Crops began to vanish mysteriously a decade ago on Lucas Criswell’s no-till farm in central Pennsylvania.
“Things just started to disappear,” the Lewisburg, Pa., farmer recalls. “You’d see a soybean plant one day, and the next day it would be gone.”
Only by creeping into his fields at dusk and dawn did Criswell finally identify the thief slipping through his 1,800 no-till acres of corn, soybean, wheat and rye. “Slugs — all different sizes, big ones, little ones,” he told DTN. “You’ll see a slime trail on the plant that kind of glistens. It’s a little bit creepy.”
The creep factor is the least of growers’ concerns, however, said Penn State University entomologist John Tooker. “Slugs are among the most challenging pests that Mid-Atlantic farmers face in their no-till fields,” he explained.
For years, the steady rise of slug populations in no-till fields had baffled farmers in the region. Recently Tooker’s research has uncovered one possible culprit: the neonicotinoid seed treatments that companies add to soybean seed for protection against seedling pests.
The discovery comes on the heels of an EPA report that concluded that neonicotinoid seed treatments in soybeans were of no economic benefit to most northern farmers. Now Tooker’s research casts new doubt on their value for no-till farmers struggling with slug infestations in the mid-Atlantic region and raises questions about their effect on non-target organisms.
“At least in fields where slugs are the biggest challenge, neonics aren’t giving growers any benefits,” Tooker concluded.
A series of lab and field studies showed that slugs, unaffected by the neonicotinoids in the plant material, pass the toxins on to the beneficial field insects that prey on them — mostly ground beetles, beetle larvae and spiders. The poisoned predators become lethargic; some prove unable to roll themselves upright, and many die. Meanwhile, the slugs continue on their slimy way.
A SLIPPERY FOE
Tooker and his graduate student, Maggie Douglas, were initially only interested in collecting potential field predators of slugs, at the request of frustrated local no-till farmers.
The pests thrive in the moist, stable, residue-rich environments of no-till fields, Tooker explained. Their numbers have risen in states like Pennsylvania, where up to 70% of farmers have switched to no-till after a major federal and state push to protect water quality in the Chesapeake Bay.
Soybeans are especially vulnerable to slugs, because their growing point is above ground and easy prey for the hungry mollusks. “In heavy slug years, I’ve heard of growers in Pennsylvania replanting three times because they can’t get a good stand because the slugs are going to town,” Tooker said.
Douglas rounded up a sample of ground beetles, beetle larvae and wolf spiders and brought them back to the lab. To mimic field conditions, she fed the slugs soybean plants treated with neonicotinoid insecticide and a fungicide, and then unleashed the predators on them.
“We saw right away that these potential predators didn’t fare so well,” Tooker recalled.
In one video from the study, the researchers documented the after-effects of a ground beetle feasting on a neonicotinoid-fed slug. The camera focuses first on the fat, freshly fed slug, sitting complacently in a jar of soil. It then pans to the other side of the jar, where the predator ground beetle lolls about on its back, twitching its legs in the throes of death.
As the researchers discovered, neonicotinoid seed treatments are very effective at targeting insects. But slugs aren’t insects — they’re mollusks, which are more closely related to clams and mussels than insects, Tooker explained. Insecticides made no visible impression on the slugs in the studies.
“Their feeding behavior doesn’t change and their weight gain doesn’t change in the presence of insecticide,” Tooker marveled. “They don’t seem to notice it at all.”
Yet shortly after feeding, the slugs toted around traces of the neonicotinoids up to 500 parts per billion per slug — more than enough to kill the predator insects that picked them as prey.
IMPLICATIONS IN THE FIELD
After ascertaining that the neonicotinoids were the culprit, and not the fungicides, the researchers took their study to the fields. “Slug-infested fields that had seed treatments had 19% lower stand counts, 5% lower yields, fewer predators and more slugs than the untreated plots,” Tooker said.
Douglas also tested parts of the field “community” for insecticide levels, including soil, plants, slugs and other field critters.
The amount of neonicotinoids decreased significantly as it traveled from plant to slug to insect, but traces of it could still be found among the field community. Even neighboring earthworms were found to contain up to 227 parts per billion of neonicotinoids. These trace amounts are plenty high enough to cause illness and death in insects that feed within this biological community, Tooker said.
The toxicity of neonicotinoids to non-target organisms has not been well studied, but Tooker and Douglas’ research suggests the area needs more attention.
Neonicotinoid seed treatments, long assumed to be an inexpensive, solely beneficial insurance policy for farmers, have now been implicated in the deaths of two important groups: honey bees and natural insect predators, Tooker said.
“It opened our eyes to the unintended consequences of blanket adoption of management tactics,” Tooker said. “Neonics traveling through slugs to get to predators cannot be the only way that neonics influence the predator community in crop fields. We don’t know for sure what other pathways are, but it’s safe to say they are probably out there.”
For his part, Criswell switched to untreated soybean seed three years ago, in response to Tooker’s growing research on the topic. The beans look rough, thanks to early bean leaf beetle feeding, but they always recover and yield well above the county average, Criswell noted. He now controls the slug populations in his field by planting his crops into live cover crops, which supplies the slugs with another food source and provides habitat for natural slug predators.
Last year, Criswell ventured into non-treated corn seed as well. The 30-acre patch he grew “looked like crap” throughout the summer but yielded the same as his treated corn hybrids — around 200 bushels per acre. This year, in an effort to find good hybrids without any seed treatment, Criswell plans to plant 90 acres of untreated, non-GMO corn seed from Pioneer.