Person spraying insecticide on plants (Photo by Tima Miroshnichenko on Unsplash)
Table of Contents
In a nutshell
Insecticides applied to crops may inadvertently help weeds thrive by killing beneficial insects that would normally eat weed seeds
Herbicide-resistant weeds like marestail grew much more abundantly in insecticide-treated plots without cover crops
Reducing unnecessary insecticide use and implementing cover crops could help maintain natural weed control, though results may vary by region
UNIVERSITY PARK, Pa. — Farmers across America face a counterintuitive problem, according to new research. The very chemicals they spray to kill insect pests might be inadvertently helping weeds survive by killing off the bugs that would normally eat weed seeds. This finding comes from a study that examined how common agricultural insecticides might be disrupting nature’s built-in weed control system.
The research team—scientists from Penn State, UC Riverside, University of New Hampshire, and Cornell—discovered that insecticide use in corn and soybean fields appears to give certain problematic weeds, especially herbicide-resistant ones, a better chance at flourishing by interfering with biological weed control.
“Insecticides may facilitate the escape of weeds from biological control” might sound like a mundane title, but for American agriculture, where roughly 90% of corn seeds and over half of soybean seeds come coated with pesticide treatments, the implications are serious.
The Hidden Cost of Chemical Protection
Most farmers apply seed treatments as a kind of insurance against early-season bugs and diseases. But this preventative approach might be backfiring when it comes to weed management, one of farming’s biggest headaches.
The research, published in PeerJ, focused on how different pest management approaches affect beneficial insects that eat weed seeds—mainly ground beetles and ants that normally consume countless weed seeds that would otherwise sprout and compete with crops.
Over three years, the scientists compared different pest control strategies: preventative management using seeds treated with neonicotinoid insecticides and fungicides; integrated pest management where insecticides were applied only when pest numbers actually threatened crops; and a control group with no insect pest management at all. They tested each approach both with and without cover crops to see if these conservation practices could help offset any negative effects of the chemicals.
What they found by the third year was eye-opening. Plots treated with insecticides but without cover crops showed dramatically higher amounts of marestail (Erigeron canadensis), a particularly troublesome weed that often resists glyphosate, the active ingredient in Roundup herbicide.
Marestail isn’t just any weed—it’s a farmer’s nightmare. One plant can blast out 200,000 tiny parachute-equipped seeds that drift in the wind. Despite using glyphosate on all experimental plots, researchers watched marestail biomass skyrocket from 2017 to 2019 in insecticide-treated fields.
Young marestail (Erigeron canadensis) plants growing in gravel. (© rosinka79 – stock.adobe.com)
The Ecological Ripple Effect of Insecticide
Why would insecticides help weeds thrive? The researchers proposed several explanations. Insecticides might kill bugs that would normally munch on young weeds. They could interfere with seed-eating beetles and ants, allowing more weed seeds to survive. The fungicides in seed treatments might even protect weed seeds from fungal attacks, giving them another pass from natural controls.
Cover crops showed somewhat helpful effects but weren’t straightforward saviors. They reduced broadleaf weed seeds in the soil by 20% in later years but had inconsistent effects on grass weeds. They reduced weed growth early in the growing season, but these benefits often vanished by mid-season.
Oddly enough, cover crops actually reduced the activity of beneficial ground beetles, contrary to what many would expect. Previous studies have usually found positive effects of cover crops on these helpful insects. The scientists think the cover crop residue might create a more complex habitat that slows beetle movement.
Herbicide resistance has become a full-blown crisis, with many farmers watching production costs climb and profits shrink as weeds develop immunity to chemical controls. The research indicates that current pest management might be working against one of nature’s free services—biological weed control—and potentially making the herbicide resistance problem worse.
Rethinking Pest Management
For farmers, this study offers good reason to reconsider automatic insecticide use, especially as seed coatings. While marketed as insurance against potential pest issues, these treatments may be creating different risks by disrupting the ecosystem services that beneficial insects provide.
The findings reinforce the value of integrated pest management approaches that use pesticides more sparingly and maintain beneficial insect populations. By allowing natural enemies to stick around, farmers might gain powerful allies against herbicide-resistant weeds.
Perhaps most troubling is that many farmers don’t even know these pesticides are on their seeds. A 2020 study found that numerous farmers were unaware their seeds came pre-treated with insecticides, and many don’t have options to choose treatments that match their specific field conditions.
As agriculture tries to balance feeding more people while reducing environmental impacts, studies like this highlight the importance of understanding ecological relationships in farming systems. Working with natural processes rather than accidentally disrupting them might offer more sustainable paths forward.
