GMOs are not contributing to the death of non-pest species of butterflies. Some types of Bt proteins are purposefully targeted to kill particular moth and butterfly species. These Bt proteins can be produced by plants when they are genetically modified. However, this targeting is intentionally aimed for moth or butterfly pest species that would be killed using insecticide sprays if Bt were not used. Bt proteins are very specific in this regard. Some non-pest species of butterflies can be killed using Bt. However, the butterflies need to eat the Bt in order to die. Only pest species, not non-pest species, eat GMO plants. Therefore, the non-target pest species are not exposed to the Bt and do not die! Studies done in the field have found few effects on non-target pest or beneficial species, including other types of butterflies that are not targeted by Bt. One infamous study found that monarch butterflies can be killed when force-fed milkweed covered in extremely high levels of pollen containing Bt. However, it was easily shown that this study was not realistic since the pollen levels were nearly 6 to 1,000 times higher than those found in the field and are only exposed to the pollen for a short time. Therefore, monarch butterflies, and other types of non-pest butterflies, are not being killed by Bt.
The decline in milkweed and monarch butterfly populations has been discussed on GMO Answers. Andrew Kniss, associate professor of weed ecology and management at the University of Wyoming, explores the factors contributing to the decline in monarchs returning to their overwintering sites in Mexico, as well as the loss of milkweed habitat. While an excerpt is below, we encourage you to read his full post, “Are herbicides responsible for the decline in monarch butterflies?”
“The monarch butterfly is in bad shape. The number of monarchs returning to their overwintering sites in Mexico has been declining steadily for at least a decade. The consensus suggests there are several reasons for this decline, including loss of their overwintering habitat and unfavorable weather patterns. But the purported cause of monarch decline that seems to get the most coverage is the loss of milkweed (Asclepias spp) in the midwestern US migratory path. The evidence seems clear that the number of milkweed plants through this region has indeed declined.
“The cause for the milkweed decline, though, is a little less certain. The same highly publicized study that documented the decline in milkweed numbers suggested that herbicide use in agricultural fields was the culprit. But the study didn’t control for herbicide use, and therefore, had no way of knowing whether it was actually the herbicide or something else associated with crop production that caused the decline in milkweed. They found a correlation between adoption of glyphosate-resistant crops and estimates of milkweed density in Iowa. Surprisingly, this limitation didn’t stop the authors from concluding herbicides were solely to blame, even titling the paper “Milkweed loss in agricultural fields because of herbicide use…” (emphasis mine).
“I think there is little doubt that using herbicides in corn and soybean fields has reduced milkweeds in those crops. But the authors also observed a decline in non-agricultural milkweed density over the same period, but discounted the importance of that loss because they observed more eggs in agricultural fields. However, they used vastly different data sources to estimate monarch use of agricultural vs non-agricultural milkweed, so this difference seems tenuous. Even so, if herbicides are the explanation for milkweed decline within agricultural fields, are herbicides also the reason for the decline in non-agricultural areas? Or could whatever is reducing milkweed in non-agricultural areas also be contributing to the decline in agricultural fields? It seems likely that there is more to this story than herbicide use. But the headlines predictably blamed GM crops and associated herbicides for killing all the monarchs.”