UNL CropWatch Aug. 10, 2010: Alfalfa Aphid Update: Identification, Thresholds, and Treatment

UNL CropWatch Aug. 10, 2010: Alfalfa Aphid Update: Identification, Thresholds, and Treatment

August 10, 2010

We are getting reports of two types of aphids in Nebraska alfalfa fields, the spotted alfalfa aphid and the cowpea aphid. The spotted alfalfa aphid has been reported from Butler County and the cowpea aphid has been reported from Red Willow and Lincoln counties. Both species are occasionally damaging to alfalfa at this time of year and warrant observation now.

A diverse group of beneficial insects commonly found in alfalfa can limit aphid populations, but won’t eliminate the potential for damage. Beneficials include lady beetles, nabids, minute pirate bugs, and parasitic wasps.

Cowpea Aphid, Aphis craccivora

Adult: Dark grey to shiny black, about 1/12 inch long. Antennae and legs are whitish with blackish tips.
Host Range: Primarily legumes including alfalfa and cowpea, but also lambsquarters, dock (Rumex spp.), and smartweeds (Polygonum spp.).
Thresholds: 5 per stem in seedling alfalfa; 40 per stem when alfalfa is less than 10 inches tall; and 75 per stem when alfalfa is more than 10 inches tall.

Spotted Alfalfa Aphid, Therioaphis maculata

Adult: Light tan to pale yellow, about 1/16 to â…› inch long with 4-6 rows of dark spots along the back of the body, about 1/16 to â…› inch long. Adult females may be either winged or wingless. Damage on alfalfa results in yellowing of the leaf veins.
Host Range: Primarily alfalfa, but also may feed on clovers.
Thresholds: 1-3 aphids per seedling, 50 aphids per stem when alfalfa is less than 10 inches tall, and 100 aphids per stem when alfalfa is more than 10 inches tall.

Treatment

See the Department of Entomology website at http://entomology.unl.edu/instabls/aphids.shtml for a list of insecticides for control of aphids in alfalfa.

See http://www.ipm.ucdavis.edu/PMG/C001/m001epaphid.html for photos of common alfalfa aphids.
 

Bob Wright
Extension Entomologist

 

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