Larvae and pupae per Bt plant 9 l HOLD TOKH 6 3 6 9 J+ Shumper 12 15 Generation -- Mosaic, Cry1Ac plant --0-- Mosaic, Cry1C plant -- Sequential, Cry1Ac plant --- Sequential, Cry1C plant Pyramid, two-gene plant 1 18 21 24 Figure 1 The figure above shows the average numbers of larvae and pupae of the moth found per Bt plant in each of the treatments. Note the logarithmic scale on the y- axis.

Question 1. You may have noticed that in each of the three treatments, a portion (20%) of the field contained plants without any toxins. Leaving parts of the planted field pesticide-free (refuges) is common practice in biological control and is mandated in the use of Bt plants. What purpose(s) would such refuges serve in slowing down the rate of evolution of resistance? (Hint: Resistance to a pesticide often incurs a cost to the resistant individual.)

Another reason for refuges is that many of the alleles that confer resistance to Bt and other pesticides are recessive. Refuges allow for the dilution of the Bt-resistance alleles such that most individuals with one Bt-resistant allele will mate with individuals with no Bt-resistance alleles, thus ensuring that homozygotes for the Bt-resistance allele are seldom produced.

Question 2. On the logarithmic scale, an increase from 0.1 to 0.5 moths per plant is the same magnitude as an increase from 1 to _______ moths.

Question 3. Why does the line for the sequential, Cry1Ac plant stop at 12 generations? Why does the line for sequential, CrylC plant start at 12 generations?

Question 4. In the mosaic treatment, which type of plant had the higher density of moths?

Question 5. Assuming that the differences in moth density between the two plants in the mosaic treatment are completely due to differences in toxicity, to which toxin were the moths most susceptible?

Question 6. Aside from differences in toxicity, what is another possible explanation for the differences in moth density seen in the two plants in the mosaic treatment?

Question 7. How might one test whether the differences in moth density in the two types of plants are due to actual differences in toxicity?

Question 8. Assuming that the differences in moth density are due to toxicity differences, which of the different strategies was most successful at impeding the evolution of resistance in the moths between generation 12 and generation 24?