(Beyond Pesticides, July 18, 2019) Pesticide products containing the weed killer dicamba become more volatile and drift-prone in hot conditions and when tank-mixed with glyphosate, according to a recent study conducted by scientists at the University of Tennessee. The findings help explain rampant complaints from farmers in the South and Midwest experiencing crop loss and economic hardship as a result of drift from new dicamba products, which are formulated with glyphosate for use on genetically engineered (GE) cotton and soy. While states have taken the lead in regulating the use of GE dicamba products, top political officials within Administrator Andrew Wheeler’s EPA overruled the findings from agency scientists urging larger buffer zones to protect neighboring crops and farm fields.

During a 60-hour window, scientists applied various GE dicamba products (Clarity and XtendiMax) over a range of temperatures and took air samples. As temperatures increased, so did the volatilization and drift of dicamba, even in formulations touted as “low volatility.”

Adding glyphosate to the mixture produced stark results, increasing concentrations of dicamba in the air up to nine times compared to dicamba alone. Tom Mueller, PhD, a professor in the UT Department of Plant Sciences, stated in a press release that glyphosate as “a contributor to dicamba volatility is not as widely accepted, but our data shows the addition of glyphosate to a dicamba spray solution increased dicamba detection in the atmosphere which would point to increased volatilization.” Dr. Mueller and his colleagues hypothesize that this increase in volatility is due to the fact that glyphosate lowers the pH of the tank mix, raising the quantity of dicamba in acid form, which is already known to boost volatility.

Dicamba is an herbicide that acts like natural plant hormones called auxins. Auxins help to control plant growth and, when dicamba is applied, the plants experience uncontrolled and abnormal growth resulting in death. Glyphosate on the other hand works by inhibiting the shikimic acid pathway (and enzyme pathway), preventing protein creation. Dicamba use is increasing due to the failure of glyphosate-tolerant GE crops, which have resulted in the widespread proliferation of glyphosate tolerant-weeds. While the major manufacture of glyphosate+dicamba pesticides, Bayer Monsanto, claims new formulas have lower volatility, millions of acres of susceptible crops, trees, and ornamental plants have been damaged in the last few years, and research finds that even trace amounts of dicamba in the air, levels in the parts per million, can damage non-resistant crops.

Mixing these two herbicides has become a popular practice in an attempt to manage weeds resistant to glyphosate, a result of repeated spraying of fGE crop fields. However, exposure to either herbicide poses a health risk and both have been linked to diseases such as cancer. These herbicides also pose a threat to pollinators, especially when drift occurs. Increasing the volatility of dicamba with the addition of summer heat and glyphosate will only increase the spread of the herbicide, resulting in more crop damage, pollinator deaths, and human health concerns. While risks to public health and the environment increase, these new formulations are certain to fail as weeds will, as they have in the past, adapt.

Since 2016, numerous lawsuits have been brought regarding dicamba, several over the issue of damage from drift. As a result, some states, such Arkansas and Missouri, have attempted to enact cutoff dates for use to avoid extreme volatility in summer heat. However, many states still lack policies, and those that are in place do not completely prevent the damage caused by drift.

Given the volatility of dicamba alone, and the increased drift when mixed with glyphosate and when exposed to heat, it is evident that more regulation is needed. Current policies in place must be reevaluated, and policies that regulate mixing of these chemicals must be put in place to ensure clean air and reduce crop loss. As opposed to the nefarious actions undertaken by political appointees under Wheeler’s EPA to undermine science and reasonable protections from these toxic pesticides, the agency must begin following the precautionary principle, taking measures to assess for harm before approval. Beyond Pesticides encourages farmers wishing to jump off the pesticide treadmill to adopt organic approaches that do not perpetuate a cycle of pesticide resistance, eliminating the need for these harmful chemicals, mixes of them, and risk of drift. Those impacted by pesticide drift can refer to Beyond Pesticides’ webpage on What to Do in a Pesticide Emergency, and contact the organization for additional information.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Science Daily, Weed Technology (peer reviewed journal)