Slugs have stripped the leaves from alfalfa seedlings under an oat-pea cover crop in Marathon County, WI, report Dan Undersander University of Wisconsin Extension forage agronomist, and Eileen Cullen, Extension entomologist.

They say this summer’s wet weather has prevented timely harvest of the cover crop. Saturated soil and the heavy crop over the seedlings have led to continued slug feeding, stripping alfalfa leaves to the stems.

Undersander and Cullen point out that slugs are gastropods, not insects. Insecticides aren’t labeled for slugs, and won’t control them.

There are a few species of slugs; most have one generation per year and overwinter in the egg stage. If winters are mild, adults can overwinter. Because field slugs can live 12-15 months and eggs are laid in early spring and fall, overlapping generations of adult and juvenile stages can be present in a field. Slug activity is at its peak in late spring and early summer, and again in early fall.

Adult slugs typically enter a period of inactivity during this time of year, when conditions are hot and dry. But this summer’s wet weather and the heavy Marathon County cover crop have provided ideal slug habitat.

“Our recommendation at this point is to remove the cover crop as soon as soil conditions permit,” Undersander and Cullen write. “This will alter the slug habitat in affected fields, exposing slugs to sun and heat. Then watch the alfalfa stand for regrowth to determine whether affected areas of the stand are permanently affected/killed or if regrowth is occurring. You should see new shoots beginning to grow within five to seven days. Lack of visible shoots will mean that something is still feeding on them or that the plants are dead or too weak to put out new shoots. In the latter case, the plants will die. Slugs, which are nocturnal, are active in the evening and early mornings.”

Commercially formulated metaldehyde slug baits can be applied. Treatments are expensive, typically in the range of $15-20/acre. One trade name is Deadline M-Ps, and most other products have “metaldehyde bait” in the trade name. Product information can be found in Crop Data Management System (CDMS) pesticide label database at

If applying baits, follow label instructions. It’s important that application takes place when slugs are still present and active, typically during periods of cooler temperatures (63-68° F) and wet conditions favorable to above-ground slug activity. For this reason, slug baits are often applied aerially.

For more information, refer to the Wisconsin Crop Manager Newsletter article, Slugs in Corn and Soybean.