IntroductionThere has recently been an increase in the number acres in which cereal rye has been broadcast into standing crops as an innovative method of cover crop establishment. Extensive slug feeding on the seed has been observed in several fields and stand losses of nearly 100% observed. Common field histories included; no-till, heavy soils, high amounts of crop residue and wet soils. However, extensive damage has also been noticed in fields that do not fit this description.

Slug Biology“Slug” is a common name given to a group of mollusks, typically referred to as “snails without a shell”. Slugs are soft-bodied, legless, slimy and may be light too dark colored depending on species present. The majority of slugs have two sets of tentacles or “feelers” on their heads. The larger pair senses light and the lower, smaller pair provides sense of smell. Immediately behind the head is the mantle, which covers sex organs, the anus and a respiratory organ. The foot is located underneath the slug and behind the mantle. Rhythmic muscle contractions of the foot allow slugs to move. There are approximately ten different species of slugs found in the state of Wisconsin. In 2016, gray field slugs and marsh slugs were commonly found in fields. Slug biology has not been well researched. However, we do know that slugs go through one generation per year and can probably overwinter as eggs, juveniles and adults. Life stages may not always be synchronized, allowing for several life stages to be present at the same time.

DamageSlugs have a “rasp-like” mouthpart called a radula and damage seeds by hollowing out the germ (growing tissue) and by scraping off leaf tissue on both seedlings and mature plants. Leaf feeding scars may be either round or longitudinal. Initially, the wax-like cuticle of the leaf may remain intact and this symptom is often call “window paining”. Eventually, the cuticle will weather and drop off, leaving long, narrow holes in the damaged leaves. Most slugs feed on a variety of seeds as well as broadleaf and grass plants. Feeding may be of economic importance under no-till conditions, high weed pressure or when field conditions are wet for prolonged periods of time. Slugs feed nocturnally and occasionally, on cool, cloudy days. During daylight hours, they hide in soil cracks and under soil clods and plant debris.

Scouting SuggestionsInitiate scouting in fields with a history of slug feeding. Slugs will hide under objects during the day; looking in these areas can be useful to determine their presence. Place flat objects (e.g. small boards) on the soil surface and check the undersides of these object during daylight hours for several days to confirm the presence/absence of slugs. Economic thresholds have not been established using this method of scouting. However, this information can be useful to confirm the presence or absence of slugs prior to seeding.

Slug Damage to Cereal Rye SeedTips for Wisconsin Landowners

nrcs.usda.gov/

ControlGeneral Comments: Before considering control options, management needs to be thought about in terms of a program or a “systems approach”. That is, short term, single tactic solutions are unlikely to be effective. Several management options should be considered to include, but not limited to, time of planting, planting method, crop rotation, residue management, use of insecticide seed treatment and foliar application of insecticides. Natural control: Slugs have several natural enemies including beetles, centipedes, spiders and several other invertebrate predators. Natural enemies, by themselves, may not always control high slug populations but are an important contributing factor.

Mechanical Control: The primary key to slug survival and population growth is habitat. Slugs require shelter from the sun during the day and do so by hiding under crop residue and in soil cracks. Reducing crop residue through primary or secondary tillage can be very effective. However, growers may be committed to reduce or no-till because of erosion, conservation plans and/or soil health benefits. Cultural Control: Broadcasting cereal rye seed into standing crops has important benefits that include early establishment and improved fall growth. However, if slug populations are known to be high, producers may find it advantageous to harvest the standing crop first before drilling cereal rye seed, as long as seeding can be accomplished by mid-October. Slug feeding on the emerged rye shoot has not been noticed, making drilling an acceptable alternative to broadcasting. Slug feeding was not observed on other broadleaf cover crop seeds or seedlings. Chemical Control: Slugs are mollusks, not insects. Therefore, insecticides will not control them. Judicious use of foliar and soil applied insecticides will preserve their natural enemies. In standing crops use foliar insecticides only when needed. Neonicotinoid insecticides are systemic seed treatments often applied to corn and soybean seed to control early season insect pests. Even if slugs feed on these seedling plants they are not affected, however, natural enemies that feed on slugs may be killed. In fields with known high densities of slugs, consider using seed that is not treated with a neonicotinoid seed treatment.

Wisconsin

USDA is an equal opportunity provider, employer, and lender. Resources • March 2017

University of Wisconsin–Extension

Information provided by NRCS Area Resource Conservationist, Brian Briski, in cooperation with Bryan Jensen, UW-Extension and Integrated Pest Management Program, Department of Entomology.