The Impact of Genetically Engineered Crops on Farm Sustainability in the United States (2010) / Chapter Skim
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2 Environmental Impacts of Genetically Engineered Crops at the Farm Level
2 Environmental Impacts of Genetically Engineered Crops at the Farm Level
Pages 59-134
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From page 59... ...
. No-tillage systems, crop rotations, integrated pest management, and other environmentally friendly management practices may ameliorate some of the adverse impacts, but the tradeoff between agricultural production and the environment remains.
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From page 60... ...
This section first examines the substitu tion of glyphosate for other herbicides that has taken place and how the use of HR crops has interacted with tillage practices. It then assesses eco logical effects of those changes on soil quality, water quality, arthropod biodiversity, and weed communities.
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From page 61... ...
Quantifying to what extent HR crops caused the adoption of conservation tillage practices, given that there is a two-way relationship, and 2. Spatially linking the adoption of HR crops with data on the occur rence of Highly Erodible Land, something not feasible without spatially explicit data.
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From page 62... ...
; however, these formulations are labeled for terrestrial uses only with restrictions with respect to waterways. The greater use of postemergence glyphosate applications has been accompanied by modifications of agronomic practices, particularly
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From page 63... ...
NOTE: The strong correlation between the rising percentage of HR cotton acres planted over time, the increased applications of glyphosate, and the decreased use of other herbicides suggests but does not confirm causation between these variables. SOURCE: USDA-NASS, 2001, 2003, 2005, 2007, 2009a, 2009b; Fernandez-Cornejo et al., 2009.
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From page 64... ...
Indeed, in the last 10 years, the use of conservation tillage has continued to increase, with the exception that it has remained constant in the case of corn (Figure 2-4) .2 The adoption of conservation tillage practices by U.S.
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10 5 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Cons ervat ion t illage: ≥30% res idue Tillage leaving < 30% res idue No-t ill HR c ott on Corn 80 Ac res planted (million) 60 40 20 0 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 Year Cons ervat ion t illage: ≥30% res idue Tillage leaving < 30% res idue No-t ill HR c orn FIGURE 2-4 Trends in conservation tillage practices and no-till for soybean, cot ton, and corn, and adoption of herbicide-resistant crops since their introduction in 1996.
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From page 66... ...
No-till prac tices increased by 4 percent over the same time period (4.3 million acres) , but this was disproportionate relative to overall increases in conservation tillage practices (1.9 million acres)
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From page 67... ...
HR cotton increased the probability that farmers would adopt conservation tillage and conversely that farmers that had previously adopted conservation tillage practices were more likely to adopt HR cotton. Thus, the adoption of no-till and the adoption of HR cotton are complementary practices.
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Soil Quality The relationship between the adoption of conservation tillage practices and the adoption of HR crops is relevant to farm sustainability because conservation tillage has fewer adverse environmental impacts than con ventional tillage (reviewed by Uri et al., 1999)
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Water Quality Conservation tillage practices can have off-farm benefits for water quality that are potentially more important than onsite productivity effects (Foster and Dabney, 1995)
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. Shifts to conservation tillage attributable to the availability of HR crops have contributed to reductions in soil loss and probably in herbi cide runoff.
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Pollinators feed on nectar or use some weeds as hosts for their larval stage; weed species can be food for herbivores that in turn are preyed on by predators that also control pests of crops. In particular, more effective weed management could decrease the abundance of beneficial organisms, depending on the mobility of a species and how closely its resource base is associated with weed abundance.
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From page 72... ...
Herbicide Resistance in Weeds The International Survey of Herbicide Resistant Weeds (ISHRW) provides a historical account and extensive list of weeds that have evolved resistance to herbicides (Heap, 2010)
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. Subsequent to that discovery in 2000, other weed species have evolved resistance to glyphosate in glyphosate-resistant crops in the United States (Table 2-1)
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aMinimum and maximum acreages are based on expert judgments provided for each state. The estimates were summed and rounded to provide an assessment of the minimum and maximum acreages in the United States.
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Other Shifts in Weed Communities Factors other than the evolution of glyphosate resistance affect the composition of weed species in the field. Changes in the tillage system used in growing HR crops are probably the most important factor in promoting weed shifts because disturbance is a primary selective force (Buhler, 1992)
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From page 76... ...
THE IMPACT OF GE CROPS ON FARM SUSTAINABILITY TABLE 2-2 Weeds Reported to Have Increased in Abundance in Glyphosate-Resistant Crops Species Crop Location Reference Acalypha spp. Soybean -- Owen and Zelaya, (copperleaf)
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From page 77... ...
. Those studies indicate that the evolution of glyphosate resistance and weed shifts could lead to two important changes in practices: increased use of herbicides generally and reductions in conservation tillage (Mueller et al., 2005)
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From page 78... ...
. Implications of Weed Shifts in Herbicide-Resistant Cropping Systems As noted above, because the adoption of HR crops has facilitated an increase in conservation tillage and reduced the number of herbicides that growers use to control weeds, the selection pressures affecting weed communities has changed.
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From page 79... ...
. However, strategies for delaying the evolution and spread of resistance are not the same because there are important underlying differences in the population genetics of herbicide resistance and insect resistance.
