Expert offers options for keeping slugs at bay
SALEM — Penn State University entomologist John Tooker didn’t provide Oregon growers with any silver bullet solutions to slug control during his visit to the Willamette Valley last week.
But Tooker shared strategies Pennsylvania growers have used to lower slug pressure and encouraged Oregon growers to consider implementing some of them.
“I would ask you to think about ways to incorporate some of these ideas, recognizing our annual cropping system in Pennsylvania is different than what you have here,” he said at an Oregon State University Extension meeting in Salem on Sept. 10. “By implementing these ideas, a couple of growers who have fully embraced them have made their slug populations go away.”
Slugs are by most accounts among the worst pests in Oregon grass seed production, if not the worst. They accounted for nearly $100 million in damage to the $500 million crop in recent years. The mollusk also is responsible for substantial crop loss in several other field and row crops.
Tooker, who has become a leading expert in slug control in recent years, said growers and researchers in Pennsylvania have found that use of cover crops and predator beetles, in the absence of insecticidal seed treatments, can be a successful formula for keeping slugs at bay.
To start with, he said, slugs prefer certain cover crops over cash crops — a preference growers can use to their advantage.
“If you give them a choice between a rye plant and a corn plant, they will choose the rye every time,” he said.
Complementing the direct benefit of keeping slugs off grower’s primary crop, rye and crimson clover plants serve as hosts for beneficial insects that feed on slugs.
“The rye distracts the slugs, allowing them to feed on something they like better than the cash crop, and it improves the ground beetle population,” he said. “Those two things together are taking the pressure off the cash crop, letting it get out of the ground and grow.”
Some growers in Pennsylvania have even started planting cash crops directly into a standing green cover crop, Tooker said. They follow that with a treatment of glyphosate, which kills off the cover crop, but while the cover crop is dying, it is still palatable to the slugs and still fostering beneficial insect populations, he said.
“It is more management intensive,” he said, noting that growers incorporating this technique are not using insecticidal seed treatments and, instead, have increased scouting for insect pests and are treating only when needed.
“But,” he said, “what growers who are doing this have found is they are getting the best yields that they’ve ever had.”
Tooker showed evidence that treating seed with neonicotinoid insecticides can reduce ground beetle populations and, subsequently, increase slug pressure.
“Slugs are consuming the insecticide and the beetle gets it from the slug,” he said.
He added: “On average, we see more slugs where you have the insecticide than where you don’t. That is the exact opposite of what a grower expects. That insecticide on the seed is supposed to protect the crop from early-season insect pests. But insects aren’t at play here.
“There is no reason to think an insecticide will kill a mollusk, and no reason to think a molluskicide will kill an insect,” he said.
Tooker also provided evidence that when applied at night, nitrogen applications at 20 gallons to the acre can reduce slug populations — sometimes as much as 75 percent.
“This is not easy on your plants,” he said, “but the general thinking is the benefit you gain by knocking back the slug population outweighs the cost of dinging up you corn or soybeans with nitrogen.”
He said farmers in Pennsylvania have grown frustrated with the efficacy and rainfastness of the common slug bait metaldehyde.