Research Highlights

Integrated Practices Manage Hard-to-Predict Slug Pressure

By Laura Temple

Beneficial soil health practices like no-till and cover crops protect the soil surface more by disturbing it less. However, slugs thrive in this environment. They can feast on soybean seeds and emerging seedlings, causing significant crop damage or the need to replant. 

To better understand slug pressure, extension educators across Pennsylvania have been monitoring populations in problem fields since 2018. This effort is part of the Pennsylvania Soybean On-Farm Network research, supported by a checkoff investment from the Pennsylvania Soybean Board.

“We are monitoring slug activity and trying to discern patterns to their emergence and damage,” says Anna Hodgson, a field and forage crops extension educator with Penn State Extension, and a leader of the research. “We want to learn what factors contribute to severe outbreaks to help farmers manage against this problem, because control options are limited and often ineffective.”

To gather data to tackle these questions, extension educators assess slug populations in about 15 fields each season. They place ten 1-foot square roofing shingles in fields identified by cooperating farmers with a history of slug pressure. From before planting until about three weeks after crop emergence, they count slugs and look for eggs under those shingle traps weekly. Though the team monitors fields during the spring, slugs can also feed on cover crop seeds in the fall, reducing stand establishment.

Grey garden slugs, a non-native species, are most common in the Mid-Atlantic region. Hodgson adds that native marsh slugs and non-native dusky and banded slugs also appear in the fields they monitor. 

She considers a severe infestation to be a field average of one or two slugs per trap with moderate crop damage. At most, Hodgson reports such outbreaks in just one or two monitored fields each year. While that hasn’t yet produced sufficient data to identify patterns, the information gathered to date does help her team sift through potential theories.

“We’ve determined that egg counts are not a good predictor of slug pressure,” she says. “That’s likely because the small, clear eggs are hard to find. They are about the size of cupcake sprinkles, and looking for them in crop residue is like looking for a needle in a haystack.”

In some cases, high slug counts occur without much crop damage. She notes that once soybeans emerge and grow past the cotyledon stage, they usually recover from slug feeding. 

“We also speculate that in addition to harboring slugs, cover crops provide habitat for their natural enemies,” Hodgson says. “Ground beetles and wolf spiders feed on slugs, which could also account for less crop damage when populations are high.”

Preventing Slug Pressure

While Hodgson continues to gather slug population data to learn more and look for patterns, she also offers recommendations to limit potential slug damage in soybeans.

• Plant soybeans at the right soil temperature, so seeds quickly germinate and grow past the stages vulnerable to slug feeding.
• Ensure good furrow closure over seeds when planting no-till fields. Furrows that aren’t fully closed actually serve as a “highway” for slugs, allowing them to easily move from seed to seed as they feed.
• Limit use of neonicotinoid insecticides at planting, including seed treatments. While this class has no activity on slugs, they can ingest the active ingredient from furrows or seed treatments. The slugs can then pass the insecticide on to the beneficial insects that feed on them, reducing the population of natural enemies that can control slugs and other insects throughout the season. 
• Choose cover crop mixtures carefully. Some evidence indicates that slugs prefer brassicas like canola and radish, so limiting their inclusion in cover crops may be helpful in fields that have a history of slug damage. 

“Tillage is a control option for slugs, but most farmers in our region rely on no-till systems, so they are not comfortable resorting to this,” Hodgson adds. “Slug bait should be considered a rescue treatment, as it is expensive and washes away easily.”

Published: Jul 3, 2023