Aphids in Alfalfa: Some Clarification

April 21, 2000
For issue no. 2 (April 7, 2000) of the Bulletin, I wrote an article about pea aphids in alfalfa. This species occurs commonly in alfalfa fields every year, but it rarely causes economic damage. Occasionally people find other species of aphids in alfalfa, and some worry about the blue alfalfa aphid, a species that occasionally causes economic damage in western states. This article discusses three species of aphids that have been or might be found in alfalfa fields in Illinois.

In the aforementioned article, I indicated that the blue alfalfa aphid, Acyrthosiphon kondoi, was not known to occur in Illinois. I was wrong. David Voegtlin, Associate Professional Scientist in the Center for Biodiversity at the Illinois Natural History Survey, informed me that blue alfalfa aphids have been captured in traps in past years in Illinois. However, no one has ever reported any economic damage caused by this pest in our state.

The blue alfalfa aphid can be confused easily with the pea aphid, Acyrthosiphon pisum. Following are the distinguishing characteristics of the two species. Pea aphids are about 1/8 inch long with light green bodies, long antennae with dark bands, and dark eyes. They have piercing-sucking mouthparts, long legs, a pointed "tail," and two long cornicles ("tailpipes" projecting from the rear of the abdomen). The blue alfalfa aphid is slightly smaller (1/10 inch) and lacks the characteristic dark bands on the antennal segments.

Pea aphid (left) and Aphis craccivora (right). (Digital photograph submitted by Robert Bellm, Edwardsville, Illinois.)

Several people also have found another species of aphid that is prevalent in some fields. Based on a digital photograph submitted by Robert Bellm, Extension Educator in Crop Systems in Edwardsville, David Voegtlin has identified the species as Aphis craccivora, commonly referred to as the cowpea aphid or black legume aphid. David indicates that A. craccivora is very common in southern Illinois in the spring on a wide variety of hosts. It is known to be polyphagous (feeds on many species of plants), with a marked preference for legumes.

Wayne Bailey, Extension entomologist at the University of Missouri and a specialist on alfalfa insects, wrote an article about aphids in alfalfa for the most recent issue of their newsletter, Integrated Pest and Crop Management. Following are a couple of paragraphs regarding the cowpea aphid:

"Of concern is the reemergence of the cowpea aphid in Missouri alfalfa fields. Although this aphid has been recorded in alfalfa from this state in the past, numbers have always been extremely low. Beginning last fall and continuing into this spring, numbers of the cowpea aphid have been moderate to high in central and northern Missouri alfalfa fields. This is of concern as the cowpea aphid has emerged over the past three years as a major pest of California alfalfa. In California, high numbers of cowpea aphids feed on alfalfa crowns during winter and early spring resulting in poor growth of the alfalfa after dormancy is broken. With heavy aphid infestations the alfalfa was unable to grow. The aphids also covered the plants with sticky honeydew which promoted the growth of sooty mold and made harvesting more difficult.

"There are no scouting methods or economic thresholds available for cowpea aphid populations. Producers are encouraged to report any aphid damage which occurs prior to first cutting. The cowpea aphid is easy to identify as it is dark black in color and has two highly visible black cornicles (look like tailpipes) located near its back end."

We also would like to receive any reports of damage caused by this aphid species, so let us know what you are finding.--Kevin Steffey

Author: Kevin Steffey