Scouting for Slugs in Field Crops

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- [Instructor] Are slugs a problem in your field crops?

If so you aren't alone.

Slugs have become one of the most challenging pests for no-till farmers.

This video will outline a practical method for scouting for slugs and identifying damage in field crops so that you can make informed management decisions.

The heavy crop residue and lack of soil disturbance in no-till fields creates an ideal habitat for slugs.

Slugs are challenging to control because there are limited control options available.

Insecticides are often not effective because slugs are mollusks, not insects.

Insecticides are used to kill insects while molluscicides are used to kill mollusks.

Research at Penn State has determined that the best approach to control slugs is to diversify your crop rotation, include using cover crops, and to practice integrate pest management.

This approach will encourage the growth of predator populations which help to control slugs.

Nevertheless you should track local populations to determine if rescue treatments are necessary.

So when you should you be scouting for slugs in your field crops?

Because slug feeding is most damaging to seeds and seedlings, we recommend scouting for slugs immediately after planting from Mid-April to June.

If you plan to establish a crop in the Fall, we recommend scouting for slugs in Mid-August to October.

An economical and easy way to scout for slugs is to use refuge traps.

A refuge trap is a shelter that sits on the soil surface and provides a place for slugs to hide during the day.

They can be made from a variety of materials including roofing shingles, bricks, plywood boards, feed bags, or even cardboard.

For research purposes we tend to use roofing shingles because they hold up better over continued use, are inexpensive and can be driven over by farming equipment.

After your crop is planted and you have your materials, place the refuge traps flat on the soil surface in your crop fields.

Moving aside residue if necessary.

Flagging the refuge traps will ensure you can find them later.

We strive for 10 shingles per field randomly distributed across the whole field.

But you should determine the number of shingles and pattern of distribution in the field that are best for your situation.

Slug populations can be higher on field edges, and lower in the middle.

So distribution of shingles needs to take this into account.

After the refuge traps have been set up for a few days, you can start scouting.

Flip over the refuge trap and count the number of slugs.

Keep a record of your monitoring efforts.

We recommend scouting refuge traps at least once per week until June or when crops are approximately knee high.

Because slugs are active at night it is best to scout your refuge traps in the early morning or at dusk.

If you consistently find an average of one to two slugs per trap or higher, continue monitoring your refuge traps but keep a close eye on crop damage.

Before making a management decision, we recommend that you scout your crops for slugs in addition to monitoring refuge traps.

Shiny slime trails like the ones shown here are a good indication of slugs.

When it comes to damage, slugs feed by scraping their mouth part across the leaf surface.

For corn seedlings the feeding usually leaves part of the membrane intact cause a windowpane-like experience, like the ones in here.

For soybean seedlings slug feeding appears as deep circular pitting.

Although slug damage can be alarming, crops usually outgrow feeding damage.

Most fields harbor populations of predators like ground beetles and wolf spiders that usually keep slug populations below economically damaging levels.

These natural enemies also feed on other crop pests, so it's a good sign if you see them in your fields.

Planting cover crops and eliminating unnecessary insecticides can protect predator populations.

Sometimes our crops can't out grow the slug damage and predators aren't enough.

So when do you need a rescue treatment?

If slug populations are steadily increasing above a one to two slug per trap average, and large portions of a field have dying seedlings or poor establishment like in this picture of soybeans, it is time to consider using a molluscicide bait.

If damage is abundant, but plants are still putting on new foliage, and you feel they are likely to survive, baits may not be needed.

If you choose to use a molluscicide or slug bait always read and follow the label.

To summarize, we discussed how to make and use refuge traps to scout for slugs.

How to identify slug damage in field crops.

And how these tools can be applied to determine if a rescue treatment is necessary.

For more information on slugs, contact your local extension office, or visit our website at extension.psu.edu.