Utah State University Extension
Alfalfa Stem Nematode (ASN) is a nearly microscopic round worm that enters the alfalfa plant and lives in the stems and leaves, usually above ground. Under ideal conditions (wet weather in late winter or early spring, at 59-70°F), ASN can complete its life cycle from egg to reproducing adult in 19-23 days.
Evans, Kent; Israelsen, Clark; and Pace, Mike, "Alfalfa Stem Nematode" (2008). All Current Publications. Paper 842.