Bt Brinjal The scope and adequacy of the GEAC environmental risk assessment

44 Bt Brinjal: The GEAC environmental risk assessmentIn some areas of India brinjal is cultivated once a year during the rainy season, and BFSB can complete up to threegenerations. In other regions brinjal is cultivated throughout the year, and BFSB can complete 8 or more generations.We assume that wherever brinjal is grown, Bt brinjal will be grown during only one time of the year, so that only threegenerations of BFSB will be exposed to Bt brinjal a year. This is probably satisfactory for those regions with singlecropping. For regions where brinjal is grown throughout the year, Bt brinjal could be grown during two or more seasons.In these cases, the time to resistance failure would be projected to occur much sooner. If two crops are grown resistancefailure would occur twice as fast, and if three crops are grown, it would occur three times as fast as what we project here.i. Based on the results from the small-scale field trials (Dossier, volume 6) and the published literature (Caprio et al. 2000,Meihls et al. 2008, Anilkumar et al. 2008) on other low dose Bt events, we suggest that when resistance is found in BFSB,that it will be partially recessive with heterozygosity (h) between 0.2 and 0.4.j. We also assume that there is strong density dependent population growth. This is because larvae are forced to interactin brinjal fruits. This results in a = 6 and b = 0.8. the stronger the density dependence the longer the time to resistancefailure. However, this effect is minor except in some special cases.k. We assume that per capita fecundity (F) is 100 offspring per female. Although most moths can produce several hundredeggs, this value is used because it counts only one of the two sexes (the females), and includes egg mortality and otherforms of density-independent mortality.l. Finally, we assume that the frequency of resistance alleles ( p 0) in the natural BFSB populations is low, less than one in10,000. This value is lower than most observed initial R allele frequencies for Bt resistance. Time to resistance failure islonger for lower values of initial resistance frequency.Using these assumptions, we can simulate evolution for two levels of adoption of Bt brinjal (50% adoption and 90%adoption), the two values of heterozygosity, and several refuge assumptions. Adoption rate is not for the entire country. Becauseevolution will occur locally, adoption is probably related to the district level within states, and possibly at finer spatial scales. Ingeneral, heterozygosity has large effects on resistance evolution, so we simulated the extreme values that are expected for BFSBand EE-1 Bt brinjal. We simulated no refuge (or 0% compliance), and 5, 10, and 20% non-Bt refuge with 100% compliance.Intermediate levels of compliance will produce results intermediate between 0% refuge and the recommended % refuge.The simulations show that resistance is projected to occur rapidly, sometime between 4 and 17 years (Fig. 2). The precise valuescan be disputed, but a robust conclusion is that under some conditions, resistance failures can occur rapidly (less than 4 years),and under other conditions, it may take many years for resistance failures to show up. Thus, efforts should be taken to ensurethat India is more likely to experience the latter conditions than the former conditions.This could be difficult. Local adoption rate has the largest effect on resistance evolution (Fig. 2). If adoption stays below50% and heterozygosity is closer to 0.2 than 0.4, then Bt brinjal may last for many years. However, if Bt brinjal is truly aseconomically beneficial as projected in EEII and the Dossier, local adoption rates are likely to be much higher than 50%,and the resistance risk is very high. The resistance risk is greatest where adoption is highest, so India should pay attention toadoption rate if Bt brinjal is commercialised.The level of heterozygosity has a large effect on the rate of resistance evolution.Unfortunately, it is not possible to know the value for heterozygosity until aresistance allele is discovered in BFSB. Thus, it should be a high researchpriority to discover resistance alleles to EE-1 Bt brinjal in BFSB (see below).A 5% refuge has little effect on the rate of resistance evolution (Fig. 2). If there is a cost to resistance a 5% refuge will have alarger effect at delaying evolution, but this is still likely to be small quantitatively. A larger percent refuge will be better at slowingresistance evolution. A 20% refuge will increase the time to resistance failure by 25% (Fig. 2). If there is a cost to resistance, theefficacy of the refuge in delaying resistance is likely to increase substantially. If India is going to manage resistance a refuge largerthan 5% seems necessary.The assumptions that we have made in these simulations are not worst-case assumptions. Indeed, BFSB may be exposed