GENETICALLY MODIFIED FOODS Are
genetically modified crops an environmental dream come-true or a disaster in the
making Scientists are looking for answers. 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.
Two years ago in Edinburgh, Scotland, eco-vandals stormed a
field, crushing canola plants. Last year in Maine, midnight raiders hacked down
more than 3000 experimental poplar trees. And in Sun 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 requiting
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. Fewer Poisons in the Soil
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, com, 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 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 (苷磷除草剂), which are less to
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) (杆菌苏立菌) 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 m 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 7 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 com 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. At What Cost to Wildlife In 1998
a Swiss study provoked widespread worry that Bt plants can inadvertently harm
unlucky creatures. In this laboratory experiment, green lacewing(草蛉)
caterpillars proved more likely to die after eating European corn-borer
caterpillars that had fed on Bt com instead of regular corn. The flames of fear
erupted again a year later, when Cornell University entomologist John Losey and
his colleagues reported that riley had fed milkweed (乳草属植物) 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
but-terries 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 com,
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 6 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. Researches in the wild field evaluating the two most common Bt corn plants showed that the toxin absorbed was too ______ .