3.29 MonarchButterfly_CommonMilkweed

Will dicamba herbicide drift destroy all the milkweed?

The question of what impact dicamba drift might have on milkweed populations is addressed.

Source: Michigan State University Extension

Dear MSU Extension,

I am wondering if you could help me solve a problem. Last year, my meadow had only a few milkweed plants whereas normally it has been filled with them. I saw an online article about dicamba drift, “EPA-Approved Dicamba Is an Airborne Menace and Some States Are Banning It” by Dan Nosowitz of Modern Farmer, July 11, 2017, and am wondering if the milkweed decrease in my meadow is related to dicamba being sprayed in the area. The field is surrounded by forest on three sides with fallow farmland on the fourth side. Thank you.

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Thank you for the question. Interestingly, when I received your email I was at a meeting where Purdue University weed specialist Bill Johnson was talking, and someone in the audience asked him a similar question. His response was that he doubted milkweed would be greatly impacted by dicamba drift—it’s challenging to kill even with a full rate of the herbicide, and particle drift would typically have 1 percent or less of that rate if it occurred.

I also came across a posting from Iowa State University’s weed specialist Bob Hartzler, “Dicamba, monarchs, and milkweeds,” who addressed the topic earlier this month. Although he said there is reason for concern in general about the use of dicamba in this new system, he essentially agreed with Johnson’s outlook.

There was a report published earlier this month, “A Menace to Monarchs (pdf)” by the Center for Biological Diversity, which addressed the impact of dicamba on milkweed and the monarch population and was very critical of the new herbicide system and herbicide use in general. Hartzler addressed portions of the report briefly in his post, but in general he said the study was flawed in several key areas.

The short answer to your question is there is no clearinghouse of information on who has sprayed what herbicides in a given area. The farmers and agribusiness professionals I have talked with in the region said dicamba applications in soybeans in 2017 were very limited in Michigan. If there was a marked reduction in milkweed in the meadow you mentioned, you would need to do some digging as to who owns the property adjacent to the field, and what, if any, herbicides may have been sprayed.

Proving plant injury from herbicide drift can be challenging even when the incident is addressed within a few weeks, but it would be even more difficult looking back to last summer. I know the drought last summer took a toll on the milkweed we had growing in our yard with most never growing larger than a foot tall, and that may be a factor in reduced populations (or at least reduced sightings) last year.

Looking ahead, you would need to make some observations this year of how many milkweed plants (coming from existing underground rhizomes) and seedlings emerge and what the fate of those plants are, and also how many become host to monarch egg masses. As you are making your observations, document any herbicide injury symptoms that may appear, when they occur, where they occur in the field, and note any patterns of injury across the field. Pictures are very helpful.

As you may know, the Environmental Protection Agency (EPA) added several restrictions to the product labels of the three dicamba herbicides that are legal to spray in soybeans this year. Michigan State University Extension field crops weed specialist Christy Sprague has prepared a fact sheet on spraying in this system, “Guidelines and Precautions for Dicamba Use in Roundup Ready 2 Xtend Soybean (pdf),” and I wrote a two-part article on these, “Application requirements for spraying dicamba in Xtend soybeans” and “Spraying dicamba in Xtend soybeans in 2018? Be sure you meet all the requirements.”

EPA will be re-evaluating the registrations for these products at the end of this year, and if we have another year like 2017 with dicamba-injury claims (mostly in the central to southern U.S., and mostly impacting non-tolerant soybean crops), I suspect there will either be additional restrictions put on the products or the registrations will be pulled.

If you are interested in doing some more reading on the subject, click on the links to the sources I mentioned above. You can also read “Native Milkweeds” by the Xerces Society and USDA-NRCS for more information on the plant’s value to wildlife, NCRS conservation practices that include milkweeds, establishment practices and numerous species that are commercially available.

Originally posted by Michigan State University Extension. 

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