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Genus: Arion

Common name: Roundback slugs 
​Family: Arionidae

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Arion distinctus, brown slug
Origin: Europe
Diet: Live and dead plants. These are some of the most significant crop and garden pests.
Other species in Oregon: Arion intermedius, A. circumscriptus circumscriptus and A. circumscriptus silvaticus
Where in Oregon: Widespread.
Similar exotics to look for: There are at least 19 additional species of Arion in Europe. They are difficult to differentiate from species already present in Oregon. Dissection and inspection of internal organs is required to confirm identification. 
Lateral of Arion hortensis next to a quarter. Size range of Arion species ranges from 30 to 140 millimeters
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Arion distinctus, brown slug
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Arion hortensis, garden Arion
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Arion rufus, red or chocolate Arion, contracted
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Arion rufus, red or chocolate arion
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Arion subfuscus, dusky slug
Quick key to Arion species (lengths refer to mature specimens)
1. Sole yellow, less than 40mm long.
Yellow foot or sole of Arion hortensis
    A. less than 20mm long.....A. intermedius
    
B. more than 20mm long....A. distinctus and hortensis

2. Sole is not primarily yellow, length variable. 
    A. With dorsal stripes
          i. Clear mucus, no vertical stripes on foot fringe, less than 45mm....A. circumscriptus

          ii. Yellow mucus, vertical stripes on foot fringe, adults 50mm or greater....A. subfuscus
    
B. No dorsal stripes and large (60mm+)....A. rufus

Back to key
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