An herbicide can “drift” from the agricultural areas where it is applied and negatively affect weeds that develop at the edges of the fields, consequently influencing pollinators.
A study conducted by the University of Michigan investigated the consequences of the herbicide dicamba and revealed that plants subjected to dicamba drift experienced a diminished presence of pollinators, and that pollinator interactions with flowers were curtailed for certain weeds, but not for others. This research, led by U-M professor of ecology and evolutionary biology Regina Baucom, is featured in the journal New Phytologist.
Dicamba was created in the late 1950s and was first approved for usage in 1962. Nevertheless, the herbicide bears moderate toxicity to humans and wildlife, according to Baucom. Its popularity waned, leading farmers to adopt RoundUp on crops genetically altered to withstand glyphosate, the active component in RoundUp. Glyphosate has generally been regarded as environmentally benign.
However, weeds have developed resistance due to the extensive application of RoundUp. The agricultural corporation Monsanto, now under Bayer’s ownership, shifted its focus to dicamba, creating crops resistant to it. In 2018, the Trump administration reinstated its approval for use. Shortly after, farmers began reporting detrimental effects on their crops, Baucom mentioned.
“One reason dicamba lost favor is its volatility. A farmer applies it, and it can rise into the air column or re-volatize, subsequently drifting and affecting unintended areas,” Baucom noted.
Farmers not yet using dicamba indicated that their crops, such as soybeans – which are particularly susceptible to dicamba – were yielding below their usual levels, she explained.
“Then we recognized that dicamba drift would not just influence crops, but also plants or weeds in the waste zones around fields that serve as critical habitats for insects,” Baucom stated. “Given the widespread decline of insect populations and our genuine worries regarding pollinator well-being, I believed this was a significant issue worthy of investigation.”
Baucom and her team aimed to determine if weeds, and ultimately, pollinators were influenced by this drift. They discovered that weeds exposed to dicamba drift had fewer pollinators nearby, and that visits from pollinators to flowers diminished for some weeds, but not for others.
Dicamba affects plants by imitating the hormone auxin, which is present during various stages of plant growth. It exists in several plant structures, including reproductive parts, and governs how plants orient toward light. Baucom explained that when a plant like soybean encounters dicamba drift, its leaves display a cup-like formation.
“We were curious if exposure to dicamba drift induced changes in flower production. Do we observe alterations in flowering timing? We were interested in any factors that could influence a pollinator’s willingness to visit a cluster of plants,” she expressed.
The team established plots where they cultivated duplicates of 11 distinct weed species. Five of these plots were subjected to dicamba just once, early in the plants’ life cycle, at drift levels, approximately 1% of a typical application. Each species was also replicated in multiple plots serving as controls that were not exposed to dicamba.
Subsequently, the researchers monitored plant growth, size, signs of damage, and the timing and quantity of flowering. They then assessed for pollinator abundance and interactions.
“In the control plots, we observed significantly more pollinator insects compared to those in the drift environment. And bear in mind, these constructed weed communities were only exposed to dicamba drift once, early in their development,” Baucom noted. “Thus, this singular exposure to a low dose of dicamba resulted in fewer pollinating insects in those constructed communities.”
Baucom believes that the reduction in pollinator numbers and visits arose from the damage caused by dicamba to the plants. Some plants were stunted in size, others developed fewer flowers, and certain plants bloomed later than their usual schedule. The researchers modeled this damage, implying that dicamba drift could alter plant characteristics. Such damage could subsequently affect the frequency of pollinator visits and approaches.
“In the control context, a positive correlation exists between the number of flowers and the number of pollinator visits. The greater the flower count, the more frequently a pollinator would arrive,” Baucom affirmed. “In the dicamba context, that correlation disappears. There’s no correlation between flower numbers and visitation counts.”
“What I found particularly intriguing is how this singular auxinic herbicide disrupts this weed community and a long-established connection between plant signals and pollinator behavior.”
Going forward, Baucom and her colleagues at U-M secured a grant through the College of Literature, Science, and the Arts’ Meet the Moment Research Initiative to explore how dicamba drift could affect prairie strips, which consist of native plants cultivated alongside agricultural fields to capture farm runoff while providing essential habitats for pollinators.