基因改造食物安全吗？
  - 鼓吹基因改造作物的人说，这类作物不像传统作物，需要的有毒农药较少，对环境有利。但令批评者担忧的是潜在的风险，他们想知道所谓的利益究竟有多少。到底基因改造作物是环保美梦的实现，还是一场正在形成中的灾难？科学家正积极寻找答案。

　 GENETICALLY MODIFIED FOODS: Are They Safe?
  - Are genetically modified crops an environmental dream come true or a disaster in the making? Scientists are looking for answers
　人们对基因改造食物的态度，似乎愈来愈壁垒分明，一边的人支持，另一边的人则是畏惧。支持者宣称，种植基因改造作物对环境伤害较小，而食用这种农作物制成的食品也完全无害。它们还说，基因工程让农作物在贫瘠的土地上也能生长，或可培育出更营养的食物。在不久的未来，全球人口快速膨胀，还得靠这方法解决粮食问题。持怀疑态度者则反驳，基因改造作物对生态环境或人体健康都有极大的风险，令人忧心，不该贸然接受。许多欧洲国家抱持这种态度，因而限制基因改造作物的种植与输入。主要的争议，集中在基因改造食物的安全性。然而，最近的科学研究又是如何看待基因改造食物的危险呢？答案，往往迷失在各种报导的争议中；但是在接下来的篇幅里，它们将呈现在你的眼前。
  The world seems increasingly divided into those who favor genetically modified (GM) foods and those who fear them. Advocates assert that growing genetically altered crops can be kinder to the environment and that eating foods from those plants is perfectly safe. And, they say, genetic engineering－which can induce plants to grow in poor soils or to produce more nutritious foods－will soon become an essential tool for helping to feed the world's burgeoning population. Skeptics contend that GM crops could pose unique risks to the environment and to health－risks too troubling to accept placidly. Taking that view, many European countries are restricting the planting and importation of GM agricultural products. Much of the debate hinges on perceptions of safety. But what exactly does recent scientific research say about the hazards? The answers, too often lost in reports on the controversy, are served up in the pages that follow.
 两年前，一群生态骇客冲进苏格兰爱丁堡的一块农田，捣毁了种植的油菜。去年，美国缅因州一处白杨树实验林遭“夜半突袭队”闯入，砍倒了三千多株树。在加州圣地牙哥，抗议人士破坏了高粱作物，并且在温室的墙上喷漆示威。
这些暴行都是针对基因改造作物而来的，但是抗议人士的行动适得其反，因为他们所破坏的，全都是传统农作物。在每个案例里，那些行动派都把一般作物误认为基因改造过的品种。
原因不难理解。世界上已有4400万公顷土地（相当于台湾面积的12.3倍）种植了基改作物，可是从某个角度来看，那些作物都是隐形的。植入农作物的基因，你一个也看不见、尝不出、摸不着，或察觉它对环境的影响。光凭外观，你无从知道含有外源基因的花粉粒是否会毒害蝶儿，或是传播到几公里外使其他植株受精。最令人担忧的正是它的“隐形”。基改作物究竟如何影响环境？我们何时才会注意到这些影响呢？
鼓吹基因改造（或基因转植）作物的人说，这类作物不像传统作物，需要的有毒农药较少，对环境有利。但令批评者担忧的是潜在的风险，他们想知道所谓的利益究竟有多少。“我们对这类作物有太多的疑问，”纽约大学土壤微生物学者史达兹基说，“我们不知道的多着呢，必须找出答案来。”
由于基因改造作物在大地上占据的面积不断倍增，已经有数量空前的科学研究人员散入田野，搜集资讯，填补我们的知识鸿沟。他们最近的发现，有些令人心安，有些则教人不得不提高警觉。
  Two years ago in Edinburgh, Scotland, eco-vandals stormed a field, crushing canola plants. Last year in Maine, midnight raiders hacked down more than 3,000 experimental poplar trees. And in San Diego, protesters smashed sorghum and sprayed paint over greenhouse walls.
This far-flung outrage took aim at genetically modified crops. But the protests backfired: all the destroyed plants were conventionally bred. In each case, activists mistook ordinary plants for GM varieties.
It's easy to understand why. In a way, GM crops－now on some 109 million acres of farmland worldwide－are invisible. You can't see, taste or touch a gene inserted into a plant or sense its effects on the environment. You can't tell, just by looking, whether pollen containing a foreign gene can poison butterflies or fertilize plants miles away. That invisibility is precisely what worries people. How, exactly, will GM crops affect the environment－and when will we notice?
