Research Highlights

Exploring the Role of Cover Crops in Long-Term Slug Management

By Laura Temple

An annual survey of crop losses from insects consistently finds slugs among the top pests in Delaware, according to David Owens, extension entomologist specialist based at the University of Delaware Carvel Research and Education Center.

“Slugs are infuriatingly nasty pests to deal with,” he says. “We don’t have good thresholds for treatment and pressure is unpredictable. They prefer cool, wet weather, but often by the time farmers realize they have a problem, emerging crops like soybeans are severely damaged.”

Owens explains that chemical management of slugs is hard. Slug bait is expensive and granular. Most regional farmers aren’t equipped to spread granular products. Applications must be timed just right, during spring when time is limited because of other fieldwork that must be completed.

Slugs favor fields that are not tilled and have high residue or cover crops. Increased slug pressure is one unpredictable challenge in the push to improve soil health with practices like no-till and cover crops. 

“This region is among the national leaders in cover crop adoption rates,” Owens says. “I am focusing on cultural practices that may help manage slug pressure. For example, do slugs respond to given cover crop species? Over time, how do individual species of cover crops affect slug populations?”

Anecdotes from farmers in Delaware and Pennsylvania suggest that slugs may favor brassica species like radish, canola, mustards or forage turnips, but no data exists to support observations like this. Some research suggests that delaying cover crop termination can help manage slugs, though Owens personally suspects delayed termination may sometimes favor slugs. He has talked to farmers who have had conflicting experiences with late cover crop termination timing and slug pressure.

The Delaware Soybean Board invested checkoff funds into a multi-year study he is leading to generate data about how adjustments to cover crop practices, like species choice and termination timing, may help manage slug populations over time. 

On-Farm Trials

Owens is conducting this study in fields of cooperating farmers with known slug pressure. In the fall of 2021, he planted various cover crop plots at three sites. In the fall of 2022, he added two more farms. 

In each field, his team is monitoring slug populations with shingle traps. They begin checking populations when cover crops are planted shortly after harvest. In the spring, monitoring runs from March until the main crops are planted. 

Soybeans are a vital part of the crop rotation in all these fields. The long-term nature of this study means that his team is monitoring slugs in fields with cover crops following full-season and double-crop soybeans and in fields that will be planted to soybeans at least once during the study. Crop rotations in Delaware often include corn, small grains and vegetables, along with soybeans.

However, he notes that the unpredictability of slug populations creates challenges. For example, during the spring of 2022, his team chose to stop monitoring a field that had very little slug pressure, despite its history.

Species Comparisons

Owens says that farmers in this region most often plant small grains as cover crops. Sometimes they choose legumes like vetch and crimson clover, and occasionally they plant tillage radish, a brassica. Multi-species mixtures are also common.

His research reflects these options, with each trial field including cover crop plots of barley, rye, crimson clover and tillage radish. 

“In the spring of 2022, the tillage radish was winter-killed for the first time in three years, which was disappointing because I have seen slugs feed on it like candy,” Owens says. “We chose to switch our brassica species to canola when planting cover crops in the fall of 2022 to avoid this potential problem.”

During the first year of the trial, his team observed no slug population differences between cover crop species, but he expects any response to be cumulative over a few years. 

“If over time we can show that specific cover crop species are more or less supportive of slugs, we can make better recommendations for cover crops in problem fields,” he explains. 

Termination Timing

Owens’ study is also looking at early and late cover crop termination timing, all done with herbicide. He targets early termination timing for three to four weeks before planting. Late termination happens as close to planting as possible, sometimes even occurring after planting but before emergence. 

He is still evaluating the data comparing termination timing, but again he expects multiple years of data will be needed to find any trends.

Owens added that the late termination timing had the unintentional benefit of allowing the crimson clover to serve as pollinator habitat after it bloomed in late April and early May. 

“The clover plots were buzzing with bees,” he says. “While we are still learning how specific cover crop practices impact slugs, we are reminded that there are always tradeoffs. The goal is that this research will provide data to help farmers understand those tradeoffs as they relate to slug management to make the best decisions for their farms.” 

Additional Resources:

• SRIN information page: https://soybeanresearchinfo.com/soybean-pest/slugs/
• David Owens profile

Published: Feb 6, 2023