The most economically important species of slug (> 90%) in the Pacific Northwest that is most associated with damaging field crops is the gray field slug (or commonly called the gray garden slug), Deroceras reticulatum, Müller. This slug species can be found in cultivated areas such as agricultural field crops, backyard gardens, roadside, parks, and meadows. This slug is much less common in natural habitats (e.g., forests). It is native to Europe, North Africa and the Atlantic Islands. The European red slug, Arion rufus, and over the last 10 years, the brown-banded Arion, Arion circumscriptus, the black greenhouse slug, Milax gagates, the large spotted garden slug, Limax maximus, the marsh slug, Deroceras laeve, and the native reticulated slug, Prophysaon andersoni, have also become important pests.
There are differences in activity patterns (seasonal, as well as responses to weather changes) and environmental/ecological factors (temperature, moisture, food preferences) between slug species. It has been noted that some Arion species are less active on soil surface, feed more on humus, decaying leaf mold, and vegetation, and prefer temperatures greater than 40 °F, compared to D. reticulatum, that can be active at lower temperatures in the winter, and is a surface feeder.
Taxonomy of slugs has traditionally been based on thier external and internal morphology. In many cases, adult slugs need to be dissected and genitalia examined to confirm species. There are over 40,000 species of Gastropods.
Oregon Department of Agriculture poster can be found here.
Color Variants of Arion ater by John Tayler
https://archive.org/stream/cu31924003132044#page/n254/mode/1up
Our most economically important species is the “gray field slug” or gray garden slug, Deroceras reticulatum.
In recent years the brown-banded arion, Arion circumscriptus, has been seen.
The large spotted garden slug, Agriolimax species.
The marsh slug, Deroceras laeve.
The gray field slug, D. reticulatum, in a winter clover field.
Arion rufus, is another European species belonging to the round back family.
It can roll itself into a ball to protect itself.
The European black or red slug, Arion subfuscus.
The black greenhouse slug, Milax gagates.
The body color of Arion circumscriptus may be influenced by their diet.
The reticulated slug, Prophysaon andersoni.
Arion circumscriptus (family Arionidae), is a round back slug. It can be gray or brown colored.
Arion circumscriptus, the brown-banded slug or white-soled slug.
It is becoming more common in grass seed fields in westerm Oregon.