It's getting hard to overestimate the problems slugs pose for no-tillers.
The University of Delaware estimates about 20% of no-till acres in the Mid-Atlantic States alone — some 600,000 acres — are affected by slug damage each year. Even mainstream Newsweek magazine recently authored an article on slug damage.
While consistent, cost-effective options to control slugs aren’t as numerous as with other crop pests, Penn State University Extension entomologist John Tooker shared several potential strategies at the National No-Tillage Conference in Cincinnati last January.
1. Feed the Beetles.
Some wolf spiders and insects like firefly larvae, soldier beetle larvae and ground beetles will kill slugs. But ground beetles have by far the largest effect, Tooker says.
He shared a study that found 100% slug survival with no predators and some mortality with soldier beetle larvae and wolf spiders. But when these predators were combined with ground beetles, very few slugs survived.
Tooker even shared a heat map from Europe showing the abundance of gray garden slugs and ground beetles in the same locations.
“The ground beetles are responding to the slug populations because that’s what they want to eat,” he says. “If we can get more of these ground beetles in our fields, our slug populations are going to go down.”
2. Don’t Create ‘Toxic’ Slugs.
Tooker shared Penn State research data from 2013 indicating that in plots where neonicotinoid insecticides were used to treat seed corn, slug populations were higher compared to plots without the treatments.
More detailed Penn State data from a 2012 study on soybeans using quarter-acre plots showed that insecticides were disrupting biological control.
Tooker notes researchers have also measured the amount of thiamethoxam, the active ingredient in Cruiser, and clothianidin, the active ingredient in Poncho, and found that levels in slugs sometimes reached 200 parts per billion — more than twice the amount of chemical that will kill ground beetles. Neurotoxicity from overexposure to these chemicals can also poison beetles, making them slow, lethargic, and vulnerable for several days to predation by birds, mice and other animals.
“Manage for the pests that you have. If you don’t have slugs and your main concern are fields where wireworms have been a problem, that’s where your insecticide will probably pay off,” Tooker says. “But if you have other fields where slugs are your biggest concern, I would encourage you to remove your seed treatments because they’re not helping you in sluggy fields, they are actually hurting you.”
3. Tried and True.
There are some products marketed specifically to kill slugs, such as metaldehyde, which is found in Deadline and other products. Slugs eat the pellets, dehydrate and die, and the product doesn’t affect natural enemies, Tooker says.
If no-tillers use a spinner spreader for application, Tooker suggests a rate of 4-6 pellets per square foot. Research at the University of Maryland by entomologist Galen Dively also found that spreading 10 pounds per acre, banded right over the row, works well.
Iron phosphate-based baits, while somewhat expensive, have worked well in organic no-till. Cans of beer and salt placed in fields work, too, but is impractical on a large scale.
4. Use Nitrogen.
Some no-tillers have experimented with spraying nitrogen (N) on slugs under the “Rule of 3.” About 30% N mixed 1:1 in water, sprayed three nights in a row at 3 a.m., Tooker says.
“The nitrogen becomes a contact poison. It actually melts the slugs,” he says. “The reason they do it three nights in a row, I believe, is because you’ll get one-third of the population each night.”
Dively once counted the number of slugs per 20 plants and organized four sets of plots: one untreated plot and three plots where he mixed 5 gallons of N in 15 gallons of water, 10 gallons N in 10 gallons of water or 20 gallons of straight 30%.
“The happy medium is 10 gallons of nitrogen in 10 gallons of water. And he knocked back his slug populations by about 75%,” Tooker says.
5. Try Cover Crops.
It might seem that adding more biomass to fields with cover crops will make slug problems worse, but Tooker says their research indicates this isn’t necessarily the case, in part because of the feeding preferences of slugs.
Given a choice between corn and cereal rye, for example, slugs will almost always choose the rye. They also prefer soybeans to corn. While the results are preliminary, Tooker says Penn State has one season of field evidence that suggests crimson clover can suppress slugs.
Both Penn State researchers and Pennsylvania no-tiller Lucas Criswell have also experimented with interseeding cereal rye into row crops to give slugs something else to eat beside the crops.
Last spring at Penn State’s research farm and at Criswell’s farm, they seeded cereal rye between rows of corn in a separate planter pass. For soybeans, they mixed rye in the planter’s fertilizer boxes and seeded the mix in one pass.
They established plots with and without rye, and found that the amount of slug damage to soybeans decreased dramatically where rye was present between the rows.
“Even better is that the ground beetles responded to this treatment as well. Where we had the rye, the beetles were at much higher levels,” Tooker says.
6. Close the Furrow.
Planter performance may also factor into slug control. No-tillers should make sure furrows are closed during planting, Tooker says.
“If you have an open furrow, that’s more or less serving as a highway that leads the slug from one plant to the next,” he says. “It’s nice and moist down there and they’re going to roll right down the furrow. Make sure you get the furrow closed.”