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. Aphid populations in seedling alfalfa are best sampled by pulling stems and estimating numbers per stem. Sampling should be conducted at least once per week after emergence of new seedlings or during the growth of the first crop in established stands.