Advocates of GM, or transgenic, crops say the plants will benefit the environment by requiring fewer toxic pesticides than conventional crops. But critics fear the potential risks and wonder how big the benefits really are. “We have so many questions about these plants,” remarks Guenther Stotzky, a soil microbiologist at New York University. “There's a lot we don't know and need to find out.”
As GM crops multiply in the landscape, unprecedented numbers of researchers have started fanning into the fields to get the missing information. Some of their recent findings are reassuring; others suggest a need for vigilance.
 土壤中的毒药可以少些？ Fewer Poisons in the Soil?根据估计，美国农夫每一年要喷洒44万公吨的农药，主要是对付昆虫、杂草以及真菌。但是农药残留在农作物上或附近土壤中，然后渗入地下水，流入河川，最后进了野生生物的腹中。这一化学药剂的涓涓之流，早就令环保人士忧虑了。
农产公司自1990年代中开始宣传基改种子，向农友保证可降低有毒农药的用量。如今大部分基改作物都含有抗害虫或耐除草剂的基因，以大豆、玉米、棉花及油菜为主。植入抗虫基因的作物会自行制造杀虫剂，因此可望减少化学药剂的喷洒。耐除草剂的基改作物可耐受广效性除草剂，农人就可以摒弃针对特定杂草且毒性更强的化学药剂。农人总是希望尽量少用比较危险的农药，不过基改作物之所以吸引人，是因为劳作手续简化了（降低施用农药的频率及复杂程度），甚至可使产量增加。
但是所谓的“对环境有好处”却不易证实。事实上，还没有任何一篇经过同行专家审查的报告，讨论过那些好处，因为植物不同、地点不同，结果必定随之而变。不过还是有些资讯可供参考，根据美国农业部统计，耐除草剂的作物不见得会降低农药的喷洒量，不过农人将使用比较温和的混合药剂。例如，农人要是种植了耐除草剂的大豆，就会避免使用最毒的杀草剂，而改用毒性弱、分解快的苷磷除草剂。
作物植入抗虫基因，也产生了优劣参半的后果。目前，抗虫害的特性是取自土壤中杆菌苏力菌（Bacillus thuringiensis，下文简称Bt）的一个基因。这个基因会促使细胞制造一种晶体状蛋白质，对某些昆虫来说是毒药，尤其是啃食作物的毛毛虫和甲虫，却不会伤害其他生物。不同的苏力菌菌株，各有不同的毒基因，影响的昆虫也不同，所以种籽生产商可以针对特定的作物，选用最适合的抗虫基因。
在所有植入Bt基因的农作物中，农药喷洒量降低的幅度以棉花最为可观。根据美国环保署的资料，1999年大量种植Bt棉花的各州，杀虫剂的喷洒量比往常减少了21%。环保署病虫害防治部门的行政人员强森说，那个数字“非常戏剧性，让人印象深刻”。通常在一个生长季中，农人要在棉花田里喷洒7~14次杀虫剂，“如果你种的是Bt棉花，你或许只需施用少量刺鼻的化学药剂，甚至根本就用不着了！”他指出。然而Bt玉米和Bt马铃薯就少有农药减量的情况，一部分的原因是：这两种作物的农药用量本来就比较少，而且它们所遭受的虫害不一定严重。
要界定基改作物对环境的害处，似乎比评估优点更为困难。多亏了几份负面的报告，目前大众注目的焦点集中在Bt作物；管理当局也正积极评估基改作物的风险。美国环保署于2001年已针对Bt作物发布重要的新规定，要求种籽生产商进一步证明这些作物的安全性，并能在农场中监控。
由于消费者的疑虑如排山倒海般而来，科学家正加速研究Bt和其他基改作物对环境的影响。他们想要知道的有：Bt作物如何影响“非目标”生物，例如无害的甲虫、鸟儿、蠕虫以及其他恰巧路过的生物？基因改造作物是否会授粉给周遭的植物，使抗虫基因流入野地，创造出不受控制的超级野草？以基因工程技术植入的抗虫与耐除草剂能力万一失效，使基改作物突然变得异常脆弱，这种机率又有多大？
  Every year u.s. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects, weeds and fungi. But pesticide residues linger on crops and the surrounding soil, leaching into groundwater, running into streams and getting gobbled up by wildlife. The constant chemical trickle is an old worry for environmentalists.
