Slugs

Description and Life History. There are many species of slugs capable of causing economic damage to greenhouse crops and field crops such as crucifers, strawberries, raspberries, potatoes, tomatoes, beans and lettuce. Most species survive the winter as eggs or adults. An adult may be a single sex or have both male and female reproductive organs (hermaphroditic) and be self-fertile. A single individual is capable of producing up to 500 eggs, which are deposited in batches of one to several dozen under stones, debris or in the soil. Eggs usually hatch in the spring or early summer. Slugs are nocturnal feeders, hiding during daylight hours.
During wet growing seasons, large numbers of slugs survive the summer months and may move into cultivated fields from weedy borders, drainage ditches or other moist, sheltered areas. Crop damage is usually most severe in late summer or early autumn of cool, wet growing seasons, especially when preceded by a mild winter. Sizable slug populations may build up in fields previously in no-till corn, alfalfa, potatoes and strawberries.

Damage and Scouting. Slugs feed by grating away the surface of the plant tissue with a tooth-covered radula, which works like a rasp. This type of feeding injury is easily distinguished from caterpillar feeding on thick-leafed cole crops like cabbage. The grating action produces a large wound on the leaf surface nearest the slug, which gradually tapers to a smaller hole through the opposite surface. Slug injury to cabbage appears ragged compared with the clean-sided incision typical of caterpillar feeding. On thin-leafed crucifers or other crops, insects produce leaf injury which is virtually indistinguishable from slug feeding. The presence of a glistening slime trail can sometimes be used to distinguish slug injury. Also, most insect pests can be found on the foliage during daylight hours while slugs tend to hide off the plant. Slugs attack the fruit of tomatoes and strawberries leaving small (1/2-inch long), shallow holes in the fruit's surface.

Scout for signs of slug feeding on crop plants near the weedy borders of fields.

Monitoring. A covered pit can be used to provide a humid, sheltered hiding place for slugs during daylight hours. The pit should be four inches in diameter and six inches deep. An aluminum foil-covered shingle or a board can be used as a cover to provide a cool refuge from the sun. Slugs tend to congregate in large numbers in these shelters and may be counted and destroyed during daylight hours. Set monitoring traps near field borders. The traps will not function as well in weedy fields or with crops like cabbage which provide adequate shelter for slugs beneath large-frame leaves close to the ground.

Action Threshold. The literature suggests applying control measures when one to five slugs per trap are found.

Cultural Control. Maintain good weed control within the field and along borders to eliminate daytime refuges of slugs.

Chemical Control. Baits are much more effective at controlling slugs than are foliar applied insecticides. Therefore, it is important to distinguish between caterpillar and slug injury. Apply baits in a clean cultivated strip along the margins of fields to intercept slugs entering fields from weedy borders. For crops that provide plenty of foliar cover near ground level (e.g., cabbage), broadcast the bait through the entire field if slug damage is a problem Weeds provide alternative food sources for slugs and will reduce the effectiveness of baits. Metaldehyde baits (e.g., Deadline Bullets) are currently registered for slug control on many crops. Metaldehyde use is prohibited after edible portions of the plant begin to form. Follow label application instructions carefully for best results.

Written by Jude Boucher, Vegetable Crops IPM Program Coordinator, University of Connecticut

The information in this material is for educational purposes. The recommendations contained are based on the best available knowledge at the time of printing. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension system does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.All agrochemicals/pesticides listed are registered for suggested uses in accordance with federal and Connecticut state laws and regulations as of the date of printing. If the information does not agree with current labeling, follow the label instructions. The label is the law.Warning! Agrochemicals/pesticides are dangerous. Read and follow all instructions and safety precautions on labels. Carefully handle and store agrochemicals/pesticides in originally labeled containers immediately in a safe manner and place. Contact the Connecticut Department of Environmental Protection for current regulations.The user of this information assumes all risks for personal injury or property damage.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kirklyn M. Kerr, Director, Cooperative Extension System, The University of Connecticut, Storrs. The Connecticut Cooperative Extension System offers its programs to persons regardless of race, color, national origin, sex, age or disability and is an equal opportunity employer.

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