AGRICULTURAL EXPERIMENT STATION • UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, LEXINGTON, KY, 40546 AGRICULTURAL EXPERIMENT STATION • UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, LEXINGTON, KY, 40546 AGRICULTURAL EXPERIMENT STATION • UNIVERSITY OF KENTUCKY COLLEGE OF AGRICULTURE, LEXINGTON, KY, 40546 SR-103 A Field Guide to the Slugs of Kentucky University of Kentucky • Lexington, Kentucky 40546 University of Kentucky • Lexington, Kentucky 40546 RESEARCH RESEARCH University of Kentucky • Lexington, Kentucky 40546 RESEARCH Cover photo: Deroceras reticulatum A Field Guide to the Slugs of Kentucky Anna K. Thomas,1 Rory J. Mc Donnell,2 Timothy D. Paine,2 and James D. Harwood1 Corresponding Author James D. Harwood Department of Entomology University of Kentucky Lexington, KY 40546-0091 E-mail: [email protected] 1 Department of Entomology, University of Kentucky, Lexington, KY 40546-0091, U.S.A. 2 Department of Entomology, University of California, Riverside, CA 92521, U.S.A. Contents Introduction ...................................................................................7 Identification Key to Family, Genus and Species ........11 Species Descriptions ...............................................................14 Glossary of Terms .....................................................................30 References ....................................................................................31 Acknowledgments ...................................................................34 Introduction Slug biology, ecology and pest status Slugs are soft bodied gastropods (Mol- ically, aestivation will occur in the soil; how- lusca) with rasping mouthparts and two ever, some slugs can aestivate while being pairs of tentacles (Figure 1). The animal’s attached to stationary objects such as rocks eyes are located on the uppermost, longer and stones. pair, called ocular tentacles; the lower pair, Terrestrial slugs usually lay their eggs in called peduncular tentacles, have a senso- holes or underneath debris, and the number ry function. A slug breathes through a small of eggs per batch, and the number of batch- hole called a pneumostome that is located es laid, varies between individuals and spe- on its mantle, the flap of tissue behind the cies.5 For example, according to Carrick,6 head.1 All species are capable of produc- Deroceras reticulatum can lay up to 500 eggs ing mucus that aids in moisture retention as per year: Davies7 states that Arion horten- well as movement, defense, and reproduc- sis lays from ten to thirty eggs per batch in tion.2 Slugs are hermaphroditic; when envi- captivity. Hatchling slugs appear very simi- ronmental conditions are adverse to sexual lar to adult slugs, with only size and differ- reproduction (e.g. when food is limited or ences in color pattern distinguishing them during periods of adverse weather such as from their mature counterparts. The time high temperature or low rainfall), a slug can necessary to reach sexual maturity and the self-fertilize and produce viable offspring.3 natural life span of slugs varies between spe- Slugs prefer cool, dark and moist habitats, cies with some, such as Deroceras reticula- and activity patterns tend to be highly vari- tum, having an annual life cycle, and others, able throughout the day but greatest imme- such as Limax maximus, having a biennial diately after sunset and during the hours or plurennial life cycle. proceeding dawn.4 When temperatures increase or conditions become too dry, slugs coat themselves with mucus to prevent des- 1 South, 1992 2 South, 1992; Gordon, 1994 iccation and undergo aestivation (a peri- 3 South, 1992 4 South, 1992 od of summer dormancy). During this time 5 South, 1992 they remain contracted and immobile. Typ- 6 Carrick, 1938 7 Davies, 1977 ocular Figure 1. External slug morphology. tentacle mantle head keel pneumostome tail genital sensory foot fringe pore tentacle 7 Slugs occur throughout temperate and In addition to direct yield losses resulting tropical regions and are present in a variety from feeding activities, slugs vector sever- of habitats including forests, grasslands, riv- al plant diseases. Wester et al.15 reported er edges and areas characterized by high lev- that slugs are capable of transporting downy els of anthropogenic disturbance (e.g. urban mildew to lima beans, and Hering16 report- gardens, plant nurseries, garden centers and ed that Deroceras reticulatum and Arion agroecosystems). Generally, invasive spe- hortensis spread the fungus Botrytis along cies are found in disturbed habitats such as grapevines, a trait also common among oth- urban and agricultural areas; native species er slug species. They also transmit brassi- dominate the malacofauna in more remote ca dark leaf spot,17 carrot licorice rot,18 and locations such as old growth forests.1 bacterial soft rot.19 Slugs can pose potential health risks to humans by vectoring certain Slugs are well-known agricultural pests pathogens. Sproston et al.20 demonstrated throughout the world and attack a wide the ability of Deroceras reticulatum to vec- range of