genetically modified maize can restrain the growth of caterpillars in the fields, shows recent studies conducted in North America's grain belt. The studies are the first to show damaging ecological effects of Bt maize in and around the fields. Diane Stanley-Horn of the University of Guelph, Ontario, found that monarch caterpillars were affected by the normal levels of pollen falling onto the leaves of milkweed plants within the fields (www.newscientist.com, September 11, 2001). They showed 60 per cent lesser survivorship and 42 per cent decrease in weight gain as compared to those exposed to leaves from outside the field. However, the experiment used one of the strongest versions of Bt maize, which is now being withdrawn from sale. This product type, known as event-176, is said to be 40 times more toxic than some other brands. Other Bt maize types appeared to have less impact. "The only transgenic corn pollen that consistently affected monarch larvae was from event-176 hybrids. Other varieties are relatively less toxic,' said Richard Hellmich of the us government's Agricultural Research Service in Ames, Iowa.
In another study, the event-176 hybrids were found to affect black swallowtail, a caterpillar. Entomologist May Berenbaum of the University of Illinois, Urbana, usa , found that there was a significant reduction in growth rates of black swallowtail larvae after exposure to Bt pollen. Berenbaum says her results appear worrying because the effects showed up despite rainfall during the experiment that removed much of the pollen from the leaves. Berenbaum said that such ecological impacts are potentially manageable, but event-176 should not be registered by the us Environmental Protection Agency. In 1999, during laboratory tests, caterpillars of monarch butterflies had died after being force-fed on leaves dusted with the pollen of maize engineered to produce the Bt insecticide.