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From page 80... ...
of weeds. The selective differential between individuals with and without resistance alleles can also be reduced by rotating the types of herbicides used to control the target weeds so that selection for resistance to a specific herbicide occurs only in alternate growing seasons (Jasieniuk et al., 1996; Roux et al., 2008)
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From page 81... ...
. In contrast with corn, soybean, and most cotton production, growing crop species like canola, in which hybridization between the crop and weedy relatives is possible, poses a risk of gene flow between the HR crop and the weedy relatives (Beckie et al., 2003; Légère, 2005; see also "Gene Flow Between Genetically Engineered Crops and Related Weed Species")
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From page 82... ...
Similarly, care should be taken to engineer crops for resistance to specific ACCase inhibitors and synthetic auxins that will still be effective in controlling weeds associated with future HR crops. If crops that are resistant to multiple herbicides -- including ALS inhibitors, ACCase inhibitors, synthetic auxins, and glyphosate -- are widely planted, continued use of the herbicides in fields that contain weeds already resistant to some of them could involve a risk of selecting for high levels of multiple herbicide resistance.
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From page 83... ...
Those changes have implications for pest populations, soil conditions, and the management of insect pests in the future. The following section evaluates the impact of insect-resistant (IR)
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From page 84... ...
NOTE: Seed-applied insecticide not included. Furthermore, the strong correlation between the rising percentage of Bt corn acres planted over time and the decrease in pounds of active ingredient per planted acre suggests but does not confirm causation between these variables.
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. Insecticide use has fallen (in pounds of active ingredient per acre)
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From page 86... ...
. Helicoerpa zea is often called cotton bollworm, corn earworm, or tomato fruitworm.
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Nevertheless, the economic consequences of the regional suppression of pests by Bt crops have been investigated only for European corn borer in five U.S. Corn Belt states (Hutchison et al., 2007)
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No Micinski et al., 2008 Ostrinia nubilalis Maryland (Eastern Shore) Up to 60% Bt corn 35 (1973–2007)
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From page 89... ...
. Reversal of Insect Resistance to Synthetic Insecticides The deployment of Bt crops is known to promote a reversal of pest resistance to synthetic insecticides, but this has not yet been observed in the United States.
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. Although the high specificity of Bt crops for the control of target pests is consistent with integrated pest management, they may have effects on beneficial organisms.
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From page 91... ...
. Bt potato crop fields without insecticide use contained higher abundances of predators, natural enemies as a whole, and nontarget pests compared to conventional potato fields, whether or not insecticides were applied to the conventional fields (Wolfenbarger et al., 2008)
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From page 92... ...
Moth larvae were responsible for a large fraction of cotton-insect losses before the adoption of Bt cotton, but cotton-insect losses caused by these larvae have become less important now that Bt cotton has been widely adopted. The five major insect pests of cotton in the United States in 2008 were lygus bugs (1 percent yield loss)
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From page 93... ...
. Over four years of continuous corn cultivation, Bt plant residues and root exudates had no consistent or persistent effect on a breadth of microorganisms or their enzymatic activity in the soil, but differences were detected according to plant species, variety, and age as well as other environmental factors (Icoz et al., 2008)
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From page 94... ...
. Similarly, the Cry1Ab insecticidal protein for European corn borer control had less effect on the bacterial community structure than other environmental factors (Baumgarte and Tebbe, 2005)
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Evolution and Management of Insect Resistance Eolution of Resistance Insects can adapt to toxins and other tactics used to control them (Palumbi, 2001; Onstad, 2008)
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. In addition, some populations of fall armyworm evolved resistance to Cry1F corn in Puerto Rico (Matten et al., 2008)
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First, DDT kills a wide array of insects regardless of their feeding habits whereas Bt cotton kills only some lepidopteran pests that feed on the cotton. Second, no refuge strategy was mandated to manage the evolution of insect resistance to DDT.
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From page 98... ...
. The refuge strategy requires the presence of refuges of non-GE host plants in or near Bt crop fields (US-EPA, 2008a, 2008b)
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Nonrecessive fitness costs can delay the evolution of resistance more effectively than recessive fitness costs because alleles that confer resistance to Bt crops are often rare (Gould et al., 1997; Andow et al., 2000; Tabashnik et al., 2006; Mahon et al., 2007) , so most resistance alleles in pests targeted by Bt crops are carried by heterozygous individuals.
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From page 100... ...
Changes in Refuge Strategy in the United States In a process that aims to use scientific knowledge to balance economic and environmental considerations, refuge strategies for Bt corn and cotton mandated by EPA have been improved since the commercialization of these GE crops in 1996. EPA specifies the area, configuration, and types of refuges to be used with specific Bt crops.
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From page 101... ...
. That could improve the durability of Bt crops if few other major insect pests targeted by Bt crops evolve resistance before the replacement of one-toxin crops by pyramids and if populations of pests that are resistant to single-toxin Bt crops remain rare.
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From page 102... ...
