Field monitoring data vs. predictions from the refuge theory
To determine if the field outcomes documented by monitoring data are consistent with the theory underlying the refuge strategy, we modeled resistance evolution in each of the six major pests listed above2. The refuge strategy is based on population genetics theory positing that refuges of non-Bt host plants delay evolution of resistance by allowing survival of susceptible insects. The refuge strategy is mandated in the U.S. and many other countries where Bt crops are grown. This strategy is expected to be especially effective when resistance is inherited as a recessive trait and most resistant adults surviving on Bt crops mate with susceptible adults from refuges. Under these conditions, the frequency of resistance is not expected to increase rapidly because Bt crops kill the hybrid progeny produced by matings between resistant and susceptible adults.
Even though large, genetically-based decreases in susceptibility to Cry1Ac are well documented for some field populations of H. zea and increased control problems have been noted anecdotally6, widespread control failures have not been reported. We think that several factors contribute to this pattern. First, even in the few states with documented cases of resistance, most populations are not resistant. Second, data from greenhouse experiments suggest that Cry1Ac in Bt cotton kills some resistant larvae, e.g., 60% of larvae in a strain with a resistance ratio of 1004. Third, insecticide sprays have been used extensively to control H. zea since the introduction of Bt cotton. Control achieved with insecticides would mask problems resulting from resistance to Cry1Ac. Finally, "pyramided" Bt cotton producing Bt toxins Cry1Ac and Cry2Ab was registered in December 2002 and planted on >1 million ha in the U.S. in 2006 and 2007. Control of Cry1Ac-resistant H. zea larvae by Cry2Ab also limits control problems associated with resistance to Cry1Ac.
The negative effects of resistance to Cry1Ac should decline further as the acreage of cotton producing only Cry1Ac decreases. This acreage decreased from 2.5 million ha in 2006 to only 1.3 million ha in 2007. In addition, Monsanto's registration for Bt cotton with only Cry1Ac is scheduled to expire in 2009. Cotton producing both Cry1Ac and Cry2Ab toxins could substantially delay evolution of resistance for pests like H. virescens that remain susceptible to both toxins. For Cry1Ac-resistant populations of H. zea, however, the resistance-delaying benefits of pyramiding these two toxins may not be fully realized.
In general, the second generation of crops genetically engineered for protection against insects offers a greatly increased diversity of toxins. The first generation of Bt crops was dominated by plants producing either Cry1Ab or Cry1Ac, two closely related toxins that kill only caterpillars. The U.S. EPA website of registered plant-incorporated protectants now lists commercially available varieties of Bt corn and Bt cotton with 12 different combinations of one to three Cry toxins that kill caterpillars, beetles, or both. In the near future, more extensive pyramiding is likely, with plans for up to six different Bt Cry proteins in single corn plants. Registration is also expected for transgenic varieties producing another type of Bt toxin called Vip (vegetative insecticidal protein). Other options for genetically engineered insect protection include modified Bt toxins specifically designed to kill insects resistant to native Bt toxins and gene-silencing technology based on RNA interference.
Looking back on the first generation of Bt crops, we think that their sustained efficacy against nearly all targeted pest populations exceeds the expectations of many scientists. An exceptional case involving field-evolved resistance to Cry1Ac in some populations of H. zea is consistent with the theory underlying the refuge strategy because this resistance is not recessive. In other words, the concentration of Cry1Ac in Bt cotton is not high enough to kill the hybrid offspring produced by matings between Cry1Ac-susceptible and -resistant adults. In reference to this concept, the refuge strategy is sometimes called the "high-dose refuge strategy." Before Bt cotton was commercialized, scientists at the U.S. EPA and elsewhere reported that the high-dose criterion of the refuge strategy was not met for Bt cotton with Cry1Ac vs. H. zea. Therefore, the relatively rapid resistance that occurred in this pest is no surprise. As the second generation of Bt crops proceeds, we can use systematic analyses of monitoring data from the first decade to maximize benefits and minimize risks. The results summarized here suggest that refuges can delay pest resistance to Bt crops, especially when resistance is recessive and refuges are abundant.