In the mid-1990s agribusinesses began advertising GM seeds that promised to reduce a farmer's use of toxic pesticides. Today most GM crops－mainly soybean, corn, cotton and canola－contain genes enabling them to either resist insect pests or tolerate weed-killing herbicides. The insect-resistant varieties make their own insecticide, a property meant to reduce the need for chemical sprays. The herbicidetolerant types survive when exposed to broad-spectrum weed killers, potentially allowing farmers to forgo more poisonous chemicals that target specific weed species. Farmers like to limit the use of more hazardous pesticides when they can, but GM crops also hold appeal because they simplify operations (reducing the frequency and complexity of pesticide applications) and, in some cases, increase yields.
But confirming environmental benefit is tricky. Virtually no peer-reviewed papers have addressed such advantages, which would be expected to vary from plant to plant and place to place. Some information is available, however. According to the U.S. Department of Agriculture, farmers who plant herbicidetolerant crops do not necessarily use fewer sprays, but they do apply a more benign mix of chemicals. For instance, those who grow herbicide-tolerant soybeans typically avoid the most noxious weed killer, turning instead to glyphosate herbicides, which are less toxic and degrade more quickly.
Insect-resistant crops also bring mixed benefits. To date, insect resistance has been provided by a gene from the soil bacterium Bacillus thuringiensis (Bt). This gene directs cells to manufacture a crystalline protein that is toxic to certain insects－especially caterpillars and beetles that gnaw on crops－but does not harm other organisms. The toxin gene in different strains of B. thuringiensis can affect different mixes of insects, so seed makers can select the version that seems best suited to a particular crop.
Of all the crops carrying Bt genes, cotton has brought the biggest drop in pesticide use. According to the Environmental Protection Agency, in 1999 growers in states using high amounts of Bt cotton sprayed 21 percent less insecticide than usual on the crop. That's a “dramatic and impressive” reduction, says Stephen Johnson, an administrator in the EPA's Office of Pesticide Programs. Typically, Johnson says, a farmer might spray insecticides on a cotton field seven to 14 times during a single growing season. “If you choose a Bt cotton product, you may have little or no use for these pretty harsh chemicals,” he notes. Growers of Bt corn and potatoes report less of a pesticide reduction, partly because those plants normally require fewer pesticides and face fluctuating numbers of pests.
Defining the environmental risks of GM crops seems even harder than calculating their benefits. At the moment, public attention is most trained on Bt crops, thanks to several negative studies. Regulators, too, are surveying the risks intensely. This spring or summer the EPA is expected to issue major new guidelines for Bt crops, ordering seed producers to show more thoroughly that the crops can be planted safely and monitored in farm fields.
In the face of mounting consumer concern, scientists are stepping up research into the consequences of Bt and other GM crops. Among their questions: How do Bt crops affect “nontarget” organisms－the innocent bugs, birds, worms and other creatures that happen to pass by the modified plants? Will GM crops pollinate nearby plants, casting their genes into the wild to create superweeds that grow unchecked? What are the odds that the genetically engineered traits will lose their ability to protect against insects and invasive weeds, leaving GM plants suddenly vulnerable?
