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.

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