The U.S. Environmental Protection Agency (EPA) announced on August 18, 2022, it released its draft ecological risk assessment for dicamba for a 60-day public review. 87 Fed. Reg. 50854. The 287-page assessment examines the potential ecological risks associated with currently registered uses of dicamba on non-target, non-listed species. Risks to federally listed threatened and endangered species are not evaluated in the assessment. EPA is reevaluating the risks of dicamba as part of a routine review process for pesticides that occurs every 15 years. Comments are due October 17, 2022. EPA expects to propose an interim decision regarding the reregistration of dicamba in 2023.

Dicamba is a systemic benzoic acid used primarily to control annual, biennial, and perennial broadleaf weeds. First registered in the United States in 1967, it is currently registered for use on a wide variety of agricultural crops, such as soybeans, cotton, corn, grains, and sorghum, as well as for non-agricultural uses, such as rangeland, fallow fields, turf, and residential premises.

The draft assessment focuses on areas where there have been updates since the most recent national-level risk assessments of dicamba by EPA (2005 and 2020) to examine if the risk picture has changed based on new data and analysis. The 2005 risk assessment was for dicamba’s Registration Eligibility Decision (RED) based on use patterns registered at that time, which were applications to non-dicamba-tolerant plants. The 2020 risk assessment was exclusively to evaluate risk associated with relatively new uses of applications to dicamba-tolerant plants (i.e., soybeans and cotton).

In general, the risk conclusions of the draft assessment are consistent with those identified in past national-level risk assessments for dicamba with a few notable exceptions:

• Lower Risks to Birds. The risk assessment incorporates recently submitted toxicity data (chronic toxicity to birds), information that eases previously identified chronic risk concern for birds in dicamba-tolerant plants.

• New Risks to Bees. Recently submitted toxicity data indicate a previously unidentified potential chronic risk concern for honeybees from all uses on non-dicamba-tolerant plants. EPA used application rates higher than those EPA evaluated in its 2020 assessment, and potential chronic risks to bees are based on weight-of-evidence and maximum single application rates. The uses with the greatest potential chronic bee risks are asparagus, soybean, dicamba-tolerant cotton, and any registered uses on unmaintained non-agricultural areas.

• Risks to Fish. Updated exposure estimates accounting for the combined residues of dicamba indicate a previously unidentified potential chronic risk concern for non-listed fish. Updated exposure estimates accounting for the combined residues of dicamba (dichlorosalicylic acid (DCSA) and 6-CSA, degradates of dicamba) indicate a previously unidentified potential chronic risk concern for non-listed fish from one use scenario.

Also of significance, there are thousands of reported incidents allegedly caused by dicamba exposure occurring at or near a wide variety of agricultural and non-agricultural use sites and affecting a wide variety of plant species. According to EPA, a pronounced increase in the overall number of reported dicamba incidents associated with damage to non-target plants started around 2016 and appears to link to the introduction of dicamba-tolerant plants and “over-the-top” (OTT) applications to those crops. The combined evidence from field studies and incident data indicates that there may be off-site movement of dicamba via runoff, spray drift, and volatility from the use of dicamba, particularly for OTT application on dicamba-tolerant plants.

Damage to plant species near areas of application presents two separate issues of concern that will have to be addressed in EPA’s eventual decisions:

1. Does routine OTT use of dicamba cause unacceptable damage to nearby commercial crops? and

2. Does any tendency to injure nearby plants represent a possible concern about possible impacts on threatened and endangered species when EPA eventually includes Endangered Species Act (ESA) assessments as part of its review?

In 2020, EPA concluded that its 2020 label restrictions of dicamba-tolerant plants would significantly reduce incident reports about damage to nearby crops. Despite the new control measures, EPA received nearly 3,500 incident reports for the 2021 growing season of damage to non-dicamba-tolerant soybean, numerous other crops, and a wide variety of non-target plants in non-crop areas, including residences, parks, and wildlife refuges. EPA continues to monitor and evaluate new incident report submissions, and the analysis will be updated as new information becomes available. Dicamba also continues to be an important issue of discussion at State FIFRA [Federal Insecticide, Fungicide, and Rodenticide Act] Issues Research and Evaluation Group (SFIREG) meetings where data, analysis, and recommendations for dicamba continue to be discussed.

Commentary

This is the first time dicamba has been identified to pose risks to bees and fish. Worth noting, the identified chronic risks to honeybees, which could indicate that dicamba affects the larvae stage as well as the adult stage of bees, in a time of increased attention to bee health, throws dicamba into another arena of controversy previously unidentified and could garner significant attention in the environmental community, industry, and EPA.

Still, the most significant risk continues to be to possible impacts on non-target terrestrial plants from spray drift and volatilization. Almost 3,500 incident reports for a single growing season are hard to ignore. EPA has received numerous complaints about dicamba damaging non-target plants since EPA allowed the herbicide OTT application on genetically modified soybeans and cotton in 2016. Spraying dicamba over the top of crops after they have emerged is typically done later in the growing season, when temperatures are hotter and other crops/plants are maturing in nearby areas. In warmer temperatures, the herbicide is more likely to volatilize from target application areas and drift, also because of the warmer temperatures. Environmental groups sued EPA to halt the approval, and a federal appeals court ordered EPA in June 2020 to cancel all registrations for use on dicamba-tolerant crops.

In October 2020, EPA issued new registrations with new controls intended to reduce and prevent dicamba movement from volatilizing and drifting onto neighboring properties. Based on the most recent incident reporting, however, the measures did not stop the complaints about harm to nearby crops, plants, and trees.

This draft ecological risk assessment puts dicamba under scrutiny once again and may signal important registration challenges for the herbicide in 2023.