Milky Slug

Deroceras reticulatum

Summary 2

Deroceras reticulatum, common names the "grey field slug" and "grey garden slug", is a species of small air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Agriolimacidae. This species is an important agricultural pest.

Distribution 3

occurs (regularly, as a native taxon) in multiple nations

Morphology 4

External: Slug very variable, creamy or light coffee cream, rarely blackish spotted (slugs with spots may appear blackish); behind the mantle the dark spots form a reticulate pattern in the skin grooves; skin thick with large tubercles; pneumostome 2/3 back from front of mantle; pale pneumostome edge; short keel; abrupt truncation of tail tip; 3-part, pale sole; mucus colourless, on irritation milky white (calcareous granules in white mucus).

Internal: Brown ovotestis; albumen gland light brown and liver-shaped; penis fleshy and with a silky sheen, in the shape of an irregular sac, in fully mature specimens divided into 2 parts by a deep lateral constriction; penial gland of very variable shape, usually consisting of a few branches or a single long branch (1-4 knobby processes); stimulator large, conical and narrow; retractor muscle inserted laterally on penis; vas deferens opens into penis wall facing the external body side; short, straight oviduct; narrow spermatheca; small atrium; rectal caecum large.

The slug cannot be distinguished from many other Deroceras species based only on its external appearance. Similar to D. agreste and D. rodnae (Kerney & Cameron 1979); different from D. laeve and D. carunae: opaque skin, white mucus when irritated, bigger size, rectal caecum presence (Quick 1960).

Eggs: 3 x 2.5 mm, translucent, begin with calcareous speckles on surface. Up to 700 eggs are produced per individual slug (Quick 1960).

Juveniles: 4 mm long, light gray and translucent at hatching (Quick 1960).

Size 5

Up to 40-60 mm long (preserved 25-30 mm); varies in size according to the habitat.

Habitat 6

Habitat Type: Terrestrial

Number of occurrences 7

Note: For many non-migratory species, occurrences are roughly equivalent to populations.

Estimated Number of Occurrences: > 300

Comments: Multiple species of Deroceras spp. were recorded in eastern Maine (11 of 101 sites) from litter samples in a variety of habitats (Nekola, 2008). In New York, Hotopp and Pearce (2007) report it from 7 counties in central and southeastern regions as introduced It was documented recently in southeastern Wisconsin (Jass, 2006). In California it occurs in 24 counties: Alameda, Butte, Calaveras, Contra Costa, Del Norte, Los Angeles, Marin, San Bernardino, San Francisco, Tulare, Santa Clara, Ventura, Humboldt, Imperial, Kern, Monterey, Sacramento, San Luis Obispo, Shasta, San Mateo, Santa Barbara, Santa Cruz, Sonoma, and Riverdale (McDonnell et al., 2009). It had been documented historically in Colorado (Cockerell, 1927) as Agriolimax agrestis. Forsyth (2006) documented introduced populations at Lac des Arcs, Alberta and museum records from Waterton Lake National Park (verified anatomically). Forsyth (2005) documented it in the Upper Fraser Basin of central British Columbia at an abandoned farm site, roadside highway pullouts, and a provincial park but is widely introduced in the rest of British Columbia; including one site (along a lake shoreline near Mackenzie) in the Peace River- northern Rockies region (Forsyth, 2005). Most recently, it was discovered in the Ktunaxa Traditional Territory in southeastern British Columbia (which extends from near Canada - U.S. border north to about 50 km north of Cranbrook) (Ovaska and Sopuck, 2009).

Ecology 8

D. reticulatum are attacked by parasites including Sciomyzid glies and nematodes. Some English populations of D. reticulatum have high rates of Sciomyzid fly infection (Stevenson & Knutson 1966, in Hunter 1978). D. reticulatum are susceptible to and experienced mortality from P. hermaphrodita (Grewal et al. 2003), which enter slugs' mantle cavities, reach adulthood, and reproduce (causing swelling of the mantle); the second generation tends to kill slug (Wilson & Grewal 2006).

In defense against predators, Deroceras reticulatum is capable of tail autotomization, followed by fleeing and hiding (Solem 1974; Pakarinen 1994a).

Behaviour 9

Not aggressive (Rollo & Wellington 1979); aggregations occur with bad moisture and temp levels (Rollo & Wellington 1981). Homes to shelter (Cook 1979).

Life cycle 10

Life cycle covers a few months, usually two generations; main reproductive phase in summer and autumn, maximum age about a year; slugs die at the first frosts. Usually only eggs hibernate, sometimes also juveniles.

In Britain, slugs mate year round; they can reach adulthood in 3 months during summer (Quick 1960). In Michigan, only (a few) adults emerged in early spring, immediately laid eggs after emergence and through the fall, and juveniles appeared in June; here, it is an annual species (Getz 1959).

Sources and Credits

  1. (c) Liam O'Brien, some rights reserved (CC BY-NC),
  2. (c) Wikipedia, some rights reserved (CC BY-SA),
  3. (c) NatureServe, some rights reserved (CC BY-NC),
  4. (c) (F. Welter Schultes); Paustian, Megan, some rights reserved (CC BY-NC),
  5. (c) (F. Welter Schultes), some rights reserved (CC BY-NC),
  6. (c) NatureServe, some rights reserved (CC BY-NC),
  7. (c) NatureServe, some rights reserved (CC BY-NC),
  8. (c) Paustian, Megan, some rights reserved (CC BY-NC),
  9. (c) Paustian, Megan, some rights reserved (CC BY-NC),
  10. (c) (F. Welter Schultes); Paustian, Megan, some rights reserved (CC BY-NC),

More Info

iNat Map

Establishment introduced
Slug or snail slug