Researchers at the University of Michigan have found that herbicides, commonly used to control weeds, may actually increase insect damage to treated plants. Their study also revealed that morning glory plants can resist damage from herbivorous insects, shedding new light on the complex interactions between plants and insects.
For more than 400 million years, plants and insects have co-evolved, shaping each other’s development. This relationship has been crucial for maintaining ecosystem stability. However, human interventions—such as the widespread use of herbicides—are now altering these evolutionary trajectories.
Herbicide Resistance and Plant Evolution
Herbicides are the primary method for controlling unwanted plants, but many plants have rapidly adapted to them, resulting in herbicide-resistant weeds. Grace Zhang, the study’s lead author, emphasized the need for a deeper understanding of how herbicides affect plant evolution beyond resistance and tolerance. “When people think about herbicides, they focus on herbicide resistance. But we don’t fully understand how herbicides influence plant evolution or how they affect interactions between plants and other species in their environment,” Zhang said.
Glyphosate, the active ingredient in many herbicides, is a key focus of the research. To explore how glyphosate affects the relationship between plants and insect herbivores, Zhang conducted an experiment with morning glory plants. These annual weeds, commonly found in agricultural fields, are an ideal model for studying herbicide effects.
The Experiment
At the Matthaei Botanical Gardens at the University of Michigan, Zhang planted 1,600 morning glory plants, half of which were sprayed with glyphosate. The other half served as a control group. Zhang then observed and recorded the growth and insect damage of both groups.
The results were striking. Glyphosate-treated plants suffered significantly more damage from insects than the untreated plants. The treated plants showed more chewed leaves, suggesting that herbicide application may weaken the plants, making them more vulnerable to insect feeding.
Interestingly, Zhang also found that glyphosate-resistant plants experienced less insect damage. In contrast, plants that were susceptible to glyphosate showed more severe damage from insects. This surprised the researchers, as they had expected a trade-off between resistance to herbicides and resistance to herbivores. Instead, they observed that plants resistant to glyphosate seemed to be better equipped to fend off insect attacks.
Unexpected Findings
Zhang noted, “Ecological theory suggests that there might be a trade-off between these two traits: a plant might put more energy into defending itself against one type of threat over another. But we didn’t see that. We saw the opposite.”
Researchers were unable to fully explain why glyphosate-resistant plants also showed resistance to insect damage. However, they hypothesize that glyphosate interferes with the plant’s ability to produce essential aromatic compounds, which play a key role in defending against herbivores. Since glyphosate disrupts the shikimate pathway, a process vital for producing these compounds, it could potentially affect the plant’s defense mechanisms.
The team also speculated that the genetic factors that make some plants resistant to glyphosate might also contribute to their ability to resist insect damage, possibly through detoxification processes.
A New Perspective on Herbicide Use
The study challenges traditional views of herbicide use and its impact on plant and insect communities. “Most research focuses on herbicide resistance in plants or examines the direct effects of herbicides on insects. But we wanted to look at the bigger picture: How does herbicide use disrupt the long-standing relationship between plants and insects?” Zhang explained.
The findings underscore the need for more research into the broader ecological consequences of herbicide use, especially as more plant species develop resistance to these chemicals. By understanding the intricate interactions between plants, insects, and herbicides, scientists can better predict and manage the ecological impacts of herbicide use.
Related topics: