What is the controversy surrounding Bt corn and the monarch butterfly?

Bt corn was specifically engineered to control the European corn borer, which in 2003 caused an estimated $1 billion worth of damage to U.S. farmers. In 1999 a study was released, based on controlled laboratory feeding experiments, that showed that corn pollen from Bt-altered plants would kill monarch butterflies. In the study, threeday- old monarch butterfly larvae were fed milkweed leaves dusted with Bt corn pollen. The larvae ate less, grew slower, and had a higher mortality rate than those fed milkweed with no corn pollen or milkweed coated with non-8t corn pollen. However, the laboratory study did not provide information on the number of 8t pollen grains that were consumed by the monarch larvae in order to observe the lethal effects. Also, no information was provided on the effects on older, larger larvae, which would be expected to have a higher tolerance to Bt toxicity. Headlines such as "Attack of the Killer Corn" and "Nature at Risk" triggered regulatory action on the part of the European Union to ban the importation and use of 8t corn varieties in Europe.
In response, other researchers described the use of 8t as a biocontrol agent since 1938 because of its selective toxicity to certain species within a given insect order. After extensive studies dealing with the likelihood that 8t corn pollen would be found on milkweed plants near cornfields that are close to the habitats of monarch butterflies, safeguards were set to decrease the risk of Bt corn pollen to monarchs: 1) farmers in monarch-rich areas should choose to grow 8t corn with [Lower toxicity levels; 2) plant a border of 000-8t corn around a Bt cornfield to decrease the problem of pollen drift to milkweed plants; and 3) plant milkweeds at sites away from cornfields to increase the probability of female monarchs encountering milkweed plants that are uncontaminated with corn pollen.