Herbicide drift impacts the growth and flowering of wildflowers in a synthetic plant community

            Each year, billions of pounds of pesticides are used to improve growing conditions for crops in the U.S., of which herbicides account for more than half of total usage rates. But what happens when herbicidal chemicals don’t stay in their intended area? How are plants and animals impacted when an herbicide infiltrates their habitats from neighboring agricultural fields? These are some of the questions I sought to address in the first chapter of my PhD thesis that was just published in the journal Annals of Botany. In particular, my research advisor, Dr. Tia-Lynn Ashman (University of Pittsburgh, Pittsburgh, PA), collaborator Dr. Regina Baucom (University of Michigan, Ann Arbor, MI), and I were interested in knowing how different wildflower species that typically live in the margins of agricultural areas respond to pollution from the herbicide dicamba, which has risen in use exponentially over the past several years and has been linked to numerous cases of ‘drift’ pollution, which occurs when herbicidal particles are taken up by the wind and carried to non-target areas at low concentrations. We focused on wildflowers because they make up diverse plant communities that are important food sources for a variety of animals, especially pollinators like bees. Therefore, by characterizing the effects of drift pollution on wildflowers, we also aimed to make predictions about how pollinators are indirectly impacted.

Experimental set-up. Twenty-five species of wildflowers were randomly distributed across benches in the University of Pittsburgh greenhouse.

            To address our research questions, we created two ‘synthetic’ communities of wildflowers in the greenhouse at the University of Pittsburgh, each comprised of 25 species that we previously identified as common inhabitants of agricultural margins in the U.S. We treated one community with an extremely small dose of dicamba to mimic what plants would be exposed to when drift pollution occurs and left the other community unexposed as a control. We tracked the growth and flowering of all plants by recording data on leaf and flower production and weighing samples of dry leaves, stems, and flowers to make size estimates. Using data on flowering, we constructed ‘networks of co-flowering interactions’ for the drift and control community, which show how frequently species overlap in flowering over time.

Signs of leaf cupping and stem damage caused by treatment with dicamba drift

            Through this experiment, we saw that there was considerable variation in how wildflower species responded to drift pollution. Many were sensitive and showed significant signs of damage immediately after drift treatment, such as stem twisting and leaf ‘cupping’ (when leaves fold inward to make a cup shape). A quarter of species were also negatively impacted by drift in terms of growth: plants of these species treated with drift were substantially smaller than their controls. Still, most species ultimately grew to comparable sizes regardless of the whether they were treated with drift or not, and a couple even grew to be significantly larger when treated with drift, possibly because the dicamba herbicide’s mode of action is to mimic the plant growth hormone auxin. The most interesting finding from this study, however, was how patterns of flowering changed. We found that most species were affected in terms of flowering time by drift—they either flowered significantly later or earlier when treated with drift than the control or flowered for shorter or longer periods of time throughout the duration of the six month-long experiment. Ultimately, this variation in flowering responses across wildflower species led to pronounced differences in the structure of coflowering networks between the control and drift community. In particular, the drift network was significantly less connected than the control, meaning that species tended to overlap in flowering less frequently. Additionally, groups of species that were more likely to overlap in flowering with each other were composed differently in the control network than the drift network. Altogether, these results suggest that herbicide drift has potential consequences for wildflowers in terms of changing which species are competing for pollinators at a given time, but also for pollinators in terms of shifting how many and which kinds of floral resources (since these are determined by which species are flowering) are available to them.

Coflwering networks of greenhouse wildflower communities that were treated with herbicide (dicamba) drift or a control solution. Each circle is a wildflower species (labels show abbreviated species names) and links between species indicate coflowering interactions (they overlap in flowering time). Thicker links mean longer periods of overlap between species. Different colors show which groups of species are more likely to overlap in flowering with each other than with others. Image was adapted from Figure 3 in Iriart et al. 2022.

            Overall, this study was important for showing how even very low levels of herbicide exposure can have profound effects on the growth and flowering of many wildflower species and also potentially the pollinators that would visit them. The next step we would need to better understand the consequences of drift pollution for wild plants and animals is to transition from ‘synthetic’ communities in the greenhouse to real ones in field conditions.

The Power of a Post-Vacation Glow

            Many people believe that getting a PhD is like an extension of the college experience: you take classes with people that have similar interests as you, you live on or near a college campus, and you still eat microwaved ramen for dinner. While all of these things are true, there are important differences between undergrad and PhD student life that, left unnoticed, could be harmful to your chances of success or even your mental health. One of these is the need to schedule your own time off from the academic grind.

            As an undergraduate, colleges or universities will schedule time off for you: along with several holidays, you typically get at least one long break in the fall, spring, and summer where classes are not in session. And while many students take on additional courses, jobs, or internships during these longer breaks, they are still often seen as opportunities for students to step away from their usual coursework. As a PhD student, you unfortunately don’t get these same pre-determined breaks. In this way, a PhD is a lot like that typical 9 to 5 job: you are expected to work on your thesis year-round and, outside of holidays, it is up to you to discuss with your advisor when you get time off. For the often overly-ambitious and probably overly-anxious PhD student like me, this set-up could quickly turn into a situation without any real vacation time at all before you even realize it. However, this past month, I took a much-needed long break away from my PhD to travel to Europe on my honeymoon, and I returned with a new appreciation for the benefits of feeling that post-vacation glow.

More energy and motivation

On average, it takes 4-6 years to earn a PhD; in my program, it takes most people at least five. This is a long time to dedicate to one research topic! So, like in many other jobs, it can be very easy to experience burnout as a PhD student, especially once you’ve reached your third or fourth year as I have. In coming back from my long break, however, I was surprised to see how eager I was to return to my thesis project. While I greatly appreciated being able to sight-see and relax on vacation, by the end of it I felt that my brain was ready to be stimulated again with new, complex problems to solve. My mind also felt much more rested physically – the lack of any pressing deadlines meant that I could get a full-night’s sleep every night. As a result, I came back to the lab feeling energized to complete the tasks that I now also had much more motivation to tackle.

A boost of happiness

Although a PhD student’s work schedule varies depending on many factors such as their personal preferences, advisor(s), and home institution, it is pretty universal that a PhD requires many hours of work per week. In my experience in biology, students work at least 40 hours per week and the hours fluctuate a lot depending on the status of their experiments. An unfortunate consequence of this is that personal relationships can suffer. In my home, my partner also has a demanding full-time job, so sometimes it feels like we end up spending little time together in a seven-day work week, despite the fact that we live together. Our vacation, however, was a time for the two of us to spend our undivided attention on each other and make our bond stronger. In addition, all the hilarious stories we collected during our travels were really fun to share with my labmates when I got back to work, so that helped me bond with them too and made me all the more happier to be in the lab again.

Closing thoughts

While it may seem obvious that taking time off from work is beneficial, many PhD students go too long without a break, worrying that it will negatively impact their productivity or cause them to fall behind. In reality, however, I have found that taking a long vacation has helped me feel refreshed and happy to continue my thesis work. The science seems to support the physical and psychological benefits of vacation time too: people who took vacations more frequently were less likely to have severe complications from heart disease or develop depression than those who vacationed infrequently. In the future, I won’t wait for a big life event like my honeymoon to take a break from all the hard work and rigor to instead spend quality time with my partner or friends, and I hope you won’t either.