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Dicamba, Monarchs, and Milkweeds

By Dr. Bob Hartzler
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.
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.  In the Cornbelt it is likely a significant percentage of fields planted with dicamba-resistant soybean acres will not be sprayed with dicamba, but it is safe to say there will be a large increase in dicamba use.
Common milkweed is one of the few native prairie plants capable of surviving 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 CBD report suggests that dicamba use will further reduce the amount of milkweed in crop fields.  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.
The CBD report also states that an additional 9 million acres of habitat will be damaged due to off-target movement of dicamba.  This number is based on extrapolating from the acres of off-target injury in 2017.  I suspect that registration for the new dicamba uses will be further restricted if similar levels of off-target damage are seen in 2018 (the EPA granted the new dicamba products a 2-year probationary label prior to the 2017 growing season).
In Iowa and surrounding states, the majority of off-target injury was limited to susceptible soybean varieties.  This is due to the inherent sensitivity of soybean to dicamba.  A dose equivalent to 0.005% of the labeled rate of dicamba can injure soybean; for comparison, it takes 0.1% of the labeled rate of 2,4-D to injure grapes (Hartzler 2016).  The sensitivity of most native plants to dicamba has not been determined, but most likely they are significantly less sensitive to dicamba than soybean.  This does not mean off-target movement to monarch habitat isn’t a concern, but that the risk and extent of injury to other plants outside of crop fields is less than the risk to sensitive soybean in the vicinity of dicamba-treated fields.
Monarch egg on common milkweed leaf damaged by dicamba.
The CBE report cites research indicating 1% of the labeled rate of dicamba reduces common milkweed biomass by 50% (Egan et al. 2014).  It is important to recognize that 4 to 6 leaf seedlings were used in this research.  Under field conditions I suspect the damage to milkweed would be much less since the majority of common milkweed in the field develops from established rootstocks, and milkweed would be exposed to dicamba at later stages of development.  We evaluated the response of common milkweed to low doses of dicamba, and the influence this injury had on ovipositioning by monarchs (Hoey et al. 2016).  Doses simulating drift equivalent to 0.1 and 1.0% of the labeled rate (0.5 lb/acre) caused severe distortion of leaves that emerged following application, but the emergence rate of leaves was not affected.  We did not determine milkweed biomass; while I suspect there was some reduction, it would have been much less than 50%.  Egg laying by monarchs was not affected by dicamba injury (Table 1).
Table 1.  Effect of simulated dicamba drift to common milkweed on monarch ovipositioning.2

% of labeled rate

Injury rating (1-5)











2Greenhouse grown common milkweed (approximately 8” in height) were sprayed with dicamba. Two weeks after application plants were placed outdoors at ISU Horticulture Farm for three weeks. Eggs were counted and removed twice weekly.
Everyone involved in production agriculture recognizes the extent of off-target injury associated with dicamba use in Xtend crops during 2017 was unacceptable.  The labels for the dicamba products registered for use in Xtend soybean include numerous restrictions intended to minimize off-target movement.  The requirement for downwind buffers at field edges is the most important strategy for protecting monarch habitat and other natural areas.  When applicators follow label requirements related to sprayer setup and wind speed, these buffers should protect natural areas that serve as habitat for monarchs, pollinators, and other organisms.  While volatility remains a concern, the quantity that leaves fields due to vapor loss typically is much less than associated with particle drift.
The decline in the monarch population is complex with many contributing factors (loss of habitat in both the overwintering and reproduction areas, climate change, disease and predators, etc.).  Expanding monarch habitat in Iowa is essential for recovery of the monarch since it is estimated that more than 45% of the overwintering monarchs in Mexico originate from Iowa and neighboring states.  I strongly believe herbicide use, including dicamba, and monarchs can co-exist, but it requires appropriate product selection and responsible application to protect resources adjacent to crop fields.  Information related to conservation efforts for the monarch in Iowa is available at the Iowa Monarch Conservation Consortium.