Biology and Life Cycle of the Gray Field Slug

Deroceras reticulatum Müller, the gray field slug (or sometimes called gray garden slug), has many different adult color morphs and damages multiple crops in the Pacific Northwest (PNW) and worldwide. In the Willamette Valley, D. reticulatum comprises more than 90% of the slugs found in grasses grown for seed. All but one of the slug species (Prophysaon andersoni)  found in grasses grown for seed are invasive or exotic. This species is an important agricultural pest in the family Agriolimacidae. Note the position of the pneumostome, the short keel at the back of the body, the mucous which is typically colorless but turns white when the slug is disturbed. Deroceras reticulatum is the only Deroceras species in the US that can change its mucous from colorless to milkey. As far as habitat choice, the gray field slug chooses cultivated areas such as agricultural field crops, backyard gardens, roadside, parks, and meadows. This slug is much less common in natural habitats like forests. It is native to Europe, North Africa and the Atlantic Islands.

ADULTS: Slugs are hermaphrodites—every slug is born with both male and female reproductive parts and any slug is capable of laying eggs, though self-fertilization can occur. In the temperate climate of the PNW, mating typically is observed in the fall (Oct-Nov) and continues in the spring (Mar-Jun). When a slug matures, which can take approximately 5-6 months over the winter, it weighs more than 200 mg (up to 500mg) and now has the capacity to produce eggs. When in motion, it is about 35 to 50 mm (> 1.5 inches long). Adult slugs  overwinter and can lay clutches of eggs when environmental conditions are right. A slug’s life expectancy is from 6 to 12 months, and some up to 18 months. Two generations of the gray field slug are possible in PNW, although information on biology is limited.

EGGS: Small, round, pearl-like, translucent (younger) eggs are laid in clusters of a dozen or more (over 500 eggs in a lifetime; average 40 eggs/cluster) in sheltered cavities near the soil surface or under residue on the soil surface, if the soil is moist. As eggs mature they turn white and can take from 2 weeks to a month to hatch, depending on the environmental conditions. It can take 5 months to hatch if eggs are laid late winter. Mature adults deposit eggs late in the season, often after mid-October. If that is the case, the eggs will overwinter, and may not hatch until the following spring. The greatest egg- laying activity in non-irrigated environments usually occurs soon after the first fall rains before temperature declines.

NEONATES: A newly-hatched slug is called a neonate, and their typical food of choice is algae and fungus. However, they can feed on vegetative parts of plants. Young neonates weigh between 1-10 mg. They don’t travel far from home.

JUVENILES: Juvenile slugs will begin feeding throughout the spring and sometimes in the summer, if moisture is present and it is not too hot. If conditions are unsuitable, juveniles and adults will rest (aestivate) under clods and debris, in burrows and soil cracks. Aestivation is a physiological response of slugs under challenging times of environmental adversity, like dryness, summer heat, scarcity of food. They are known to survive without food for several months. Juveniles weigh between 11-100 mg.