Got slugs? Don’t give up on no-till

At a meeting of the Northeast Cover Crops Council, John Tooker, a Penn State entomologist, showed a photograph of a hay mower in southern Pennsylvania teeming with slugs – tens of thousands of slugs, maybe hundreds of thousands. A photograph of a corn planter taken in Illinois showed similar quantities. “They’re the most common pest I get phone calls about, and they’re the most common source of frustration for growers,” Tooker said. Most of these growers use no-till practices. According to Tooker, the University of Delaware estimates that every year about 20% of Mid-Atlantic no-till acreage sees yield loss or stand establishment challenges from slugs. Slugs are generalists, feeding on whatever is available to them. “As long as it’s green, they’re happy,” Tooker said. Cost-effective management options are limited. Whereas a grower can turn to 20 insecticides for a bean leaf beetle or caterpillar problem, there are only a few bait options for slugs. Some Pennsylvania farmers are so frustrated with the slugs that they’ve returned to tillage for control. Tooker, however, wants them to think twice. The benefits of no-till are well-researched – decreased labor and fuel costs and, importantly, soil and water conservation. No-till practices have been so widely adopted in Pennsylvania that Tooker referred to it as a “no-till state.” No-till accounts for 75% of Pennsylvania’s soybean crop and close to 70% of its corn. An oft-overlooked benefit of no-till fields is that they host a greater abundance of beneficial insects and spiders. Tooker relishes the sight of green soybeans emerging from a mass of corn stubble. From an entomology perspective, he sees habitat and potential; the more of this type of habitat, the better it can help promote insect predator populations which can decrease pest populations. Predators include ground beetles (both adults and the larvae), rove beetles, ants, spiders and firefly larvae. As slug populations have increased, so has the use of neonicotinoid insecticides which are highly water soluble. This means that plants can absorb them and move them through their “circulatory systems” from the root zone up into leaves and other tissues. “Anything that bites into one of those leaves is going to get dosed with the insecticide. It’s just a question of whether they’re susceptible to the insecticide and the dose that they get. These are the most toxic insecticides that have ever been put on the market,” Tooker said. In 2011, more than 80% of corn, more than 50% of cotton and about 40% of soybean acres were planted with neonic-treated seed, a total area roughly the size of California. Tooker showed data that neonic usage continued to increase until 2015 when the U.S. stopped collecting data. Tooker’s field research shows that the use of neonics reduces beneficial insect populations in both corn and soybean fields. These beneficials can help limit populations of slugs and other pests. “Where we have the insecticides coated on the seeds, we have fewer slug predators and we have less predation. The more predators the better, and they can control things even as slimy as slugs,” Tooker said. Some studies from Tooker’s research group show that soybeans and corn grown without neonic seed treatments had higher plant populations and yields than their treated counterparts. Ongoing studies are showing that in no-till systems with diverse rotations, include cover crops, which are managed with integrated pest management, pest populations decrease as an effect of increasing beneficials. The research has been comparing two no-till systems. The first is a repeated two-year corn and soybean rotation using Bt corn seed and neonic-treated soybean and corn seed, plus a pyrethroid broadcasted shortly after planting. This is being compared to a low input, six-year rotation utilizing IPM. Any insecticide use is based on scouting and economic thresholds, key principles of IPM. This rotation does not use Bt corn seed or seed treatments. For the first three years of the project, the number of predators in these two treatments were more or less the same. Then in years four through six, the number of predators in the low input system significantly increased and appear to continue increasing. “Predators can protect corn and soybeans from slugs is the take-home message,” Tooker said. The practice of establishing a cash crop into living cover crops (typically cereal rye) rather than killing the cover crop weeks before planting – called planting green – may also be key to reducing slug populations. Research shows that planting green resulted in the lowest slug numbers compared to bare plots which had the most slugs and brown plots (where the cover crop was killed two to three weeks prior to planting). In the planting green trials, farmers sprayed glyphosate one to seven days after planting to kill the cover crop. It’s hypothesized that the slugs prefer to feed on the glyphosate-killed cover crop rather than the cash crop. It contains more free protein, making it more attractive to the slugs. Planting green has the added benefit of creating habitat for natural slug predators by extending the season when predators are more comfortable and providing continuous habitat coverage. An increase of beneficials over a longer period appears to reduce the need for insecticides, which have a negative impact on the beneficials. by Sonja Heyck-Merlin