Greenhouse tests on resistance management of Bt transgenic plants using refuge strategies

Experimental evaluation of the effectiveness of resistance management tacti cs is vital to help provide guidelines for the deployment of transgenic ins ecticidal crops. Transgenic broccoli expressing a Cry1Ac gene of Bacillus t huringiensis (Bt) and the diamondback moth, Plutella xylostella (L.), were used in greenhouse tests to evaluate the influence of size and placement of nontransgenic refuge plants on changes in resistance allele frequency and pest population growth. In the first test with an initial Cry1Ac-resistance (R) allele frequency of 0.007, P. xylostella were introduced into cages wi th the following treatments: 0, 3.3, 10, 20, and 100% refuge plants. Result s after four generations showed that resistance could be delayed by increas ing the proportion of refuge plants in the cage. Population growth was also influenced by refuge size with the highest populations occurring in treatm ents that had either no refuge plants or all refuge plants. In the second t ests, we evaluated the effect of refuge placement by comparing 20% separate and 20% mixed refuges. P. xylostella with an initial frequency of resistan t alleles at 0.0125 were introduced into cages and allowed to cycle; later generations were evaluated for resistance and population growth. Separating the refuge had a pronounced effect on delaying resistance and slowing esta blishment of resistant larvae on Bt plants. Combining information from both trials, we found a strong negative correlation between the number of larva e on Bt plants and the mortality of the population in leaf dip bioassays. R esults from larval movement studies showed that separate refuges delayed re sistance better than mixed refuges because they conserved relatively more s usceptible alleles than R alleles and did not increase the effective domina nce of resistance.