When Pest Control Fails to Deliver Yields

By Andi Anderson

Insecticides are important tools used to manage insect pests in soybean farming. However, in regions like Illinois, pest populations are usually low and do not often reach levels that harm crop yields.

While this may seem reassuring, it creates challenges for farmers who must decide whether pest control measures are necessary.

Since insect populations are unpredictable and scouting fields can be costly, many farmers apply insecticides as a preventive measure. These treatments are commonly applied to seeds or sprayed during specific soybean growth stages, especially at the R3 stage, often along with fungicides. This practice is often seen as a low-cost insurance strategy rather than a response to actual pest outbreaks.

To understand the effectiveness of such practices, researchers conducted 15 field experiments during 2024 and 2025. These studies took place on both research farms and commercial fields. The goal was to measure whether insecticide applications improved soybean yields compared to untreated plots.

The results showed that pest populations remained below economic thresholds at all sites. As a result, there was no consistent increase in yield due to insecticide use.

In some cases, large populations of bean leaf beetles were successfully controlled when insecticides were applied at the R5 growth stage. However, even in untreated plots, leaf damage stayed below the critical level of 10 percent, and no yield benefit was observed.

Additionally, seed treatments using imidacloprid were tested in several experiments. While some differences in plant stand and insect feeding were noted, none of these led to higher yields. Similarly, insect counts during the R3 stage were very low, suggesting that spraying at this stage often has little impact. This is especially true since common insecticides remain effective for only 7 to 10 days.

These findings highlight the need for careful decision-making in pest management. Rather than applying insecticides routinely, farmers may achieve better results by monitoring pest levels and using treatments only when necessary.

Future research aims to study conditions where pest populations are high enough to justify insecticide use and provide clearer recommendations for improving both yield and profitability.

Photo Credit: soybean-board