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Biology and Ecology of Slugs in Mid-Atlantic Crop Fields

Slugs covering a hay mower that was cutting in early May 2012 in Franklin Co., PA (photo by Kent Martin)
Slugs covering a hay mower that was cutting in early May 2012 in Franklin Co., PA (photo by Kent Martin)

Description:
Of the 20 or so slug species that occur in Mid-Atlantic states, four are common in field crops (see the Gallery link at left for images):  the gray garden slug (Deroceras reticulatum), the marsh slug (Deroceras laeve), the dusky slug (Arion subfuscus), and the banded slug (Arion fasciatus). While the relative importance of each species is not well understood, the gray garden slug often occurs in the largest numbers and is most often associated with crop damage. This is a medium-sized, light to dark gray slug that produces sticky, white mucus when disturbed. All of these slug species were introduced from Europe with the exception of the marsh slug, which is thought to have mixed native and introduced populations. 
 
Slugs are close relatives of snails - essentially snails without a shell. They are legless, soft-bodied creatures with four front tentacles and a covering of slimy mucus all over the body. They secrete this mucus wherever they go, leaving a characteristic “slime trail” that can be a valuable clue of their presence. Different species vary in color and pattern, but all are various earth tones such as gray, brown, or orange. Again varying by species and age, slugs can range in size from a fraction of an inch to several inches.  Juvenile slugs resemble adults but are smaller. Slug eggs are small, gelatinous spheres or ovals found under residue or in the soil. The eggs are often found in clumps but may also occur singly.

Life cycle:
All slugs are hermaphrodites, meaning individuals have both male and female reproductive organs, but they usually mate with one another to reproduce. Mating, egg-laying, hatching, and development are not well synchronized even within a single species, so slugs of various stages of development can be found at many times of year. This makes slug activity difficult to predict reliably. Slugs can overwinter in all stages, except in extremely cold winters when adults and juveniles may be killed, but thick snow packs can insulate slugs against the cold. Timing of the life cycle varies by species and is somewhat unclear in the Mid-Atlantic region.

The gray garden slug appears to have a more synchronized life cycle than some other species. In Mid-Atlantic states, large numbers of eggs of this species hatch in mid-spring and are often associated with crop damage. Juveniles produced from spring egg hatch grow through the spring and summer, mature in the late summer or early fall, mate, and lay eggs in the fall. These are the eggs that overwinter and hatch the following spring. Given mild winter conditions, some adults and juveniles can also overwinter. A single gray garden slug can lay several hundred eggs in its lifetime. Slugs die soon after laying eggs.