Dicamba, monarchs, and milkweed

March 29, 2018

By Bob Hartzler, ISU Extension weed scientist

The Center for Biological Diversity (CBD) recently released a report (A menace to monarchs) describing the threat to monarch butterflies posed by an increased use of dicamba.  While I have been fairly critical of the Xtend system1 due to off-target movement of dicamba and subsequent injury, I think CBD has exaggerated the threat to monarchs posed by dicamba. I would like to make it clear that while I think their analysis is flawed, I do believe additional restrictions on dicamba use are required to minimize risks associated with volatilization. 

The CBD reported that within two years 60 million acres of habitat will be sprayed with dicamba. This is based on Monsanto’s estimates of acres planted with dicamba-resistant cotton and soybean, and that all these acres will be treated with dicamba. In the Cornbelt it is likely a significant percentage of fields planted with dicamba-resistant soybean acres will not be sprayed with dicamba, therefore reducing the area treated with dicamba. However, it is safe to say there will be a large increase in dicamba use.

The CBD report suggests that dicamba use will further reduce the amount of common milkweed in crop fields. Common milkweed is one of the few native prairie plants capable of surviving the management practices used in producing agricultural crops. Prior to the introduction of Roundup Ready crops, about 50% of Iowa’s crop fields were infested with low densities of milkweed. A decade later both the number of fields infested and the amount of milkweed in fields declined by more than 80% (Hartzler, 2010). The loss of common milkweed from crop fields in the Cornbelt is a contributing factor in the monarch’s decline. However, to expect farmers to intentionally allow milkweed to survive within their fields is unrealistic. Farmers use the most cost-effective practices available to control weeds. Decreasing the intensity of management to allow milkweed to survive within fields would increase the likelihood of other weeds surviving, and therefore negatively impact yield and profitability...

Click to read the full blog post from ISU Integrated Crop Management.