Bryan Jensen
UW Extension
I have been getting some calls and emails regarding slug damage in corn and soybean. With the way the weather has been going it was only a question of when, not if.
Slugs are herbivorous and will feed on a variety of broadleaf and grass plants. Both soybean and corn are attractive hosts. Slugs are be also found feeding in small grains and alfalfa, however, their damage rarely is of economic importance.
Slugs have a “rasp-like” mouthpart and damage seedling plants by scraping off leaf tissue. These feeding scares are usually longitudinal (especially in corn) and may 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 characteristic that finding slugs to confirm their damage (vs. other insects) may not be needed.
Initiate scouting for slugs in fields with a history of slug feeding or in fields with significant residue. Economic thresholds based on % defoliation or slug numbers have not been developed.
Water conservation is extremely important for slug survival. Conditions that lead to greater slug survival include cool/wet weather, excessive crop residue and high weed pressure. Prevention is important and controlling residue is probably the most effective cultural control practice. However, many growers are committed to reduce or no-till systems and are unable to change the amount of residue
Other cultural practices may, or may not provide adequate levels of prevention. Use of row cleaners/trash whippers when planting may help under low to moderate slug populations. Also, early planting may give corn and soybeans a head start and allow them to outgrow damage. Planting corn instead of soybean into suspected slug hotspots may also be an option because corn’s growing point is below ground for a period of time. However, make sure the seed furrow is completely closed during planting. Open seed furrows allow slugs to feed on the emerging plant and consume the growing point. Open seed furrows also serves as another protected site to hide during daytime hours.
Chemical control using baits in corn can a rescue option. Baits using the active ingredient methadehyde can be an effective treatment but they should not be considered a “silver bullet”. Bait formulations have gotten better in recent years. Both in terms of higher % active ingredient and their persistence. A ballpark cost estimate is $1.90- $2.00/lb of product. However, that price is dependent on several factors including availability, incentives, amount purchased, etc. Your costs will vary. Per acre costs will of course vary depending on use rates. Accurate distribution of product is very important. Read and follow label directions for assistance with timing, application type(s), rates and product distribution.
I have heard anecdotal evidence of people using liquid fertilizers in an attempt to control slugs. However, results have not repeatable. Furthermore, I would have concerns over crop injury at rates necessary to get adequate coverage to control slugs.