 野外生物要付出什么代价？ At What Cost to Wildlife?1998年瑞士的一份研究报告激起了广泛的疑虑，大家担心Bt作物可能会在无意中伤害运气不好的生物。研究是在实验室中进行的，科学家以玉米螟幼虫喂食蚜狮幼虫，发现吃Bt玉米长大的玉米螟会使蚜狮死亡，而普通玉米则否。一年之后，美国康乃尔大学的昆虫学者洛西等人提出报告，他们以沾有Bt玉米花粉粒的马利筋叶喂食大桦斑蝶幼虫，结果那些幼虫都死了。疑惧之火再度燃起。
“这是压垮骆驼的那根稻草。”康乃尔另一位昆虫学者皮门特尔说。一时之间，所有的目光都集中在那些大口嚼食基改作物叶片、小口品尝基改花粉的生物，或在基改作物下的土壤中蠕动的生物──牠们是维持植物族群恒定的重要角色。2000年8月，另一个关于大桦斑蝶的研究，也发出了警讯。
然而，实验室可不比农田，许多科学家怀疑这些先期实验有何用处。他们指出，昆虫在实验室里摄取的Bt毒素，远超过牠们在外面的真实世界所摄取的量。因此研究人员亲下田野，到栽种基因改造作物的玉米田里测量花粉中的毒素，估计有多少毒素会飘落到马利筋之类的植物上，最后还需确定蛾、蝶幼虫的毒素接触量。大部分调查已经在2000年的生长季里完成，随后会向环保署提出报告。
不过当局透露，针对两种最常见Bt玉米（由诺瓦蒂斯与孟山都两家公司出品）的初步调查结果显示，大桦斑蝶幼虫的确会在马利筋叶片上接触到Bt玉米花粉粒，但花粉数量很少，所以不至于有毒性。然而，有毒的是什么？环保署估计，即使马利筋叶片上每平方公分有高达150颗的玉米花粉，昆虫食用后也没有明显的危害。最近在美国马里兰州、内布拉斯加州和加拿大安大略省的玉米田所作的研究发现，不管在田里或附近，马利筋叶片上的玉米花粉粒更少，每平方公分只有6~78颗。环保署Bt作物环境评估小组的负责人卫图吉斯总结道：“现有证据意味着，田野里的Bt玉米花粉不会威胁大桦斑蝶幼虫的生存。”
但是事情还未尘埃落定。“证据根本就不够，”环保科学家协会的瑞斯乐指出，“基改作物对非目标生物的影响这个问题根本就是个黑洞，环保署目前握有的数据太少，根本无法判断大桦斑蝶的问题是否严重，更遑论长期的评估。”
2000年秋天，在环保署的一个基因改造作物会议中，卫图吉斯承认他们缺乏Bt作物和昆虫族群的长期研究资料。他评论道：“这需要更多的时间，因为Bt作物问世至今才不过几年呢！”他补充说，环保署会继续搜集数据，但是目前还没有证据显示这类农作物会对野外的昆虫造成“意想不到的恶果”。
  In 1998 a swiss study provoked widespread worry that Bt plants can inadvertently harm unlucky creatures. In this laboratory experiment, green lacewing caterpillars proved more likely to die after eating European corn-borer caterpillars that had fed on Bt corn instead of regular corn. The flames of fear erupted again a year later, when Cornell University entomologist John Losey and his colleagues reported that they had fed milkweed leaves dusted with Bt corn pollen to monarch butterfly larvae in the lab and that those larvae, too, had died.
“That was the straw that broke the camel's back,” says David Pimentel, also an entomologist at Cornell. Suddenly, all eyes turned to the organisms munching GM plant leaves, nipping modified pollen or wriggling around in the soil below the plants－organisms that play vital roles in sustaining plant populations. Another alarming study relating to monarch butterflies appeared last August.
But the lab bench is not a farm field, and many scientists question the usefulness of these early experiments. The lab insects, they note, consumed far higher doses of Bt toxin than they would outside, in the real world. So researchers have headed into nature themselves, measuring the toxin in pollen from plots of GM corn, estimating how much of it drifts onto plants such as milkweed and, finally, determining the exposure of butterfly and moth larvae to the protein. Much of that work, done during the 2000 growing season, is slated to be reported to the EPA shortly.
According to the agency, however, preliminary studies evaluating the two most common Bt corn plants (from Novartis and Monsanto) already indicate that monarch larvae encounter Bt corn pollen on milkweed plants－but at levels too low to be toxic. What is toxic? The EPA estimates that the insects face no observable harm when consuming milkweed leaves laden with up to 150 corn pollen grains per square centimeter of leaf surface. Recent studies of milkweed plants in and around the cornfields of Maryland, Nebraska and Ontario report far lower levels of Bt pollen, ranging from just six to 78 grains of Bt corn pollen per square centimeter of milkweed leaf surface. “The weight of the evidence suggests Bt corn pollen in the field does not pose a hazard to monarch larvae,” concludes EPA scientist Zigfridas Vaituzis, who heads the agency's team studying the ecological effects of Bt crops.
But the jury is still out. “There's not much evidence to weigh,” notes Jane Rissler of the Union of Concerned Scientists. “This issue of nontarget effects is just a black hole, and EPA has very little good data at this point to conclude whether the monarch butterfly problem is real, particularly in the long term.”