This work was supported by NRI, CSREES, USDA grant 2006-35302-17365.
1. James C. Global status of commercialized biotech/GM crops: 2007. ISAAA Brief No. 37, International Service for the Acquisition of Agri-Biotech Applications, Ithaca, NY, USA (2007)
2. Tabashnik BE, Gassmann AJ, Crowder DW, Carrière Y. 2008. Insect resistance to Bt crops: evidence versus theory. Nat. Biotech. 26, 199-202
3. Tabashnik BE. 1994. Evolution of resistance to Bacillus thuringiensis. Annu. Rev. Entomol. 39, 47-94
4. Jackson RE, Bradley JR Jr., Van Duyn JW. 2004. Performance of feral and Cry1Ac-selected Helicoverpa zea (Lepidoptera: Noctuidae) strains on transgenic cottons expressing either one or two Bacillus thuringiensis ssp. kurstaki proteins under greenhouse conditions. J. Entomol. Sci. 39, 46-55
5. Luttrell RG, Ali MI. 2007. Exploring selection for Bt resistance in Heliothines: results of laboratory and field studies, in Proceedings of the 2007 Beltwide Cotton Conferences, New Orleans, LA, January 9–12, 2007, 1073-1086 (National Cotton Council of America, Memphis, TN; 2007).
7. Tabashnik BE, Cushing NL, Finson N, Johnson MW. 1990. Field development of resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae). J. Econ. Entomol. 83, 1671-1676
8. Janmaat AF, Myers JH. 2003. Rapid evolution and the cost of resistance to Bacillus thuringiensis in greenhouse populations of cabbage loopers, Trichoplusia ni. Proc. Roy. Soc. B 270, 2263-2270
9. Luttrell RG, Wan L, Knighten K. 1999. Variation in susceptibility of Noctuid (Lepidoptera) larvae attacking cotton and soybean to purified endotoxin proteins and commercial formulations of Bacillus thuringiensis. J. Econ. Entomol. 92, 21-32
10. Luttrell RG et al. 2004. Resistance to Bt in Arkansas populations of cotton bollworm, in Proceedings of the 2004 Beltwide Cotton Conferences, San Antonio, TX, January 5-9, 2004 (ed. Richter, D.A.) 1373-1383 (National Cotton Council of America, Memphis, TN; 2004)
11. Ali MI, Luttrell RG, Young SY. 2006. Susceptibilities of Helicoverpa zea and Heliothis virescens (Lepidoptera: Noctuidae) populations to Cry1Ac insecticidal protein. J. Econ. Entomol. 99, 164-175
12. Ali MI et al. 2007. Monitoring Bt susceptibilities in Helicoverpa zea and Heliothis virescens: results of 2006 studies, in Proceedings of the 2007 Beltwide Cotton Conferences, New Orleans, LA, January 9–12, 2007, 1062-1072 (National Cotton Council of America, Memphis, TN; 2007)
TRENDS IN PESTICIDE USE ON TRANSGENIC VERSUS CONVENTIONAL CROPS
Gijs A. Kleter et al.
Characteristics and commercialization of transgenic crops
Since 1996 the large-scale cultivation of commercial transgenic (genetically engineered) crops has grown continuously, with a total of 114 million hectares cultivated in 23 countries in 20071. Most crops are modified with one or both of two main agronomic input traits—herbicide tolerance and insect resistance.
