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Controlling animal pests in a grain field often requires a combination of practices.

In the case of slugs and voles, wild animals and cover crops are two of a farmer’s best allies, said James Hoorman, a regional soil health specialist for the USDA Natural Resources Conservation Service in Ohio and Michigan.

Hoorman spoke in a March 13 NRCS webinar.

Though they are quite different animals, slugs and voles cause broadly similar damage.

Both feed on corn and soybeans, killing plants and damaging stands. Voles, which find soybean cotyledons magnetically attractive, can also disturb a field by tunneling and will girdle the trunks of fruit trees.

The all-star predator of slugs is the ground beetle, a black insect that also harries black cutworms, wireworms, stalk borers and true armyworms.

“They really love the slug eggs. That’s what they go after, so to them it’s like caviar,” Hoorman said.

Fireflies, which also prey mainly on the eggs, have suffered in recent years because artificial lights can confuse them at night.

“Where you have a lot of very diverse cover crops, you’ll see that the fireflies are coming back,” Hoorman said.

Plenty of other creepy-crawlies &tstr; rove beetles, centipedes, daddy long legs, wolf spiders, frogs and toads &tstr; also eat slugs at some point in their life cycle.

To promote beneficial insects, especially ground beetles, farmers should consider whether they can omit neonicotinoid seed treatments.

The seed treatments don’t harm slugs, but a ground beetle can die if it attacks a slug that has consumed the insecticide.

The treatment is intended to control seed corn maggots and wireworms. If those pests are not a problem in the field &tstr; and they rarely are &tstr; a neonicotinoid might not be necessary, Hoorman said,

Untreated seed usually has to be requested well in advance, often the summer or fall before planting.

Slugs are sometimes food for large animals, such as raccoons, opossums, ducks and robins.

A few predators &tstr; owls, snakes and especially shrews &tstr; eat both slugs and voles.

Voles spell dinner for everything from hawks to coyotes.

“They are a favorite food of just about everything,” Hoorman said.

Some predators, like barn owls and coyotes, eat lots of voles but roam across a large territory.

American kestrels and foxes hunt in highly localized areas, so they are among the top vole killers per acre.

Attracting and keeping these localized predators can help keep vole populations in check.

For kestrels, that means erecting perches and nesting boxes on the farm.

A perch, which can be as simple as a pole with a short crosspiece at the top, should be painted a bright color. That’s to prevent driving equipment over a perch that has been toppled by deer, which use the poles as rubs, Hoorman said.

Some small dog breeds, especially rat terriers, can also control voles.

Feral cats are less reliable predators, and their predilection for beneficial songbirds outweighs whatever vole suppression they provide, Hoorman said.

Like animals, plants can play an important role in fending off crop pests.

Perhaps the easiest strategy is to plant cover crops that slugs and voles dislike.

Crimson clover is the biggest turn-off for both pests, but sunflower and chicory aren’t hits either.

Based on field observations, Hoorman suspects that daikon radishes can reduce slug populations.

“The slugs are attracted to the radish. It’s very sweet, but it also is very high in sulfur,” which can build up in the slugs and kill them, he said.

Some researchers he’s checked with are not sure the radishes kill the slugs.

Another option is to plant cover crops like cereal rye and winter peas to distract the pests till the crop is up and able to withstand damage.

But trap cropping can backfire in a long, cold spring when the corn takes a long time to get growing.

Some researchers have even considered spreading cracked grain on the field to occupy the voles, though it might be hard for farmers to justify directly feeding pests.

Spreading grain probably makes the most sense if a farmer has a low pest population and wants to ensure the resident predators stick around.

Voles do spread mycorrhizal fungi, which grow around plant roots and help plants obtain water and nutrients.

“We don’t want to kill (all the voles), but we just want to keep them so that they don’t overpopulate and cause a problem,” Hoorman said.

Still, spreading grain only works in a corn field. Those soybean cotyledons are just too tantalizing for a vole to pass up, Hoorman said.

Stand management can degrade slug and vole habitat.

For example, including in the mix some cover crops that winter kill can open up the stand and give the predators extra room to hunt can help.

Rotary hoeing or harrowing the crop residue can disturb vole nests and slug egg deposits. Dusk and dawn, when the pests are active, are the best times to hit the field.

“Just a little fluffing of that residue can dry it out and alter the environment. That’s what we’re trying to do,” Hoorman said.

Spraying an early preplanting herbicide a month before planting reduces food and shelter for pests, but planting green, in which farmers seed into living cover crops, can also work.

If the field has low slug numbers, no crop damage and high predation, “it seems like your system is ecologically balanced. I would say don’t mess with success,” Hoorman said.

To scout for slugs before the season, put a shingle, board or newspaper in the field. Wait a few days, and see how many appear.

If there are four to five slugs on the underside of the shingle, action may be warranted.

Mowing on field edges reduces harborage where pests enter the field, though farmers should check with their government programs to see if mowing is allowed, Hoorman said.

In heavy slug infestations, farmers may resort to poison baits. A number of these products exist, but they are fairly expensive, and their effectiveness depends on the slugs eating a fatal dose.

Slug baits are often needed for two to three years. “About probably the fourth year, that’s when we probably get a cold winter, so I kind of question how effective they really are,” Hoorman said.

To repel voles, farmers can use the bad-smelling insecticide Lorsban or capsaicin, the chemical that gives chilies their heat.

Capsaicin does wash off in the rain, and voles may get used to it if their populations are high enough.

But between predators, cover crops and chemicals, farmers have some tools to deal with these pests.


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