In an EPA meeting on GM crops last fall, Vaituzis acknowledged the lack of long-term data on Bt crops and insect populations. Such studies “require more time than has been available since the registration of Bt crops,” Vaituzis remarked. The EPA, he added, continues to collect Bt crop data－but so far without evidence of “unreasonable adverse effects” on insects in the field.
 我们会创造超级杂草吗？ Seeding Superweeds?担心基因从基改植物流入其他植物，是围绕着基改作物的另一类忧虑。不知情的昆虫，或者来得不是时候的一阵风，都可能将基改作物的花粉带到它们的野草亲戚身上，使之受精。一旦如此，获得新基因的植物可能挣脱原有的生态阶层，变成“超级野草”，不惧原本的天敌或农药。科学家已经不再怀疑这样的基因流通是否可能发生。康乃尔大学的生态学者包尔说：“很多案例显示，基因流通终将发生。现在的问题则是：基因流通的后果是什么？”
到目前为止，还没有科学研究发现基改作物导致超级野草的出现。2001年2月《自然》杂志上有篇报告指出，在一个长达十年的研究里，英格兰栽种的基改马铃薯、甜菜、玉米或油菜，都没发现像野草那样能使近亲种受精的情形。然而令人忧心的耳语已经出现，尤其是加拿大农人，他们说基改油菜已经溜出农田，如野草般侵入小麦田。这种油菜也可以抵抗农药。
包尔研究的是抗病毒基改作物的基因流通，他的发现让人心生警惕。现在抗病毒基改作物只占基改作物版图的一小块，但将来可能会更普遍，特别是在开发中国家。包尔正在调查小麦、大麦和燕麦等谷类作物的基因流通，它们都植入了抵抗“大麦黄矮病毒”的基因，此种病毒会侵犯约100种草类植物。这些基因改造谷物预估可在十年内上市。
包尔在实验室所作的研究显示，野生燕麦（燕麦的野草亲戚）可以“攫取”抗大麦黄矮病毒的基因。她说，这种情形如果发生在野外，获得抗病毒基因的野生燕麦便可能以燎原之势席卷美国西部，将其他原生草类逼得走投无路。包尔警告说，每一种基改作物都有其独特的环境性格，独特的风险。
在美国，至少还有生物地理的屏障，Bt作物不太可能将植入的基因传播给野草，因为美国的基改作物多半种在没有近亲的地区。大多数植物要相互授粉，彼此之间必须有些共通之处，例如相同的染色体数目、相同的生命周期或适合的栖地。美国的“没有近亲”法则的唯一例外，是夏威夷和弗罗里达州南部的野生棉花，它们和基改棉花相似得离奇，所以可以接受基改棉花的花粉。为了区隔野生物种与生技物种，美国环保署已要求生产商不得在弗州60号州际公路以南或夏威夷出售基改棉花。
而在北美以外的地区想避免超级野草的产生，恐怕就难了，因为在这些地区，农作物的野草亲戚颇为常见。举例来说，野生棉花已经蔓延过弗罗里达群岛，横越墨西哥湾进入墨西哥；在南美洲的玉米田周围，长着它们的野生亲戚，蜀黍。这两种植物都很容易接受它们基改亲戚的花粉。事实上科学家认为，在许多国家，基改作物最后都可能栽种在它们的原始物种附近，它们共享的，可不只是头顶上的阳光，还有祖传的基因呢。“几乎每种农作物，在地球的某个角落都有野草亲戚。”植物生理学者杜克说，“你要怎么防止基改作物出现在不该出现的地方？”他在美国农业部领导一个研究团队，驻在密西西比大学牛津校区。
  Worries about the flow of genes from the original plant to others also surround GM crops. Unwitting insects or the right wind might carry GM crop pollen to weedy plant relatives, fertilizing them. And if that happens, the newly endowed plants could break ecological rank, becoming “superweeds” that are unusually resistant to eradication by natural predators or pesticides. Scientists have stopped asking if such gene flow is possible. “In many cases,” says Cornell ecologist Allison Power, “we know gene flow will occur. The question now is, What will the consequences be?”
So far no scientific studies have found evidence of GM crops causing superweeds, and a 10-year study reported in Nature in February found no weedlike behavior by GM potatoes, beets, corn or canola planted in England. But worrisome anecdotes have appeared. Canadian farmers, in particular, have described GM canola escaping from farm fields and invading wheat crops like a weed. This canola also resisted pesticide sprays.