agricultural and horticultural crops.2 tor Escherichia coli 0157. The slugs came For example, Deroceras reticulatum can be into contact with the pathogen on sheep responsible for up to one third of winter feces and then transferred it to vegetables wheat seed and seedling losses in the Unit- where the bacteria remained viable for sev- ed Kingdom and other temperate regions.3 eral days. This ability to vector disease and In North America, evidence for the agro- bacteria further highlights the potential nomic impact of slugs is less widely reported, adverse impacts of invasive slugs in the but invasive mollusks are responsible for sig- U.S. Finally, important commodities grown nificant losses to soybean,4 legumes,5 corn,6 throughout Kentucky have been reported alfalfa,7 tobacco,8 and strawberries.9 Such as food resources for many invasive species losses are likely to increase in significance as known to occur within the state. Therefore, production systems diversify and agricul- economic losses are likely to increase in tural practices increasingly adopt organic, future years because of changing agricultur- low-input and minimum tillage approaches. al production practices and more favorable In 1985, losses equivalent to $14 million to climatic conditions. potato crops and $5 million to winter wheat were recorded in Great Britain alone.10 Slugs 1 Kappes, 2006 are also widely regarded as important pests 2 Barker, 2002; Godan, 1983; South, 1992 of leeks, potatoes, onions, artichoke, brassi- 3 Port & Port, 1986; Glen, 1989; Brooks et al., 2005 4 Hammond et al., 1999 cas, carrots, celery, cucumbers, chicory and 5 Byers et al., 1985 beans.11 Moreover, slugs are also important 6 Hammond & Stinner, 1987 7 Barratt et al., 1989 pests in the horticultural and floricultural 8 Mistic & Morrison, 1979 9 Prystupa et al., 1987; Duval & Banville, 1989 industries with primulas, campanulas, sax- 10 Port & Port, 1986 ifrage and Michaelmas daisy often sustain- 11 South, 1992 12 12 Barnes & Weil, 1945 ing significant levels of aesthetic damage. 13 Eaton & Tomsett, 1976; Ministry of Agriculture, 1979 14 Ministry of Agriculture, 1979 Although they sometimes consume mature 15 Wester et al., 1964 foliage and flowers,13 damage typically occurs 16 Hering, 1969 14 17 Hasan and Vago, 1966 during the seedling stage. On flowering 18 Dawkins et al., 1985 19 Dawkins et al., 1986 plants and ornamentals, slugs will also often 20 Sproston et al., 2005 feed on bulbs, corms and tubers. 8 Slug collecting A range of sampling techniques can be uti- ly subterranean. Baited traps, consisting of lized to collect slugs. Although the vast cat/dog food or organic vegetables covered majority of species are nocturnal, speci- with black plastic sheeting, can also be used mens can easily be found during the day by to collect specimens.2 The food provides searching under sources of cover in suitable an attractant for slugs; the plastic sheet- habitats (e.g., flower pots in garden centers, ing provides suitable shelter and maintains decaying wood in forests and in sprinkler a high humidity. Since certain behavioral valve boxes in urban gardens). A number characteristics (e.g., response to a continu- of authors1 have successfully used soil cores ous disturbance) are important in identify- to sample slug communities. This method ing some species, specimens should be kept involves removing a turf-soil core (e.g. 10 alive in containers (e.g. 15 cm x 15 cm x 15 cm x 50 cm) and gently washing it in a fine- cm) lined with damp paper towel and fed meshed sieve with water. This technique on organic vegetables, oatmeal and/or pet is an important tool for collecting species food. For additional information on estab- such as Tandonia budapestensis and Tes- lishing and maintaining slug colonies, see tacella haliotidea, which are predominant- Sivik3 and Stephenson.4 Slug preservation and dissection Some slug species, such as Arion horten- and preserve in 100 percent ethanol.5 The sis, are difficult to identify solely on external remainder of the slug should be handled as morphological traits; dissection and exami- described above. nation of genitalia is necessary in some cas- To prepare a Petri dish for the dissection, es to make positive identifications. Before pour hot wax to a depth of approximate- specimens are preserved, make detailed ly 5 mm and allow to solidify. Place the slug notes on collection locale, date, slug size on the wax surface, straighten its body and and the color of both the general body and insert pins through the head close to the mucus. Such information can prove very tentacles, and near the tip of the tail. Add useful at a later date, as colors tend to fade water until the specimen is covered. Use a over time when a specimen is stored in alco- fine scalpel to make an incision just above hol. If possible, take a photograph of the the genital pore (Figure 1). Continue the dorsal, ventral and right sides of the slug. dissection beneath the entire mantle and Place specimens for preservation in a jar upwards to a point just posterior to the end that has been filled to the brim with boiled of the mantle.