Pea aphids are present in alfalfa during the cooler parts of the year, spring and fall. Females produce live young that mature in 10 to 14 days. These aphids do best when temperatures are in the mid 60's with humidity in the range of 80%. There is time for several generations in the spring and occasionally large populations can develop in the spring.
Populations in the range of 50 aphids or more per stem would be needed to justify control. An insecticide application to control pea aphids is seldom justified. Treat only if plants lose their dark green color and are wilting. Products such as Ambush, Baythroid, Cythion, Dimethoate, Furadan, Lannate, Lorsban, Mustang Max, Pounce or Warrior 1 CS may be used.
To protect honey bees, do not make insecticide applications when the crop or weeds are blooming.