Don Wirth is searching for slimy vampires that steal the lifeblood from his fields. With Mt. Hood and the majestic Cascades over his shoulder, Wirth lifts a roof shingle and checks the improvised trap to reveal several 1”-long creatures sheltered from the early afternoon sun. Hiding by day and feeding by night, the sleeping assassins are the most deceptive of crop killers.
Slugs are common on many farms, but as numbers rise, particularly in the Midwest, producers are reckoning with a new level of damage. A mild slug presence, sheltered by increasing no-till acreage, can usher in a wave
of replants, major yield loss and expensive bait control. The precise financial cost of slugs to U.S. agriculture is unknown, but it’s more than several hundred million dollars each year and is consistently going up.
Across 3,000 acres on the floor of northwest Oregon’s gravid Willamette Valley, Wirth grows a dizzying variety of crops for his Saddlebutte Ag seed company: annual ryegrass, tall fescue, peas, mustard seed, kale, clover, chicory, plantain, forage cabbage and many more. He can drive by a crop at 40 mph and spot the whitish cast of a slug infestation.
“Corn, soybeans or grass, I don’t think there is a crop slugs can’t hurt. I’ve seen an entire 160-acre field of perennial ryegrass get killed,” he says. “The whole block faded to brown as the slugs moved across.”
Wirth has spent up to $60,000 per year on slug control and attributes an increased gray slug presence to multiple factors, one of which traces back to a benchmark 1988 field burn. On an Aug. 3 afternoon, post-harvest burning cast a dark cloud of smoke across I-5 outside of Eugene, essentially creating a blind tunnel for drivers. The thick smoke led to a chain reaction of collisions resulting in seven deaths and 37 injuries. The 21-vehicle pileup led to a near-total burn ban on Oregon farmland.
Wirth believes the absence of fire to control pest numbers is only a partial explanation for the rise in slug populations. “I don’t know the whole answer to bigger slug numbers, but I think cover crops and no-till are creating a protection blanket,” he says.
Slugs take a $60 million bite out of the Oregon seed grass industry each year, and Wirth says vigilance is an absolute necessity. “If you sit put and don’t scout, you’ll soon have a mega problem,” he warns. “This is a prehistoric creature that knows how to survive. Put a slug in the freezer for a few weeks and when you take him out, don’t be surprised if he crawls off.”
Creatures Of Habit. Slugs become active at dusk, moving to feed or mate during the night. With a typical lifespan of one year, slugs lay eggs in autumn and spring, and offspring are ready to mate in six months. Slugs are hermaphrodites and when necessary, can self-fertilize and don’t need a mating partner.
Two molluscicides are labeled for slugs in Oregon, metaldehyde and iron-based pellets. However, Wirth hasn’t had great success with either treatment. “If we kill 15% of the slugs, that’s a success, even though we’re really only killing the big ones, and nothing ever seems to work on the eggs,” he explains.
In the wild, slugs feed on preferred plants, but in crop fields slugs aren’t mobile enough to pick and choose. Far beyond feeding damage, slugs also vector plant viruses and disease. (In addition, slug slime and feces reduce market value of some crops.)
Rory McDonnell, a renowned slug and snail expert with Oregon State University, says slugs are notoriously difficult to manage: “It’s either metaldehyde, iron phosphate or chelated iron pellets. With the advent of no-till and conservation tillage, tillage is out for many producers.”
No-till residue buildup is beneficial to slugs. It keeps the ground relatively moist for the slimy creatures, provides cover during daylight hours and offers excellent habitat for egg-laying.
“It’s critical for growers to monitor populations,” McDonnell adds. “A couple of slugs under a piece of wood are no problem, but when numbers rise, you better put out bait. Walk around after dark with a flashlight and you’ll get a good idea if you need to bait.”
The Threat Spreads. Roughly 2,400 miles away from the Willamette Valley, Kelley Tilmon anticipates increased farmer concerns over slugs in the Midwest. Tilmon, an entomologist with Ohio State University Extension, sometimes sees 20% losses in corn and soybean infestations, but has noted plenty of occasions when replanting was necessary, particularly when slugs feed on seeds.
“I expect the trend for no-till to increase and therefore slug populations to increase,” Tilmon says. “Growers should consistently scout. A fall population means a spring presence.”
She recommends simple trapping stations in fields: Roof shingles spray painted white ensure a relatively cool temperature and attract slugs to the underside. “On crops, check for holes in leaves. Slugs feed in long, narrow strips in corn and on the edges of leaves in soybeans,” Tilmon describes. “There are no official thresholds to rely on, but keep an eye on defoliation and slug numbers to roughly measure problem levels.”
Unprecedented slug damage hit Kentucky in 2017 and caused heavy replanting in some counties, notes Raul Villanueva, an entomologist with the University of Kentucky Extension. At least 30,000 acres were replanted in Kentucky due to slug damage, and Ohio County producers were forced to replant 70% of soybean acreage, Villanueva says. He recorded up to 90% damage on soybean fields across six counties. Villanueva attributes part of the slug population jump to a warm winter in 2016 and the lack of a natural cold-weather kill.
Daniel Fleishman, BASF innovation specialist, says if harvest is followed by a wet fall and mild winter, expect Kentucky fields to host unprecedented slug numbers: “Slugs are a completely different problem than anything we’ve dealt with before, and we’ve got a lot to learn.”