As the Losey study demonstrated, there is a significant risk to the Monarch butterfly from Bt corn. The only source of food for the Monarch larvae is milkweed, which grows adjacent to many corn fields. The pollen of Bt corn plants can be toxic to Monarch Larvae, and pollen concentrations of greater than 1000 grains/cm2 are lethal.
Thankfully, the risk is not as high as it seemed after Dr. Losey's study. There are two main factors that protect the Monarchs from this lethal pollen.
The first is that Monarchs are rarely in the Corn Belt when Corn is pollinating. The Monarchs feed for only a few days, and the corn plants only pollenate for a few days. This overlap needs to be met before any threat is realised.
The second is that even during pollination the concentration of Corn pollen on milkweed is less than 170 grains/cm2, well below the lethal dosage. The Monarchs only begin to see negative reactions at 200 grains/cm2, and they will die at 1000 grains/cm2. The odds of attaining such a high concentration in wild milkweed are unlikely.
It must also be noted that the contentration drops off dramatically when one moves away from the corn field. At 1-2 metres the concentration will drop to about 100 grains/cm2, and at 30 metres, the concentration is nearing inconsequential. In addition to this, the pollen tends to accumulate on the middle leaves of the milkweed plant. Most Monarch feeding
takes place on the upper leaves.
There is also a third factor when looking at the potential risk to the Monarchs, and that is to compare Bt to conventional pesticides. All of the studies done to date have shown smaller than a 50% mortality rate, including the Losey study, when Monarchs were placed in environments with Bt. Meanwhile, most conventional Corn pesticides cause a mortality rate of 75% or higher.
Regular synthetic pesticides are hard to control, expensive, and they affect a host of non-target organisms. Toxins will move up the food chain, accumulating in birds and mammals. Monarch butterflies exposed to synthetic pesticides blown off corn fields are far worse off than those who might happen to ingest
a few mouthfuls of Bt corn pollen.