Power's studies of gene flow from virus-resistant GM plants give further reason for precaution. For now, virus-resistant crops stake a small share of the GM landscape, but they are likely to become more prevalent, particularly in the developing world. Power investigates gene flow in cultivated grain crops－ wheat, barley and oats－engineered to contain genes that make the plants resistant to the barley yellow dwarf virus (which damages some 100 grass species). These GM grain crops could be on the market within the next decade.
Power's work, carried out in the laboratory, indicates that wild oats－a weedy relative of cultivated oats－can “catch” the genes conferring resistance to barley yellow dwarf virus. If that happened in the field, she says, wild oats might run amok in the western U.S., outcompeting native grasses with kudzu-like intensity. Every GM crop, Power cautions, brings its own environmental personality and its own risks.
In the U.S., at least, landscape logistics make it rather unlikely that herbicide-tolerant or Bt crops will spread their biotech genes to weeds. That's because the GM crops sown in this country have no close relatives in the regions where they grow; most plants can pollinate others only if the recipients and the donors have certain features in common, such as the same chromosome number, life cycle or preferred habitat. A known exception to the “no relatives” rule in the U.S. is wild cotton growing in Hawaii and southern Florida, which, by virtue of its unusual similarity to GM cotton, can accept the GM pollen. To separate the wild and biotech plants from each other, the EPA has ordered companies not to sell GM cotton south of Florida's Interstate 60 or in Hawaii.
But it may prove harder to avoid creating superweeds outside North America, where weedy relatives of cultivated crops are common. Wild cotton, for instance, creeps past the Florida Keys, across the Gulf of Mexico and into Mexico. In South America, a weedy corn relative, teosinte, dresses the edges of domesticated cornfields. Either plant would readily accept the pollen from a GM relative. Indeed, scientists say, GM crops in many countries could end up growing near their ancestral plants－and sharing more than the sunshine overhead. “Almost every crop has weedy relatives somewhere in the world,” says Stephen Duke, a USDA plant physiologist in Oxford, Miss. “How do you keep GM crops out of places where they're not supposed to be?”
 设立收容所 Taking Refuge最后，不管基改作物种在什么地方，永远有个风险尾随，那就是演化。定期喷洒的农药，只要时间一久，害虫和杂草都会产生抗药性。在生技时代，这势必一样会发生：最后，昆虫可以不为所动，津津有味嚼着基改抗虫植物；耐除草剂作物周围的杂草，也会对农人选用的除草剂视若无物。“农业，是农作物保护之道与病虫害两者间的演化军备竞赛。”爱荷华州立大学的植物学者温德尔评论道，“而基改作物只是我们想要战胜虫害的另一种尝试！即使只是短暂的。”
为使除草剂能有效对付杂草，孟山都等公司要求农人以负责任的态度使用农药，只在必要时才喷洒。为了延缓昆虫对Bt毒素产生抗药性，环保署规定，种植Bt作物的人必须挪出部分农地种植传统作物。举例来说，这些“收容所”可以种在Bt作物栽植区外的某个角落，也可以种成一排，把Bt作物一分为二。在收容所里，已经具备一点抗Bt毒性的昆虫与没有抵抗力的个体交配繁殖，就会稀释抗毒能力。根据孟山都的说法，Bt作物的商业栽植已经五年了，还没有发现能抗Bt毒性的昆虫。这家公司声称，种植Bt玉米和棉花的农人，约有90%遵守规定设立收容所。
但是，有些环保人士怀疑情况是否真的这么乐观，他们认为，那些非Bt作物收容所不是种植面积太小，就是设计太差，要期望昆虫长期不产生抵抗力，实在很难。纽约市非营利组织“捍卫环境”的资深科学家戈德伯格说：“2000年秋天的环保署会议中，科学家似乎都同意应设置更大更好的收容所，但是棉花农绝对不会同意。”更广泛的说，戈德伯格质疑基改作物究竟能为环境带来什么好处？她说：“不管要经过多少年，我们终会失去Bt这个对抗虫害的利器，然后必须寻找另一种化学武器。不少人把这一代生技作物当成某种新玩意儿，而非农业上的实质进展。”她支持比较持久的方案，包括仔细规划作物轮耕与有机农耕法，而非一味喷洒农药或改造作物基因。
抗病毒的基改作物还未成为大众关切的焦点，但它们跟其他基改作物一样会带来类似的风险。有些科学家担心，病毒会从抗病毒基改作物那里得到抗药性，演化成难以对付的品种，危害更多植物。有些批评者也质疑新兴基改作物对生态的冲击，特别是具有抗旱、耐盐、高营养成分等特性的作物。例如说，“环保科学家协会”的梅隆指出，耐盐稻米一旦落到湿地上，可能会像野草般蔓延，破坏脆弱的湿地生态系。
“如果说每一种基改作物都会变成问题，其实是不公平的。”瑞斯乐说，“但我们现在仍须花心思研究那些风险，免得以后得收拾残局。”
不过有些农人仍旧充满信心，他们认为基改技术为农业带来新生，造福多过闯祸。住在密苏里州格兰帕斯的乌特劳特，30年来在密苏里河畔经营了1400公顷的农地。2000年起，他首度在所有田地只种植耐除草剂的玉米和大豆，而且用免耕法耕作以保护土壤。结果，他声称喷洒农药的次数少了一半，产量却比往常多。他说：“如果态度最强硬的环保人士能看到我的耕作方式，我想他们就会了解基改作物的好处。我可是这个技术的狂热拥护者！”现在他必须等待，看看科学是否能证实他的信念。
