COLUMBUS, Ohio – A slug monitoring project funded by the United Soybean Board wrapped up the first week of July. Ohio was one of several states in the Mid-Atlantic that participated in the project with data collection headed by the Ohio State University Extension team.

Slug pressure has increased in recent years largely due to the greater adoption of agronomic practices like no-till and cover crop adoption, said Kelley Tilmon, state specialist in field crop entomology and professor of entomology at OSU.

“Those are really good agronomic practices, and it’s great that people are adopting them on more acreage, but one of the downsides is that this creates better slug environments,” she said. “Slugs need things to be a little bit cool and moist, and when you have no-till conditions, you have a lot more residue, which creates that refuge for slugs to hide under. Likewise, cover crops also provide that cover and protection for slugs.”

The multistate slug monitoring project aims to quantify the problem and understand the variation of slug pressure. From early April to the first week of July, Extension specialists monitored for slugs in 15 counties in Ohio across 17 soybean fields. Fields included in the project were no-till with no foliar applications of insecticide and no slug bait applied within 10 feet of the monitoring area.

Slugs were monitored and quantified using shingle traps, a 1-sq.ft. white roofing shingle secured to the ground with tent stakes that provided refuge for slugs to hide during the daylight. Each week, traps were checked in the morning by lifting the shingle and counting the adult and juvenile slugs underneath each trap.

The gray garden slug is the most common slug in Ohio associated with field crops. Eggs typically hatch in late April in central Ohio and mid to late May in northern Ohio. Noticeable feeding begins one to two weeks after hatching.

While slugs aren’t picky about what they eat, they pose a bigger threat to soybeans than other crops. This is because the growing point of soybeans is above ground during the seedling stage. A hungry slug can munch on the apical meristem, the plant’s growing point, killing the entire plant. In comparison, corn’s growing point is underground, and new leaves will continue to be put on once above ground, even if defoliated.

Data analysis is ongoing, and it will be some time before researchers can recommend management advice. However, raw data has been shared through the Crop Observation and Recommendation Network Newsletter, a product of the OSU Extension agronomics crops team. The week of May 27th saw the highest average number of slugs, 9.93, across all monitored sites. For some producers, this was about a month after planting. During the last week of monitoring, Seneca and Wayne counties reported the highest average number of slugs.

While part of the project is to assess how many slugs producers are seeing, another key component is researching ways to manage slugs. Slugs are a type of mollusk, so insecticides do not affect their populations. In fact, researchers at the Pennsylvania State University (PSU) have found that using insecticidal seed treatments on soybean crops can actually increase slug populations.

Tilmon said neonicotinoids are applied to the seed, and when the seed germinates, the product is taken up into the young seedling plant.

“As the slugs eat these young plants that are full of this insecticide, they excrete this insecticide through their skin into their slime to get rid of it,” she explained. “The major natural predators of slugs are ground beetles, predatory insects found in many crop habitats, especially in no-till and cover crop situations. They’re very good at killing slugs. You can see how, if this slug is chock full of this insecticide and the ground beetle eats the slug, what happens to the ground beetle? The ground beetle dies.”

In this case, the slugs slowly kill off their main predator, which increases their population. Tilmon said that John Tooker, professor of entomology at PSU, is investigating how rotated corn and soybean crops impact the ground beetle and slug populations. The same insecticides are used in both corn and soybean growing. Tooker is also investigating farms where soybeans are planted into rye cover. In these fields, slugs could feed on the rye, which could help protect the soybean crop. 

Additionally, Tilmon said that slug samples have been collected and sent to the University of Delaware to study a more natural way of controlling slugs. Parasitic nematodes are worms that can infect species and cause disease. Some of these worms are specific to mollusks or slugs.

“All slug populations, to some extent, will have these parasitic nematodes,” Tilmon said. “But we know very little about them. By learning what parasitic nematodes attack and kill slugs, we may be able to manipulate conditions so that we get more natural control of the slugs.”

Tilmon and her team members have applied for a second year of funding with the United Soybean Board. The proposal is still pending, but she is confident that a second field season would lead to more comprehensive field management advice.

“Before you can tackle solving a problem, you have to document it,” she said. “This is important because we are in the phase of documenting this problem with real, scientific data.”