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Ground Beetles Key Allies for Slug Control in Conservation Agriculture

Nine-part 3x3 collage image with overhead views of nine dark-colored beetles. While varying in exact shape and color, they are all mostly black in color with long abdomens covered by vertical-lined wing shields. Some of the beetles are slightly iridescent.

Slugs are a common headache for corn and soybean growers following the conservation agriculture practices of limited tillage and frequent cover cropping. A new study finds that, among slugs’ natural enemies, ground beetles (such as those shown here) provide the best control, and growers should consider practices that attract ground beetles to their fields. (Photos by Thabu Mugala)

By Carolyn Bernhardt

Encouraging the presence of ground beetles and other natural enemies can enhance slug suppression in corn and soybean fields grown through conservation agriculture practices, according to a recent study. The research was published in May in Environmental Entomology. As farmers increasingly adopt conservation agriculture worldwide, a deeper understanding of slug dynamics and effective mitigation methods becomes imperative for sustainable and resilient farming systems.

Slugs in Conservation Agriculture

Conservation agriculture practices focus on eliminating tillage and planting high-residue cover crops, which offer advantages such as increased land productivity, reduced labor requirements, decreased water usage, improved nutrient cycling, enhanced soil biota, higher farm profits, environmental sustainability, and equitable access for small-scale farmers. After a slow start, conservation agriculture is gaining traction worldwide as its benefits become more widely recognized and appreciated by farmers and researchers alike.

However, the crop residues—i.e., the plant matter left on the land after harvest—in these contexts create moist and shielded environments, which means slugs can thrive. And most farmers will tell you: Where there are slugs, there is crop damage. Researchers have seen how big of a stumbling block slug damage can be for years. A 2013 survey in Virginia found that no-till corn and soybean acres were 13 times more likely to have slug damage than conventional acres. That same survey showed slug damage was related to insecticide use, with 65 percent of farmers who “always use insecticide” at planting reporting slug damage—possibly a result of the insecticide affecting natural enemies of slugs—compared to 13 percent of farmers who “never use insecticide” at planting.

Additionally, chemical molluscicides can be expensive, less effective in wet conditions, and harmful to wildlife. They notoriously impact birds, small mammals like rodents and shrews, and aquatic creatures such as fish, frogs, newts, salamanders, and invertebrates. “When the molluscicide seeps into water bodies, these aquatic animals can also suffer the consequences,” says Thabu Mugala, a Ph.D. student in the Department of Entomology and Wildlife Ecology at the University of Delaware (UD), who led the new study. “Beneficial insects are not spared either and can be harmed by these chemicals.”

Alternative management strategies include using row cleaners to remove crop residue above the seedbed and planting the cash crop into a standing, green cover crop, which exploits slugs’ preference for certain cover crops and recruits slug natural enemies in the field. Overall, researchers are interested in pursuing integrated pest management, which involves using many tactics to reduce pest populations. Conservation agriculture is crucial to farms in the Mid-Atlantic region. “Many farmers are committed to no-till for the benefits to soil conservation and water quality, among others,” says Michael Crossley, an assistant professor and agricultural entomologist at UD who helped lead the study. “And promotion of natural enemies is complementary to this approach.”

“We just need more tools,” says Bobby Clark, a senior extension agent in Virginia, who collaborated on the study. “We are at our limit of technology, and slugs are still a big problem.” Based on dialogue with multiple farm groups, Clark says the damage caused by slugs is the primary factor stopping farmers from no-tilling corn and soybean. But no-till is far better environmentally than conventional farming practices, like disking or plowing and disking. “We know if farmers disk fields, slug damage almost goes to zero,” he says. “We need alternatives to manage slugs in no-till systems. The current management tools, which include using slug bait and minimizing insecticide, are inadequate. Experts I speak to do not anticipate another chemical option being developed, so we are turning to biological control technologies.”

The Search for Natural Control

Although there are currently no commercially available biological control agents for slugs in North America, conservation agriculture practices can promote native and exotic natural enemies of slugs such as ground beetles, harvestmen, and wolf spiders. So, the researchers set out to understand the effects of various factors on the activity-density (a measure of both abundance and movement) of both slugs and natural enemies.

The team placed tile and pitfall traps in 41 corn and soybean fields in the Northern Shenandoah Valley in Virignia during the spring of 2018 and 2019, measuring slug and natural enemy activity. They built a model to estimate the effects of different factors on ground beetle activity-density and tested factors like cover cropping, tillage, pre-plant insecticide use, weather, and natural enemies.

“These were priority ideas identified by multiple researchers across the Mid-Atlantic region,” says Clark. “Prior to this research, cover cropping appeared to be one factor contributing to slug injury. We also had a strong hunch that pre-plant insecticides were harming beneficials. However, some experts still doubted that a single application of insecticide in the spring would harm beneficials. We also wanted to know if natural enemies were present, and the research proved they were.”

Ultimately, the team found that cover crops positively affected slug activity-density, but this effect was reduced by tillage. Slug activity-density decreased with higher activity-density of ground beetles, as well as with less rainfall and higher average temperature. Based on the study’s findings, weather was the only significant predictor of ground beetle activity-density, which was reduced in hot and dry or cool and wet sites and weeks.

Pre-plant insecticides had a marginally significant adverse effect on ground beetles, and the study found a substantial impact of ground beetles on slug activity-density. These findings suggest that cover crops and tillage interact to create favorable conditions for slugs, but even low levels of tillage can help mitigate this. Implementing practices that promote the recruitment of ground beetles in crop fields can improve the natural suppression of slugs in corn and soybean grown using conservation agriculture practices.

“Ground beetles are providing an important ecological service and helping us produce food sustainably,” says Mugala.

Clark says, “Some beneficial insects already exist in our fields. Working with these beneficials reduces the incidence of injury due to slug feeding.”

From Research Into Action

Previous research has reported significant effects of cover-crop residues and tillage on ground beetle activity-density and community composition. But Crossley was surprised that the team didn’t see an effect of tillage or cover crops on ground beetle activity-densities. “But I think it is important to remember that most farms in the Shenandoah Valley are doing reduced or no-till,” he says, “so our beetles are not experiencing the level of intensity of conventional tillage.”

“While this research may not be groundbreaking, it is highly valuable in comprehending overall patterns within the Shenandoah region,” says Mugala. “The research provided a substantial dataset from diverse locations. Also, unlike other studies, it considers additional factors like pre-plant insecticides, tillage, and covering crops simultaneously.”

Before this research, Clark says he was primarily opposed to using shingle traps as a method of scouting for slugs because he figured it would be too difficult. But he has been using shingle traps ever since these findings. For future research, he says, “We need to build a threshold, such as a number of slugs per square foot, for using shingle traps.”

And Crossley says, “Identifying predictors of slug activity-density is one thing, but it still isn’t clear how those slug numbers translate into crop damage and yield loss. This kind of information could help translate slug counts under shingles into actionable outcomes.”

Carolyn Bernhardt, M.A., is a freelance science writer and editor based in Portland, Oregon. Email: carolynbernhardt11@gmail.com.

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