Author: Kari Arnold, PhD, Western Region IR-4 Associate Director, Environmental Toxicology Dept., UC Davis
On October 22, 2024, EPA prohibited the use of all products containing the herbicide dimethyl tetrachloroterephthalate (Dacthal or DCPA) under the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). Although the reason for the cancellation was based in science, the decision left many vegetable growers in California at a loss for how to manage weeds at planting. CDFA’s Office of Pesticide Consultation and Analysis (OPCA) has been supporting research into herbicide alternatives through the IR-4 Project, a federal research program ensuring that specialty crop farmers have legal access to safe and effective crop protection products as regulations evolve. Through IR-4, OPCA has funded researchers spearheading the effort to register pyraflufen-ethyl, an alternative to Dacthal.
EPA’s cancellation came in response to robust studies demonstrating thyroid toxicity related to unborn babies of mothers exposed to Dacthal via handling or working in areas where it was recently applied. Prior to cancellation, Dacthal filled a niche use pattern most herbicides couldn’t. As a “pre-emergent” herbicide, it could limit the emergence of existing weed seed populations in the soil. Dacthal provided this type of protection long enough for a vegetable crop canopy to grow in, but with little to no injury to the various crops listed allowed to use it. This unique capability made many vegetable crops reliant on Dacthal as an integral step in their Integrated Pest/Weed Management (often referred to as IPM) Program.
Steven Fennimore, a retired UC Davis Professor of Cooperative Extension, invited me down in early 2023 to visit his USDA Salinas station weed management studies. While walking to the trials, he remarked that many specialty crops have been reliant upon the same herbicide chemistries for 60 years, and that we need to see if there are applicable alternatives. Part of Steven’s career consisted of collaborations with registrants and IR-4 to broaden the range of options for vegetable growers to accomplish that goal. In terms of Dacthal alternatives, pyraflufen-ethyl was one material of interest. Although this is a contact herbicide, meaning it must contact the weeds to work and cannot be used before weeds emergeSteven and the registrant, Nichino America Inc., had been testing varying rates and application times to determine an acceptable crop injury use pattern for an over-the-top application to onions and garlic. If successful, this material could control early emerging weed populations during the initial stages of crop development.
Although there was till on-going research at that time, I encouraged Steven to submit the request in 2023. With support from CDFA-OPCA, the registrant, and Western Growers, submission was prioritized at the IR-4 Food Use Workshop and moved forward for the 2024 field research season. Given our current timeline, the product should be available to growers by 2030.