Crop tolerance to otherwise phytotoxic herbicides generally improves weed management, reduces the number and strength of herbicide applications, and allows topical application of herbicide to crop and weeds. Topical herbicide application can replace directed spraying between crop rows and mechanical weed removal, without damaging crops. Replacing mechanical treatment not only reduces fuel consumption but also helps conserve soil types vulnerable to erosion and compaction. Transgenic herbicide-tolerant crops also permit more flexibility in the timing of herbicide application. The new "post-emergence" herbicide treatments for herbicide-tolerant crops may actually replace both "pre-emergence" and "post-emergence" sprays with conventional herbicides.
Herbicide tolerance is achieved through application of one or both of two strategies: the introduction and expression of a gene that codes for a target enzyme that is insensitive to the herbicide; and/or an enzyme that inactivates the herbicide of interest. Herbicide-tolerant soybeans are widely adopted in both North and South American countries, including the United States of America (USA), Argentina, Brazil, and Paraguay. Other herbicide-tolerant crops include maize and cotton in the USA, and canola, which is particularly important in Canada.
In most transgenic insect-resistant crops, resistance is achieved through the introduction of genes for Bacillus thuringiensis (Bt) proteins. These proteins, designated Cry (crystal) proteins, occur as crystal-like inclusions in Bt bacterial cells and are insecticidal to specific target insects that ingest them, but not against humans and animals. By introducing the genes for producing very low levels of these proteins in plant tissues, the plants have internal protection against target pests. This strategy has been employed for the control of corn ear worm and stem borers in maize. The larvae of these pests reduce plant performance and harvested product quality, and require careful timing of externally-applied conventional pesticides. Another example, Bt cotton, has genes encoding Bt Cry proteins for prevention of yield and quality problems caused by the cotton bollworm.
Herbicide tolerance and insect resistance can affect pesticide use in transgenic crops carrying these traits. Moreover, these changes in pesticide use may have implications for the environment, given that each pesticide has its own characteristics, with different environmental behavior and toxicity profiles. Thus, the International Union for Pure and Applied Chemistry (IUPAC) instigated a five-year project from 2002 to 2007 to develop an inventory of pesticide use in transgenic crops in order to estimate changes in the environmental impacts of pest management. The outcome of this project is published in various media, including three scientific articles2-4 of which some details are highlighted below.
Impact of herbicide- and insect-resistant crops on pesticide use
Relevant data on pesticide use are generated by various organizations, such as the United States Department of Agriculture (USDA) and the National Center for Food and Agriculture Policy (NCFAP). These institutions collect data on the application of pesticides to transgenic and conventional crops within the USA, employing surveys of farmer crop protection practices conducted by the USDA's Economic Research Service (ERS), as well as consultation with agricultural extensionists and other experts from industry and academia.
One major trend observed by the USDA is the rapid adoption of herbicide-tolerant soybeans in the USA—89% of all soybeans planted in 2006—coupled with a shift towards the substitution of glyphosate for the other most popular herbicides. For other transgenic crops, including herbicide-tolerant canola, cotton, and maize, the NCFAP surveys show that less herbicide active ingredient is used on transgenic herbicide-resistant crops compared to the amount applied to conventional crops grown in the U.S. The reductions observed by NCFAP ranged from 25% for herbicide-tolerant soybean to 33% for herbicide-tolerant maize in 20043. Another survey focused on the adoption of herbicide-tolerant canola in Canada2. From 1995 to 2000, the adoption of herbicide-tolerant canola increased to comprise more than 90% of all canola planted, and broadcast applications of soil-active herbicides plus post-emergence herbicides were replaced with one application of either glyphosate or glufosinate. As a result, total quantities of herbicides applied decreased approximately 40%2. These observations are in line with the generally observed trend towards lower pesticide use on transgenic crops3.
Other developments that occur simultaneously with the increased use of transgenic crops may affect the relative impact of transgenic crops on pesticide use. For example, in conventional crops, there is now an increasing trend towards using low-rate herbicides and insecticides. Another development that may affect the observed decrease in herbicide use on transgenic crops is the emergence of herbicide-tolerant weeds, as has been previously observed with other herbicides. Thus, increased weed tolerance to the target herbicide may require the use of other, more potent herbicides, potentially reversing the trend to increased rather than to decreased usage of herbicides.
