The gray field slug, Deroceras reticulatum, is a mollusk found globally in gardens and various agricultural environments. This widespread species adapts to cultivated areas like fields, gardens, and roadsides. Understanding its characteristics and behaviors is relevant for managing cultivated land.
Identifying the Gray Field Slug
The gray field slug measures 35 to 50 mm in length. Coloration varies from white, cream, or pale brown to gray, sometimes with dark markings. A distinguishing feature is the network-like pattern (reticulation) of dark spots on its mantle, which covers about half its body. It also has a short keel at the back of its body. When disturbed, it produces milky white mucus, a unique trait among Deroceras species in the U.S. This milky secretion and the reticulated pattern help differentiate it from other common slugs.
Biology and Habitat
The gray field slug prefers moist, cool environments, found in cultivated areas like fields, gardens, and meadows, avoiding dense forests. During the day, it seeks shelter under debris, stones, and in soil cracks to avoid dry conditions. It is primarily nocturnal, feeding at night. Its diet includes fresh leaves, fruits, seedlings, and decaying organic matter like mushrooms, dead slugs, or earthworms.
Gray field slugs are hermaphroditic, possessing both male and female reproductive organs, and can self-fertilize or cross-fertilize. They lay clusters of small, pearl-like, translucent eggs in sheltered, moist locations near the soil surface. Optimal egg development occurs between 10-16°C. Eggs hatch in 3-5 weeks, though this can extend to 5 months in late winter. Adults can lay over 500 eggs.
Management and Impact
The gray field slug is a significant agricultural and garden pest worldwide, particularly impacting crops at the seedling stage. Damage appears as irregular holes in leaves, shredded leaf edges, or complete clipping of succulent plant parts. They also feed on seeds in open drill rows. High slug populations, especially in direct-drilled fields or wet conditions, can lead to substantial crop loss and necessitate replanting.
Management involves a combination of strategies.
Cultural Controls
Cultural controls modify the environment to be less favorable for slugs. This includes improving soil drainage, removing surface residues and weeds that provide shelter and food, and cultivating the soil, as tillage can reduce slug numbers. Creating a fine, firm seedbed and increasing sowing depth to 3-5 cm can protect seeds and seedlings from slug access.
Physical Barriers and Traps
Physical barriers, such as copper tape around pots or slug fences, can deter slugs by reacting with their slime to create an unpleasant sensation. Non-toxic traps, like squares of roofing shingles or heavy cardboard placed in the field and checked in the morning, can also be used to monitor and collect slugs.
Biological Controls
Biological controls involve encouraging natural predators like ground beetles, rove beetles, spiders, and certain parasitic nematodes, such as Phasmarhabditis hermaphrodita, which can infect and kill slugs.
Chemical Options
Chemical options, specifically molluscicide baits, are available. Ferric phosphate is a common active ingredient, as metaldehyde has been banned in some regions due to environmental concerns. These baits are most effective when applied after sowing and before crop emergence, particularly after irrigation when slugs are active.