In this issue:

ECB and CEW Moth Flight Update

Cabbage Looper Alert

Soybean Aphid Update

Cowpea Aphids Arrive, in Alfalfa

Business Planning Guide Available

Swedish Machine for Fresh-Market Growers

Cowpea Aphids on Alfalfa in Wisconsin and Minnesota

Bill Hutchison, University of Minnesota, and Eileen Cullen, University of Wisconsin-Madison

We have observed “black” aphids, identified as cowpea aphids, on alfalfa in Minnesota this year for the first time. Cowpea aphids (common in California), are also present in Wisconsin, and has spread throughout much of Iowa since 2002. Mark Abrahamson, Minnesota Department of Agriculture, has provided a map (left) showing the infestation of cowpea aphid throughout Minnesota during 2003. The following article, from Wisconsin, is reprinted here for a good overview of the pest status of this species.

The black aphid several Wisconsin producers and consultants are currently seeing on alfalfa is the cowpea aphid, Aphis craccivora. Several reports and questions have come in about this black aphid on alfalfa. The cowpea aphid is easily distinguished from other aphids in alfalfa because it is the only black aphid found infesting the crop. This is a relatively small aphid, less than 2mm long. Both winged and wingless adults are shiny black, while the smaller nymphs tend to have a slate gray appearance. Using a hand lens, look at the antennae and legs. The first half of the antennae, nearest the head, will be white leading into a black tip near the end of each antenna. The legs are also multi-colored, usually a creamy white color with blackish tips.

Two excellent color photo images for identification purposes can be found on the internet: Phil Sloderbeck’s photo from the Kansas State University Entomology Dept. at: http://www.oznet.ksu.edu/entomology/extension/Photos/Alfalfa%20Insects%20Folder/cpa.jpg

The UC IPM website at: http://www.ipm.ucdavis.edu/PMG/A/I-HO-ACRA-AD.004.html

Cowpea aphid has been present in California alfalfa for many years. It is usually present in low numbers, and has rarely reached population levels that cause damage. In the winter of 1999 alfalfa grown in the high and low desert regions of California did sustain serious injury and stunting due to cowpea aphid. Large populations of cowpea aphid have been reported throughout Arizona, several counties in Texas, and sporadically throughout Kansas and Oklahoma since 2001. Closer to home, Iowa producers began seeing the cowpea aphid in alfalfa during the summer of 2002.

In alfalfa, cowpea aphids feed on young terminal growth, forming colonies that look very obvious with their dark color congregating on the plants. You’ll often find white shed skins in the midst of colonies as the insects molt. Because cowpea aphid has only recently and sporadically become associated with alfalfa, no guidelines or economic thresholds have been established for this situation. Normally, producers do not need to be too concerned about cowpea aphids in alfalfa, since populations rarely reach damaging levels and predators feed heavily on them.

Keep the following factors in mind when tracking cowpea aphid in alfalfa:

1--If alfalfa is not growing properly and cowpea aphids are the causative factor (stunted, yellowing and sticky with sooty mold), consider taking control measures.

2--Observations from Oklahoma State University note that cowpea aphids damage alfalfa and feed on the plant similar to the pea aphid. Sweep net samples can be used to assess aphid populations by walking an M-shaped pattern in the field and taking 20-consecutive sweeps in each of five randomly selected areas. Avoid taking sweep samples within 75 feet of the field edge, concentrating instead on interior portions. Total the number of aphids found in these five sets of 20 sweeps and divide by 100, the total number of sweeps for each field. Pea aphid populations exceeding 100/sweep are at threshold levels, particularly if plants show signs of wilting. Although pea aphid thresholds have not been locally verified against cowpea aphid populations, this is a general starting point.

3--Cowpea aphid may be present in combination with other alfalfa pests such as potato leaf hopper and pea aphid at this time of year. While cowpea aphid in alfalfa is an occasional pest, not usually expected to cause significant damage, it can be of greater concern in fields reaching threshold levels for other pests simultaneously.

Note: For more information on this species, from Iowa State, see Dr. Marlin Rice’s recent newsletter article, at: http://www.ipm.iastate.edu/ipm/icm/2003/7-14-2003/cowpea.html

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Suzanne Wold, Research Fellow, Department of Entomology, University of Minnesota, woldx018@tc.umn.edu

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Last Revised January 23, 2004.
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