YELLOW SPOTS IN NEW STANDS 
SPOTTED ALFALFA APHIDS? 

Yellow spots in seedling alfalfa during the fall may be caused by infestations of spotted alfalfa aphids. Careful identification and scouting should be conducted to determine the levels of infestation of the various aphids. Newly established stands are highly susceptible to even low numbers of spotted and blue alfalfa aphids (1 or 2 per seedling).   

When present in alfalfa during fall and early winter, aphids are likely spotted alfalfa aphids, which are generally yellow in color with dark spots. When aphids are present in late winter or spring and are a green or blue-green color, they are probably pea aphids or blue alfalfa aphids.   

The spotted alfalfa aphid is the most devastating aphid pest of seedling alfalfa. It causes a severe toxic reaction in susceptible varieties that often results in discoloration (yellowing) along veins of leaves near plant terminals. This characteristic symptom is called "veinbanding." Heavily infested plants turn yellow and often some leaves have reddish discoloration. Eventually, these plants may take on a bleached appearance and affected spots across a field may die. New stands of susceptible varieties are particularly vulnerable to stand loss because of the size of plants and lack of genetic resistance.   

During winter and spring, following establishment of an alfalfa stand in fall, other alfalfa aphids may be present that can also account for thinning of stands or death of plants. The blue alfalfa aphid can be devastating because, like spotted alfalfa aphids, it causes a toxic reaction that can kill plants. Generally, blue alfalfa aphids do not cause yellowing. Plants remain green until just before death. High numbers of pea aphids cause wilting similar to drought. Spotted alfalfa aphids cause yellowing. Differences in reaction of alfalfa to feeding by aphids stresses the importance of identification and timely control applications.