Insecticides may be helping weeds thrive

Farmers rely on insecticides to protect their crops from damage by pests. But using them too often may lead to an unexpected side effect – more weeds.

A new study reveals that the way insecticides are applied can shape weed growth, especially in corn and soybean fields.

Researchers from Penn State conducted a three-year study to understand the bigger picture. They compared two insecticide strategies: applying them preventively at planting, versus only applying them when insects were actually present.

The experts also tested how cover crops – plants grown after harvest to protect the soil – interact with both approaches.

Insecticides may harm helpful insects

By the third year, the results showed a clear pattern. Fields treated with insecticides but left without cover crops ended up with more weeds, especially a type called marestail. In contrast, fields with cover crops didn’t have this issue – even if insecticides were used.

This finding raised questions. One possible reason, the researchers suggest, is that insecticides may be hurting more than just the pests.

Some insects – like beetles, crickets, and ants – actually help by eating weed seeds. If these helpful insects are harmed by the insecticides, the weeds have more of a chance to thrive.

“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 a relatively uncommon problem,” said John Tooker, a professor of entomology in the College of Agricultural Sciences at Penn State.

A case for smarter pest control

The study supports the idea of integrated pest management (IPM). Instead of spraying chemicals by default, IPM involves monitoring fields and only treating them when necessary. This reduces harm to helpful insects and may keep weed populations in check.

“When taking an IPM approach, we advocate for using the right products at the right time to control the right pests, and that will also then help reduce these negative consequences of using these treatments too much,” said Tooker.

This message may be especially important now. Elizabeth Rowen, lead author of the study and assistant professor at the University of California, Riverside, said that weeds are getting harder to manage.

“Many of the seeds growers use were developed to not be killed by herbicides,” explained Rowen.

“This allows growers to use glyphosate to control weeds; however, this also results in the evolution of herbicide-resistant weeds, which makes it much harder to control weeds without killing the crops. So, having multiple strategies to help manage weeds is really important.”

Benefits of less insecticide use

The study was carried out at Penn State’s Russell E. Larson Agricultural Research Center.

Corn and soybean plots were assigned one of three pest control treatments: preventive insecticides at the time of planting, IPM-based insecticides used only when needed, or no insecticides at all. Each treatment was tested with and without cover crops.

Over the course of three years, the researchers tracked many details. They measured cover crop growth, insect communities, seed predation, weed density, and crop yields.

The results showed that applying insecticides only when needed did not reduce productivity. In fact, it came with some added benefits – less pesticide use and better weed control when cover crops were involved.

A limited approach pays off

“Oftentimes corn and soybean fields are so big that growers are inclined to do all of the management up front so they don’t have to go back and walk the fields,” Tooker said.

“But our evidence suggests that walking these fields to identify problems as they happen can provide clear benefits in terms of not needing certain pesticides, namely many of the fungicides and insecticides.”

Taking time to observe what’s actually happening in the soil and among the plants can reveal issues that automated treatments might overlook.

This study reminds us that nature is full of connections. Choosing when and how to use insecticides may not just control pests – it can also change the entire balance of a field.

The research suggests that a slower, more watchful approach might lead to better results in the long run.

The full study was published in the journal PeerJ.

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