The interest in transgenic crop technology is expanding to other crops, such as transgenic herbicide-tolerant beets, e.g., sugar beet and fodder beet. These beets are not yet commercialized within the European Union (EU); nonetheless, beets resistant to glyphosate and glufosinate are the subject of a number of investigations into their agronomic practices and/or environmental impact in Europe. Beet crops are particularly sensitive to weed infestation, requiring multiple herbicide applications in conventional beet fields to prevent yield losses. Various prospective studies indicate that the introduction of herbicide-tolerant beet crops could result in a savings in the number of and quantity of herbicide applications needed for adequate weed control4.
Similarly, adoption of insect-resistant crops reduces pesticide use; reductions in the number and quantity of insecticide applications are noted, particularly for Bt cotton in the USA, as well as in Australia, India, China, and South Africa. Bt cotton is also incorporated into integrated pest management practices because growers can avoid broad-spectrum insecticides that kill both target and beneficial insects3. Reduced insecticide usage in new transgenic insect-resistant cotton and maize lines also directly benefits growers.
Environmental impact assessment of altered pesticide use
The environmental behavior and toxicity profiles of most pesticides differ from each other. Therefore, merely a reduction in the amount of pesticides applied to crops, by itself, may not provide sufficient insight into their environmental impacts. Various indicators are available that predict the environmental impact of pesticide use. These indicators usually equate the quantities of an active ingredient per acre of crop to the environmental impacts, the form of which may differ, ranging from predicted levels of water contamination to the toxicity to environmental organisms.
In a previous study3, we used the Environmental Impact Quotient (EIQ) to translate the amounts of pesticides used on transgenic and conventional crops into comparable figures for predicted environmental impacts. The EIQ, originally developed by Cornell University for the extension of integrated pesticide management in horticulture, is also widely used to assess the impact of policies on pesticide use in general. The EIQ addresses three factors: the consumer; the agricultural worker; and the ecosystem (fish, birds, bees, and beneficial insects), separately or in combination. The input data include pesticide toxicity and environmental behavior (e.g., uptake by plants, persistence in soil, and leaching), and the result is an abstract number (environmental impact [EI] per surface area). EI values allow for the comparison of outcomes for different pesticide treatments, where lower values indicate less predicted environmental impact.
EIQ methodology has been applied to data obtained from the previously mentioned NCFAP study on pesticide use on transgenic and conventional crops in the USA in 2004. As mentioned above, studies by NCFAP show that less herbicide is applied to herbicide-tolerant crops. Therefore, the reduction in environmental impact for herbicide-tolerant crops is greater than the reduction in quantities of pesticides applied, corresponding to less impact per herbicide quantity applied. These environmental impact reductions are found whether consumers, agricultural workers, and the ecosystem are considered together or separately. This positive impact is most pronounced in soybean, showing a 68% reduction in the predicted adverse impact on farm workers3.
Similarly, Brimner and co-workers2 also used the EIQ model to evaluate data collected on Canadian herbicide-resistant canola. Similar to the reduced herbicide quantities used in canola fields mentioned above, the predicted EI of herbicides (EI per hectare) applied to herbicide-tolerant canola is consistently lower than that for herbicides applied to non-tolerant canola. With the increasing level of adoption of herbicide-tolerant canola, this also leads to an overall decrease in predicted environmental impact for weed management in that crop2.
Concluding remarks
In addition to the currently marketed herbicide-tolerant and insect-resistant crops, transgenic crops are being developed with other traits that may also have a large impact on pesticide use. For example, planting blight-resistant potato could substantially reduce fungicide use on the considerable area of potato crop land. In addition to the further evaluation of environmental impacts, it would be worthwhile to investigate the potential impacts of altered pesticide use on pesticide residues within transgenic crops. This issue is being explored within an IUPAC follow-up project.