  Finally, one risk follows GM crops wherever they're planted: evolution. Over time, insect pests and weeds can become resistant to killing by routine chemical sprays. The same is bound to happen in the biotech age: eventually, impervious insects will munch away on GM insect-resistant plants, and the weeds surrounding herbicide-tolerant crops will shrug off the herbicide of choice. “Agriculture is an evolutionary arms race between plant protections and pests,” comments botanist Jonathan Wendel of Iowa State University. “And GM crops are just one more way that we're trying to outsmart pests－temporarily.”
To keep weeds vulnerable to herbicides, Monsanto and other companies urge growers to use the sprays responsibly, only when necessary. To slow insect resistance to the Bt toxin, the EPA requires Bt crop growers to set aside some part of their farmland for crops that have not been genetically modified. These “refuges” may be a corner of a field outside a Bt crop, for instance, or rows of standard plants that break up a Bt plot. Inside the refuges, insects that have acquired some Bt resistance breed with those that have not, diluting the resistance trait.
After five years of commercial Bt crop use, no reports of insect resistance to the crops have emerged, according to Monsanto. The company contends that roughly 90 percent of Bt corn and cotton growers comply with refuge requirements.
But some environmentalists question that rosy scenario and also argue that non-Bt refuges are either too small or too poorly designed to keep insect resistance at bay for long. “At the EPA meeting last fall, scientists seemed to agree that bigger, better refuges were the way to go but that cotton farmers would never agree to big refuges,” says Rebecca Goldburg, a senior scientist at Environmental Defense, a nonprofit organization based in New York City. More broadly, Goldburg questions how much GM crops really do for the environment. “In however many years,” she says, “we'll lose Bt as an effective control against insects, and then we'll be on to another chemical control. Many of us view this current generation of biotech crops as a kind of diversion, rather than a substantive gain for agriculture.” She favors sustainable agriculture alternatives, including careful crop rotation and organic farming methods, over pesticides sprayed on or engineered into plants.
Virus-resistant GM crops have escaped widespread public concern, but they, too, pose some of the same risks as other GM crops. Some scientists worry that viruses will pick up resistance traits from virus-fighting GM crops and evolve into hard-tobeat strains that infect a newly expanded repertoire of plants. Some critics also question the ecological safety of emerging crops designed to resist drought, tolerate salt or deliver an extra nutritional punch. For example, Margaret Mellon of the Union of Concerned Scientists notes that salt-tolerant rice could potentially behave like a disruptive weed if it found its way into vulnerable wetlands.
“I don't think it's fair to say that every single GM crop is going to be a problem,” Rissler remarks. “But we need to devote the research to risks now, rather than deal with repercussions later.” Still, some farmers are confident that GM technology can revolutionize agriculture for the better. For 30 years, Ryland Utlaut of Grand Pass, Mo., has been sowing and reaping 3,500 acres along the Missouri River. Last year, for the first time, he planted only herbicide-tolerant corn and soybeans across his entire, soil-friendly, no-till farm. As a result, he claims, he sprayed the crops half as often as he did before and got bigger yields. “If even the strongest environmentalist could see my farming practices now, I think they'd understand the benefits,” Utlaut says. “I'm a fervent believer in this technology.” Now he has to wait and see whether science confirms that belief.