A treatment is seldom, if ever, needed.
Spittlebugs overwinter as eggs laid on grasses in late summer. The nymphs are active for about a month in the spring. They stay within the frothy white masses that they produce on the plants. The adults will feed for a time on alfalfa before moving to other plants. There is one generation each year.
Spittlebugs use their sucking mouthparts to remove plant sap. Large numbers must be present to cause any stunting of plants.
If spittle masses and nymphs average more than 1 per stem and alfalfa is less than 6" tall, then an insecticide application of products such as Ambush, Baythroid, Cythion, Guthion, Lorsban, Methoxychlor, Mustang Max,Pounce, Sniper, or Warrior 1 CS may be considered. Control is likely to be poor due to the protection of the insect by the spittle mass.