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Research Highlights

Research Highlights
Investigating Slug Egg Hatch for Patterns

Slugs have become a major pest in no-till soybeans in the Mid-Atlantic. Photo: Cory Whaley, University of Delaware Extension

By Laura Temple

No-till, conservation tillage and cover crops can reduce erosion, minimize runoff and improve soil health. In the mid-Atlantic, these practices also create favorable microclimates for slugs, which have become one of the most frustrating pests in the region.

Slugs feed on swelling soybean seeds as they begin to germinate. They can cause enough damage to require replanting. Slug bait is expensive, and it’s hard to time rescue treatments effectively. Often when a slug problem is noticeable, the field already needs to be replanted.

“Slug damage is difficult to predict,” says Dr. David Owens, Extension Entomologist Specialist based at the University of Delaware Carvel Research and Education Center. “We have lots of questions about them that need to be answered. A better understanding of slug activity may help farmers make management decisions.”

Owens says slug research suggests recently hatched slugs do the most damage to the crop, and that bait applications that coincide with egg hatch are most effective to prevent soybean stand loss. However, very little information exists about slug egg hatch. The Delaware Soybean Board invested checkoff funds into a study he led to learn if relationships exist between weather factors, slug populations and hatching.

“We are investing soybean checkoff dollars to find a solution, so we don’t have to replant soybeans because of slugs,” says Robbie Emerson, a Delaware Soybean Board director who farms near Middletown, Delaware. “With this type of research, we can learn more about ways to slow or minimize slug damage.”

Emerson adds that slugs have caused major issues, especially in his soybeans, during the past two cool, wet springs. He has replanted some areas of fields due to slug pressure.

Monitoring Slug Populations

Owens and his team identified farmer cooperators, including Emerson, with soybean fields that had a history of slug damage or high populations. In both 2020 and 2021, they monitored nine fields for slugs, air temperature, relative humidity and soil surface temperature under residue. His team recorded weekly slug counts from March until early June or the soybeans had outgrown the threat of slug damage.

“Sampling slugs can be challenging because they hide in the soil or under residue during the day,” he says.

They established shingle shelter traps in each sample location. They carefully sifted residue in the square meter around the traps and recorded all stages and species of slugs found, primarily marsh slugs and gray garden slugs. They counted slug eggs, neonates or newly hatched slugs, juveniles and adult slugs.

The team returned to the fields in November and December to monitor slug populations again and learn more about how they overwinter.

As a farmer cooperator, Emerson gained new perspectives on slug management. He saw that some fields weren’t as bad as expected. In some areas, high pressure caused some damage, but he learned he didn’t have to replant. However, in other areas, replanting was clearly the best decision.

Observations for Management

Slug pressure remains difficult to predict, but Owens gathered new, useful information. The data collected from this study will feed into future state and regional slug research efforts. He also discussed slug updates and management in the Delaware Weekly Crop Update.

According to Owens, weather can prevent high slug populations from causing problems. Weather can also create the right conditions for a low slug population to damage enough seedlings to force replanting.

Figure 1. 2020 Gray Garden Slug Count

“Cold winters reduce overwintering slug populations, and populations can change significantly from year to year,” Owens says. “Cool, moist spring weather that slows soybean emergence and growth creates ideal conditions for slug activity.”

His team found that slug pressure varied significantly throughout a field, reinforcing the importance of scouting. Knowing field history also helps farmers decide how to manage slugs.

The preliminary data suggests there may be a pattern to gray garden slug hatch, which begins the first week of April and continues for a few weeks. And in fields with high populations, newly hatched slugs are likely to come into contact with bait applied during the first two weeks of the month.

According to Owens, marsh slugs were more common, and this species was active at all growth stages anytime temperatures reached the low- to mid-40s. That makes them unpredictable.

Figure 2. 2020 Garden Slug Count

While some farmers may mix slug bait with the potash application in March, these observations indicate that may not give the best return on that investment. Owens says weather conditions immediately prior to and during planting should probably have the most influence on planting and slug bait application decisions. For example, he recommends waiting for warm, dry conditions to plant problem fields.

“Any cultural practices that encourage quick soybean seedling emergence and growth will help minimize slug damage in fields with high populations or a history of damage,” he adds. “Open seed furrows serve as a highway for slug activity, so row cleaners can minimize that problem. Soil disturbance also decreases slug feeding.”

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.