Points of Contention
When evaluating this study on insecticides and weed control, several important considerations might influence how we interpret the findings:
Study Design Limitations
Initial Plot Differences: Despite random assignment, plots started with different weed compositions, which might have influenced the results.
Geographic Specificity: The study was conducted only in Pennsylvania, so findings might not apply to regions with different climates, soil types, or weed species.
Three-Year Timeframe: While longer than many agricultural studies, three years might still be too short to capture long-term ecological effects that develop over extended periods.
Measurement Considerations
Activity vs. Abundance: The pitfall traps measured insect activity-density rather than absolute population numbers, which can be affected by factors like habitat complexity.
Indirect Observations: The study didn’t directly measure insect feeding on weeds or fungal attacks on weed seeds, leaving some mechanisms as reasonable but unproven hypotheses.
Alternative Explanations
Glyphosate Resistance: The increase in marestail might be due to pre-existing herbicide resistance rather than insecticide effects.
Weather Variables: Year-to-year climate variations could have influenced weed growth patterns independently of the treatments.
Ecosystem Complexity: Farm ecosystems involve countless interacting factors beyond those measured in the study, making it difficult to isolate cause and effect.
Practical Context
Economic Tradeoffs: The study doesn’t fully address the economic balance between potential yield losses from not using insecticides versus the long-term costs of increased weed pressure.
Implementation Challenges: Adopting integrated pest management might require expertise, monitoring tools, or flexibility that isn’t equally available to all farmers.
Regional Applicability: Benefits of reducing insecticide use may vary significantly based on local pest pressures and weed species compositions.
Funding and Support Considerations
Public Funding Source: The study was funded by USDA grants and Penn State’s College of Agricultural Sciences, which might prioritize certain research questions aligned with public agricultural policy objectives.
Focus on Alternative Practices: The study’s examination of integrated pest management and cover crops reflects research interest in alternatives to conventional chemical agriculture, which represents a particular perspective in agricultural science.
These points of contention don’t invalidate the study’s findings but provide important context for understanding its limitations and applicability to different farming situations.
Paper Summary
Methodology
Researchers established a three-year field experiment in two 1.5-hectare fields at the Penn State Russell E. Larson Agricultural Research Center in Pennsylvania. They divided each field into plots laid out in a randomized complete block design with six treatments (three levels of pest management by two levels of cover cropping) each replicated six times in each field. The experiment was established as a soy-corn-soy rotation on one field and as a corn-soy-corn rotation on the other field.
The pest management treatments consisted of preventative pest management (PPM) with seeds treated with neonicotinoid insecticides and fungicides, integrated pest management (IPM) where fields were scouted and treated if pest populations exceeded economic thresholds, and a control with no pest management (NPM). Cover crop treatments included oats planted in spring of certain years and cereal rye planted in fall.
Throughout the three years, researchers measured several key factors: weed and cover crop biomass before planting, the weed seed bank through germination assays, weed biomass in mid-season, the activity and abundance of seed-eating insects (primarily carabid beetles and ants) using pitfall traps, and weed seed predation using sentinel seed cards or dishes placed in the field.
Results
The results revealed several interesting patterns. First, by the final year of the study, plots treated with insecticides (both PPM and IPM) without cover crops showed significantly higher biomass of marestail (Erigeron canadensis), suggesting these chemicals may be disrupting natural weed control. This was particularly notable because marestail appeared to be resistant to glyphosate in this experiment.
Cover crops reduced the weed seed bank for broadleaf weeds by 20% in later years but had inconsistent effects on grass weeds. They also reduced weed biomass early in the growing season, though by mid-season, these differences often disappeared.
Surprisingly, cover crops consistently decreased the activity of carabid beetles, contrary to expectations. Weed seed predation varied across seasons, fields, and years, with predation generally increasing through the growing season but showing inconsistent responses to treatments.
The study also found that carabid activity was higher later in the growing season, with one species (Harpalus pennsylvanicus) dominating the community and representing 81% of all granivorous carabids collected.
Funding and Disclosures
The research was funded by USDA AFRI competitive grants, a USDA NIFA pre-doctoral fellowship to Elizabeth K. Rowen, and the College of Agricultural Sciences at Penn State via the National Institute of Food and Agriculture and Hatch Appropriations. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors declared no competing interests.
Publication Information
The study “Insecticides may facilitate the escape of weeds from biological control” was published in PeerJ on January 13, 2025. It was authored by Elizabeth K. Rowen from the Pennsylvania State University and University of California, Riverside; Kirsten Ann Pearsons from the Pennsylvania State University; Richard G. Smith from the University of New Hampshire; Kyle Wickings from Cornell University; and John F. Tooker from the Pennsylvania State University. The paper was submitted on June 14, 2024, accepted on November 5, 2024, and published on January 13, 2025.