Two aphid species typically attack alfalfa in Georgia. The pea aphid is light green and usually occurs on the first crop of the season. The spotted alfalfa aphid is smaller than the pea aphid and is pale yellow to white with several rows of dark spots along its back. Damage symptoms appear as a yellowing of the foliage between the leaf veins. Some alfalfa varieties have resistance to these aphids. Alfalfa is fairly tolerant of aphid injury, and treatment thresholds are listed in the table. If alfalfa is within one week of cutting, harvest early and treat stubble as needed. Natural enemies such as lady beetle larvae and adults and various parasites usually keep aphid populations in check. Lady beetles are not active during the winter, so large aphid populations may build up in late winter on non-dormant alfalfa before lady beetles become active in spring. If spotted alfalfa aphids appear in seedling stands, treat when they can be easily found in the field, because this aphid is capable of severely stunting or destroying alfalfa seedlings. Dimethoate and cyhalothrin products are recommended for aphid control. Zeta-cypermethrin and Baythroid are not effective against alfalfa aphids.

Several other insects, such as leaf-feeding caterpillars, cutworms and grasshoppers, defoliated alfalfa and occasionally reach damaging levels. Control measures are recommended when defoliation exceeds 10%.

Blister beetles feed on foliage but rarely cause significant defoliation. Instead, they contain a very toxic chemical in their body that contaminates hay. This compound, cantharidin, can be very toxic to livestock – especially horses. The stripped blister beetle is the species usually associated with alfalfa hay poisonings. For adult horses, several dozen blister beetles are needed to cause acute poisoning. Generally, the first cutting is at lowest risk of infestation, because beetle populations peak in late summer. But the risk of contamination is present for every cutting. Blister beetles can aggregate in very large number often along the field margin.  Modified harvesting methods, such as not driving over previously cut hay, can reduce the risk of blister beetles contamination. But with current technology it is not possible to completely eliminate the risk of contamination by blister beetles.

For more detailed information on blister beetle management in alfalfa hay, see UGA Extension Circular 917 (http://pubs.caes.uga.edu/caespubs/pubs/PDF/C917.pdf).

Finally, proper crop management can insure high alfalfa yields and quality. Following recommended agronomic practices (fertilization liming, weed control, seeding, etc.) to achieve and maintain vigorous, healthy stands allows alfalfa to tolerate and/or outgrow insect damage more efficiently than plants not receiving proper care.

Recommendations listed in this article apply to Georgia, and may not applicable to other states. Alfalfa producers should contact their local extension service for specific recommendations for their area.