Bryan Jensen, Dept. of Entomology and Integrated Pest Management Program, University of Wisconsin-Madison
You don’t need to be clairvoyant to be concerned about slugs in corn and soybean. It has been raining, continues to rain and frankly I don’t want to look at the forecast. The majority of the Wisconsin corn/soybean growing area has had significant rainfall and cool temperatures. All which can contribute to higher than normal slug populations.
Slugs have a “rasp-like” mouthpart and damage seedling plants by scraping off leaf tissue. Soybeans are more susceptible than corn because the growing point is above ground in the seedling stage. Slug feeding scars are usually longitudinal (especially in corn) and initially leave the wax-like cuticle intact. This symptom is often call “window paneing”. Eventually the cuticle will weather and drop off. Slugs may be difficult to find because they are nocturnal. However, they may be active on cool, cloudy days. During daylight hours, they hide under soil clods and plant debris. Slug injury is often so diagnostic that finding slugs to confirm their damage (vs. other insects) may not be needed.
Slugs are vulnerable to desiccation and prefer habitats which help protect them. Which includes high crop residue and/or weed growth. These are fields where I would concentrate my scouting efforts.
Unfortunately, many of the effective slug management practices are behind us which would have included practices like making sure the seed furrow was closed, residue management and planting date (the sooner the better but difficult to accomplish this year!) to name a few.
What can be done short of hoping for dry weather? Effective weed management is certainly at the top of the list and likely a goal regardless if you have slugs or not.
Slug baits can be effective but given tight profit margins they might not be an option for entire fields. Economic thresholds have not been developed for slugs. Before baits are considered, thoroughly read the label including all applicable footnotes! The metaldehyde-based bait labels excludes use on soybean in Wisconsin. This is not obvious because a you must read a footnote which indicates approved states. The Sodium Ferric EDTA containing product (Iron Fist) comes with precautions as well. Although labeled for corn and soybean production in Wisconsin it must be applied between the rows at the seedling and later stages of crop development. Furthermore, it’s availability is restricted in some Midwestern states
Sometimes knowing what will not work can be beneficial. Slugs are mollusks and insecticides are non-toxic to them. Even if contact is make or the slug consumes treated foliage. Attempting to try insecticides as a last resort is likely to increase slug damage because those products kill non-target beneficial insects like ground beetles.
Anecdotal information suggests that high salt fertilizer applied at night may work. However, this control tactic is not based on sound science, is not always effective and these fertilizers maybe phytotoxic to plants.
A final suggestion would be to make detailed field histories whiles scouting this spring. Use this information to make preventive management decisions next year.