Insecticides are often viewed as a farmer’s reliable sidekick in the age-old battle against crop pests. After all, few things strike fear into a grower’s heart like the prospect of insects devouring precious plants before they ever get a chance to bear fruit. But what if the same chemical weapons farmers wield to protect their crops are quietly opening the door for another enemy—one that doesn’t crawl or fly, but sprouts quietly from the soil? That’s exactly what researchers at Penn State have uncovered in a recent study, published in the journal PeerJ. Their findings challenge long-held assumptions about pest management, highlighting the unintended consequences of routine insecticide use on weed populations.
The Insecticide-Weed Connection: An Unlikely Partnership?
At the heart of this study lies a surprising twist: using insecticides preventively—before insects even show up—may actually increase the number of certain weeds in crop fields. Over a three-year experiment, researchers observed that fields treated with preventive insecticides and left without a protective cover crop ended up with a higher number of weeds, particularly the troublesome marestail (also known as horseweed).
But why would killing off insects result in more weeds? John Tooker, a professor of entomology at Penn State and one of the study’s authors, has a compelling theory. “Our best guess is that the insecticides suppressed the insect populations that usually help eat weed seeds,” Tooker explains. In other words, some insects aren’t just pests—they’re unsung heroes in the fight against weeds. Beetles, ants, and crickets, for example, consume large quantities of weed seeds as part of their natural diets. But when those helpful critters are collateral damage in broad insecticide applications, weeds can thrive without their natural predators keeping them in check.
Integrated Pest Management: Smarter, Not Harder
This revelation is part of a broader conversation around integrated pest management (IPM), a strategy that calls for a more nuanced, responsive approach to pest control. Unlike the blanket application of insecticides at planting time—a common practice in many large-scale farming operations—IPM relies on careful monitoring. Farmers treat fields only when pest populations cross a harmful threshold.
Elizabeth Rowen, the study’s lead author and an assistant professor at the University of California, Riverside, emphasizes the importance of IPM, especially as weeds become increasingly resistant to herbicides like glyphosate. “Many seeds are genetically modified to survive herbicides, which has unfortunately sped up the evolution of herbicide-resistant weeds,” Rowen explains. “We need multiple strategies, and beneficial insects are one of them.”
In this context, IPM isn’t just about protecting crops from insects—it’s about protecting the entire field ecosystem. By reducing unnecessary insecticide use, growers can preserve insect populations that provide essential services like weed seed predation, soil aeration, and even pollination.
Cover Crops: Nature’s Weed Defense
The Penn State study also explored how cover crops fit into this puzzle. Cover crops are planted after the cash crop (like corn or soybeans) is harvested, covering and protecting the soil during the off-season. They’re often used to prevent erosion, retain soil nutrients, and suppress weeds by outcompeting them for light and resources.
Researchers found that when cover crops were used in combination with either pest management strategy (preventive insecticides or IPM), they helped prevent the weed explosion seen in insecticide-only fields. “Cover crops really made a difference,” says Tooker. “They’re a powerful tool for reducing weed pressure, and they work synergistically with IPM to keep weed populations under control.”
The High Cost of Convenience
Large-scale agriculture often leans toward preventive pest control because it simplifies field management. On massive farms with hundreds or thousands of acres, it can be logistically challenging—if not impossible—to scout each field regularly for pests. “Oftentimes, growers are inclined to do all of the management upfront,” Tooker admits. “But our evidence suggests that taking the time to scout and respond only when necessary can bring real benefits—both economically and ecologically.”
And the costs of unnecessary insecticide use extend beyond lost time and money. Insecticides that wipe out beneficial insects can disrupt soil health, reduce biodiversity, and even affect pollinator populations. All of these factors can chip away at long-term sustainability and productivity.
A Growing Resistance Problem
The study’s findings come at a time when weed resistance to herbicides is a mounting concern. Glyphosate, the backbone of many weed management programs since its introduction in the 1970s, is losing its effectiveness against an increasing number of “superweeds.” Marestail, which featured prominently in the Penn State study, is one such weed that’s developed resistance to glyphosate in many parts of the United States.
As Rowen points out, when herbicides stop working, farmers often respond by increasing chemical applications or turning to other, potentially more toxic herbicides. “It’s a vicious cycle,” she says. “The more we rely on chemicals alone, the faster resistance develops. That’s why we need integrated approaches that include biological controls like beneficial insects, mechanical weed control, crop rotation, and cover cropping.”
Reimagining Pest and Weed Control
The implications of the Penn State research are profound. It calls for a rethinking of modern pest and weed control strategies, urging farmers to consider not just immediate outcomes but also long-term sustainability. “When you use insecticides too often or unnecessarily, you lose the natural allies that help manage weeds,” Tooker summarizes.
By adopting IPM principles and planting cover crops, growers can reduce their reliance on chemical controls while promoting healthier, more resilient ecosystems. It’s a holistic approach that not only safeguards crop yields but also nurtures the environmental health of the farmland itself.
Takeaways for Growers
So, what does this mean for farmers looking to protect their crops and their bottom lines? Here are some key lessons from the study:
- Don’t default to insecticides: Insects aren’t always a problem, and routine insecticide use may hurt more than help by eliminating beneficial insects.
- Scout your fields: Walk your fields regularly to monitor for pests. Only apply insecticides when pest populations hit a damaging threshold.
- Plant cover crops: They offer multiple benefits, including suppressing weeds, improving soil health, and reducing the need for chemical inputs.
- Diversify weed management strategies: Relying solely on herbicides can backfire as resistance builds. Combine mechanical, chemical, and biological control methods.
- Think long-term: Sustainable pest and weed management takes time and effort, but it leads to healthier crops, healthier soil, and a healthier environment.
A Call to Action
The message from Penn State’s researchers is clear: smarter pest management is not just possible—it’s essential. As Tooker puts it, “Always using an insecticide at planting does not seem to be the best approach in Pennsylvania, considering that early-season insect pests tend to be relatively uncommon.”
For farmers, agronomists, and policymakers alike, the challenge is to embrace integrated, science-based solutions that balance productivity with ecological responsibility. It’s a shift that will require time, education, and support, but the payoff—healthier crops, fewer weeds, and a more sustainable agricultural system—is well worth the effort.
Insects, it turns out, might just be some of a farmer’s best friends. All they need is a chance to do their job.
Reference: Elizabeth K. Rowen et al, Insecticides may facilitate the escape of weeds from biological control, PeerJ (2025). DOI: 10.7717/peerj.18597