Research Highlights

Unintended Consequences Link Seed-Applied Insecticides to Slug Pressure

In this article, you’ll find details on:

• When slugs are a primary pest, seed-applied neonicotinoid insecticides can reduce soybean stands and yields, rather than protecting them.
• Neonicotinoids reduce natural slug predator populations, allowing slug pressure to grow.
• Farmers must weigh the costs and benefits of insecticide seed treatments in each field to protect yield and profitability.

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By Laura Temple

Changing a complex system — like soybean production — often achieves the stated goal. But any change also triggers unanticipated responses in other parts of the system. Solutions to address these unintended consequences require evaluating and prioritizing crop production goals.  

The spring of 2024 brought the heaviest slug pressure John Tooker, entomology professor and extension specialist with Penn State University, had seen to date in soybean fields. While many factors contributed, his research shows that unintended consequences of insect control account for one of those factors.

“Years of in-field research have shown that insecticides add value only when targeted insects are present,” he says. “We’ve also seen that when insecticides are used unnecessarily, they disproportionately kill non-target predators of other pests, like slugs, and with fewer predators present, slug populations can increase and become more problematic than usual.”

That research has been conducted through the Pennsylvania Soybean On-Farm Network, multi-faceted research funded by the Pennsylvania Soybean Board. Tooker’s work connects to research monitoring both slug and insect populations. 

His research connects neonicotinoid use to higher slug pressure, especially in fields where seeds are planted into residue in cool, wet conditions.

“Neonicotinoid insecticides have become popular seed treatments,” he explains. “They target pests like bean leaf beetles and wireworms. However, these are secondary pests in Pennsylvania, and rarely exceed economic thresholds when soybeans are young and vulnerable.”

His research shows these seed-applied insecticides subsequently provide little value. The neonicotinoid class of insecticides also kills ground beetles and other predatory species, including firefly larvae. These predatory insects don’t feed on plants, but they do prey on slugs.

“In fields where slugs become a problem, our research shows that a seed-applied neonicotinoid insecticide leads to 20% less stand and 5% less yield due to slug damage,” he reports.

That begs the question, why do such seed treatments remain popular in the region?

Innovative Insect Control

For background to answer that question, Tooker shares the background and development of this insecticide class, first created in the 1990s. 

“Neonicotinoid” means “new, nicotine-like product.” These insecticides, inspired by the structure of nicotine, affect the nerves of target insects, with minimal impact on other animals.

The active ingredients in this class also dissolve and carry well in water. This provides a big advantage because the active ingredients can travel with water within plant tissues. However, that’s a drawback when those active ingredients are carried away by rain before entering plants.

In the mid-2000s, active ingredients in this class became available as seed treatments, because they also effectively control wireworms, white grubs and seed corn maggots. Some seed companies tied the use of such seed treatments to replant policies, increasing their popularity.

“Our trials show limited evidence of the value of these seed treatments,” Tooker says. “We found slightly better stands and less insect damage, but that rarely impacted yields.

“Wireworms, white grubs and plant-feeding beetles are not common in no-till Pennsylvania fields where slugs thrive,” he continues. “Seed corn maggots can be found in tilled fields with cover crops, but their impact can be mitigated if crops are planted in good conditions and with good furrow closure.”

Unintended Consequences

One strength of neonicotinoids is that they can be very effective at controlling insects, but can be mild on some types of animals. Unfortunately, that includes slugs, which can be a major pest in no-till fields and cover crop systems. 

Tooker’s research found that slugs eat treated plants grown from insecticide-treated seeds with no effect, damaging field stands. However, the active neonicotinoid ingredient remains in their system, so when a predator attacks those slugs, that predator tends to be poisoned by the insecticides in the slugs. As a result, predator populations can be inadvertently controlled by the insecticides targeting insect pests. With lower predator populations, slugs cause worse damage to soybeans. 

“Our research has found that as predator populations increase in fields, slug damage decreases,” he reports. 

But the opposite is also true, contributing to the intense slug pressure faced in 2024.

Slugs are difficult to control. Natural predators have proven to be one of the few truly effective options to limit slug damage.

Weighing Costs and Benefits

A seed-applied insecticide can benefit soybeans, especially in fields with known insect pressure in the soil. They can be considered an insurance policy that reduces the need for scouting. However, insecticides should be considered separately from fungicide seed treatments when evaluating options, even though they may be packaged together.

In fields where slugs are the main concern, that insurance policy against insects actually works against soybean stands and yields. Incentives to include insecticide on seed must be weighed against the potential for slug pressure.

Because it’s difficult to predict slug pressure and the likelihood of needing to replant when purchasing soybean seeds, farmers should consider field history and management when determining if the cost of seed-applied neonicotinoids will be outweighed by benefits, or compound problems.

Tooker explains that in addition to nicotinoid seed treatments, factors that contributed to heavy slug pressure in 2024 included a mild winter that didn’t kill overwintering slugs and early soybean planting.

“Pressure was so heavy in early planted soybeans that slugs were still around for multiple replants,” he says. 

With low prices and high input costs, farmers need to understand all potential costs of every input when deciding how best to protect soybean yield and profitability in each field. When slugs are a threat, lower input costs from forgoing a neonicotinoid seed treatment have potential to improve yield and profitability.

Additional Resources:

Integrated Practices Manage Hard-to-Predict Slug Pressure – SRIN article

Sentinel Plots Encourage Scouting, Integrated Pest Management – SRIN article

Effectiveness of Neonicotinoid Seed Treatments in Soybean – Penn State Extension publication

Insecticides foster ‘toxic’ slugs, reduce crop yields – Penn State article

Making Soybean Replanting Decisions – YouTube video

Meet the Principal Investigator: John Tooker

Published: Mar 17, 2025