Oklahoma Alfalfa
Oklahoma Cooperative Extension Service
Oklahoma Agricultural Experiment Station

DON'T FORGET SPOTTED ALFALFA APHIDS

In Oklahoma, spotted alfalfa aphid populations can be present in alfalfa fields throughout the year. These insects are particularly devastating to seedling stands (infestations as low as 1-2 aphids per stem can cause plant death), and they may also cause delays in regrowth after harvest operations. See OSU Fact Sheet No. 7184 for detailed thresholds of aphid species in alfalfa.  

Old alfalfa plants can tolerate higher numbers of these aphids; however, freshly cut hay, trying to regrow, may not withstand similar infestations. Keep in mind, that these thresholds do not always take into account the additional stresses on the stand. Mild July-August weather and lack of rainfall in some areas may accentuate the problem.  

Spotted alfalfa aphids can be distinguished from other aphid species by the presence of several rows of black spots along their backs. These spots are not easily seen with the naked eye, therefore, some magnification may be required. The aphid also has a general yellowish color, unlike the other species which are green or bluish-green.  

Prolonged feeding by spotted alfalfa aphids causes leaves to curl, yellow, die and drop. Death of the leaves, and ultimately the plant, is due to the injection of a toxin by the aphid. Infestations generally start in localized spots across a field. These spots may first appear yellow in color but turn white eventually.  

An integrated program for alfalfa aphids involves using many facets of pest management. These include; planting resistant varieties, biological, cultural and chemical control practices. On established stands, that are suffering from drought, with no immediate end in sight, simply cutting the hay may not control the aphids completely. This is particularly true if aphids remain uncontrolled and dry conditions persist. Spot spraying of affected areas, however, may be an alternative to whole field treatments. Spot treatments versus whole field application of insecticides may allow for lower costs, preservation of natural enemies and time for a "possible" rain, before harvest.  

Pay careful attention to seedling stands planted next to established alfalfa that may be infested with these insects. Just a few aphids in newly established alfalfa can quickly devastate a stand.