bt broccoli, low-rate weekly OX4319L releases (identical release rates to Treatment 2); and Treatment 4, non-Bt broccoli, high-rate weekly OX4319L releases (release rate of 20:1 in Generation 1, increased to 40:1 in subsequent generations). Means were calculated from three experimental cage replicates, with the exception of Treatments 2 and 4, which were reduced to two and one cage replicates in Generation 3, respectively. Treatment 3 cages were terminated in Generation 3 as the insect populations had reached maximum capacity. Error bars represent standard error of the mean. b Bt survival assays. Mean survival of third-instar larvae from three experimental cage treatments and the founder strain used to begin these experimental treatments when exposed to a discriminating dose of Bt in artificial diet assays. Bt dose in this assay is high enough to ensure that only homozygous Bt-resistant individuals will survive (as in the high-dose/refuge strategy). This assay therefore indicates the proportion of each population remaining Bt-resistant (homozygous) and Bt-susceptible (heterozygous or homozygous-susceptible). For each cage, two Bt assays and one no-Bt control assay were performed. Bt assays in each cage were summed and means represent averages of each set of treatment cages corrected for control mortality. The assays took place using individuals from the final generation in which each treatment was run or, in the case of the founder strain, in the generation prior to the start of the experiment. Survival was corrected for control mortality prior to analysis and error bars represent Pearson’s exact confidence intervals. Survival of insects from the low OX4319L release cages (Treatment 2) was not significantly different from the founder strain; other pairwise comparisons are significantly different (Table 1)