, and thus that caution should be used to manage the evolution of cotton bollworm resistance to Cry1Ac and Cry2Ab cotton in Australia. Agricultural and Enironmental Impacts of Insect Resistance to Bt Crops The refuge strategy was mandated in the United States not only to slow the evolution of resistance to Bt cultivars but also to protect the effectiveness of Bt sprays.
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From page 103... ...
Furthermore, the economic and environmental consequences of Bt resistance are difficult to assess because little information is available on the profitability of Cry1F corn in Puerto Rico and on how withdrawal of Cry1F corn in Puerto Rico has affected insecticide use. In contrast, the evolution of resistance of cotton bollworm (H.
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From page 104... ...
. The movement of herbicide resistance into weedy relatives present on farm fields can influence farmers' weed-management strategies.
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From page 105... ...
by Bt cotton, both the density of flower-foraging honey bees in seed production fields and the area of Bt cotton at a distance of 2,460 ft from the non-Bt cotton fields affected cross-pollination (Heuberger and Carrière, 2009)
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From page 106... ...
cultivars with the GE trait that confer resistance to glyphosate have been commercialized and were widely adopted by growers in the United States. However, in September 2009, the North ern California District Court ruled that USDA violated the National Envi ronmental Protection Act when it deregulated HR sugar beets, and USDA is required by the court to prepare an environmental impact statement to adequately consider the impacts of GE sugar beets on other sugar beet growers as well as farmers growing table beets and swiss chard, two crops with which sugar beets may cross pollinate (Center for Food Safety .
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From page 107... ...
. For HR traits, hybridization is a mechanism by which herbicide resistance might evolve in related weeds if HR crops are able to interbreed with related weedy species occurring in the same location (see the canola example in "Developing Weed-Management Strategies in Herbicide-Resistant Crops" earlier in this chapter)
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From page 108... ...
. For sunflower, the potential for transgene movement to weedy relatives is quite high and thus the consequences of gene flow on weed management presents an environ
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In the case of rice, GE glufosinate-resistant rice cultivars have been developed to improve weed management of red rice (Oryza satia L.) , a common and important weed in commercial rice production.
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From page 110... ...
. For HR crops, the introgression of herbicide resistance from GE crops into weedy near-relatives is likely to have consequences for weed management when weeds with the resistance trait occur in fields or other ecosystems treated with the herbicide.
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From page 111... ...
After 14 years of use of Bt crops, two insect pests have evolved resistance to Bt crops in the United States: Cotton bollworm (Helicoerpa zea) evolved resistance to Cry1Ac and Cry2Ab in Bt cotton, and fall armyworm evolved resistance to Cry1F in Bt corn.
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From page 112... ...
Gene flow is unlikely for Bt cotton and wild relatives because of planting restrictions, but there are no planting restrictions for HR cotton. Other crops in which gene flow with wild or weedy relatives is possible include canola, alfalfa, sunflower, creeping bentgrass, wheat, and rice.
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From page 113... ...
2000. Frequency of resistance to Bacillus thuringiensis toxin Cry1Ab in an Iowa population of European corn borer (Lepidoptera: Crambidae)
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2002. Ostrinia nubilalis parasitism and the field abundance of non-target insects in transgenic Bacillus thuringiensis corn (Zea mays)
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2004b. Resistance management for sustain able use of Bacillus thuringiensis crops in integrated pest management.
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2009. Bacillus thuringiensis toxin nomenclature.
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2002. Biochemistry and genetics of insect resistance to Bacillus thuringiensis.
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2009. Fitness costs of insect resistance to Bacillus thuringiensis.
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1997. Initial frequency of alleles for resistance to Bacillus thuringiensis toxins in field populations of Heliothis irescens.
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2003. Field performance of transgenic cottons expressing one or two Bacillus thuringiensis endotoxins against bollworm, Helicoerpa zea (Boddie)
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North Central Weed Science Society. Available online at http://www.ncwss.org/proceed/2005/proc05/ abstracts/200.pdf.
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1996. Field-evolved resistance to Bacillus thuringiensis toxin Cry1C in diamondback moth (Lepidoptera: Plutellidae)
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. International Weed Science Society.
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From page 124... ...
2005. Season-long variation in expres sion of Cry1Ac gene and efficacy of Bacillus thuringiensis toxin in transgenic cotton against Helicoerpa armigera (Lepidoptera: Noctuidae)
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Pest Management Science 64(4)
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2006. Transgenic crops expressing Bacillus thuringiensis toxins and biological control.
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Pest Management Science 56(4)
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From page 128... ...
males in transgenic cotton that produces a Bacillus thuringiensis toxin. Journal of Economic Entomology 92(4)
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2008a. Insect resistance management fact sheet for Bacillus thuringiensis (Bt)
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2002. The Bacillus thuringiensis toxin specificity da tabase.
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2007. Mechanism of resistance to Bacillus thuringiensis toxin Cry1Ac in a greenhouse population of the cabbage looper, Trichoplusia ni.
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2007. Mutated cadherin alleles from a field population of Helicoerpa armigera confer resistance to Bacillus thuringiensis toxin Cry1Ac.
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ENVIRONMENTAL IMPACTS OF GE CROPS AT THE FARM LEVEL Zwahlen, C., A Hilbeck, R
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