References
1. James C. 2007. Global Status of Commercialized Biotech/GM Crops: 2007. ISAAA Brief 37-2007: Executive Summary. International Service for Acquisition of Agribiotech Applications, Ithaca. http://www.isaaa.org/resources/publications/briefs/ 37/executivesummary/default.html
2. Brimner TA, Gallivan GJ, Stephenson GR. 2005 Influence of herbicide-resistant canola on the environmental impact of weed management. Pest Manag. Sci. 61, 47–52 DOI: 10.1002/ps.967
3. Kleter GA, Bhula R, Bodnaruk K, Carazo E, Felsot AS, Harris CA, Katayama A, Kuiper HA, Racke KD, Rubin B, Shevah Y, Stephenson GR, Tanaka K, Unsworth J, Wauchope RD, Wong SS. 2007. Altered pesticide use on transgenic crops and the associated general impact from an environmental perspective. Pest Manag. Sci. 63, 1107–1115 DOI: 10.1002/ps.1448
4. Kleter GA, Harris C, Stephenson G, Unsworth J. 2008. Comparison of herbicide regimes and the associated potential environmental effects of glyphosate-resistant crops vs. what they replace in Europe. Pest Manag. Sci. 64, 479-488 DOI: 10.1002/ps.1513
Authored by: Gijs A. Kleter*, Raj Bhula, Kevin Bodnaruk, Elizabeth Carazo, Allan S. Felsot, Caroline A. Harris, Arata Katayama, Harry A. Kuiper, Kenneth D. Racke, Baruch Rubin, Yehuda Shevah, Gerald R. Stephenson, Keiji Tanaka, John Unsworth, R. Donald Wauchope, Sue-Sun Wong.
Gijs A. Kleter*
RIKILT – Institute of Food Safety
Wageningen University and Research Center
gijs.kleter@wur.nl
ENGINEERING GRASSES WITH REDUCED CROSS-LINKING OF CELL WALLS:
IMPACT FOR ANIMAL AND BIOFUEL INDUSTRIES
Marcia M de O. Buanafina
Cell walls are complex structures that serve several important functions in the life of plants. They provide shape and strength to cells, glue cells together, give rigidity to the whole plant, and function as a physical barrier to pathogen attack. Moreover, as a major sink for photosynthates, they factor heavily in the nutrition of farm animals, providing the major source of carbon and energy. In a more industrial vein, the need for new forms of renewable and eco-friendly energy has gained wide recognition in recent years, partly due to the economic and political hazards of U.S. dependence on foreign oil and partly due to the growing realization that burning fossil fuels contributes to global warming1.
Recalcitrance of grass cell wall
Plant cell walls—particularly of grasses—are expected to supply most of the biomass for the production of renewable biofuels in the U.S. by 20301. The potential use of grasses as feedstock material stems from their high yield, low cost, suitability for marginal land, and minimal environmental impact.
However, grass cell walls are characterized by a large quantity of esterified ferulates. Ferulic acid (FA), the most abundant hydroxycinnamic acid (HCA) identified in grass cell wall, is attached to the cell wall via an ester linkage to the arabinose side chain of arabinoxylans (AX)2 (Fig. 1). HCAs can be oxidatively coupled to form a variety of dehydrodiferulate dimers, cross-linking hemicellulose polysaccharide chains3 (Fig.2). Feruloylation of AX is important because it directly cross-links xylans and because ferulate esters become incorporated into lignin through ether and C-C bonds, acting as nucleating sites for the formation of lignin. The arabinoxylans in grasses serve a major structural role by binding to cellulose microfibrils. Feruloylation is also responsible for the linkage of lignin to the xylan/cellulose network via lignin-ferulate-xylan complexes4. FA ester-ether bridges between lignin and arabinoxylan are ascribed a significant role for limiting cell wall degradability of grasses5.
