Issue No. 5, Article 3/April 28, 2006
Heavy Infestations of Cowpea and Pea Aphids in Alfalfa Continue
Impressive densities of cowpea aphids (Aphis craccivora) and pea aphids (Acyrthosiphon pisum) are being reported in some areas of Illinois. Robert Bellm, crop systems educator, Edwardsville Extension Center, observed a heavily infested alfalfa field in which the plants were "covered in sooty mold and are wilting." Although cowpea aphids have infested alfalfa stands in California for years, they have been reported less frequently in midwestern fields.
Wayne Bailey, extension entomologist, University of Missouri, indicated that cowpea aphids have been observed in some areas of Missouri since the early 1990s. Marlin Rice, extension entomologist, Iowa State University, reported in the Integrated Crop Management Newsletter (July 14, 2003) on an infestation of cowpea aphids in Iowa: "Last July, Joel DeJong, ISU field specialist-crops, and I visited an alfalfa field near LeMars (western Iowa) that had large populations of blackish aphids. This year, Brian Lang, ISU field specialist-crops, reports finding large populations of this dark aphid near Decorah (northeastern Iowa). I suspect that these aphids are cowpea aphids, Aphis craccivora, based on their color. Within 1 year, this aphid species has moved across Iowa. It has recently become a serious pest of alfalfa in California. Nothing is known about this insect's behavior and population dynamics in Iowa, other than it occurs on alfalfa, it seems to stunt alfalfa plants, and it moved quickly from counties near the Missouri River to counties near the Mississippi River in northeastern Iowa."
Cowpea aphids can easily be distinguished from other aphids infesting alfalfa due to their black coloration. Adults are shiny black, and the nymphs are slate gray. The aphids have a large host range and may be found feeding on many types of legumes and weeds, such as shepherd's purse, lambsquarters, and pepperweed. The cowpea aphid is able to inject a toxin into plants, which may result in stunting of plants (especially when aphid densities are high). If alfalfa stands are heavily infested with aphids, sooty mold may begin to grow on the excreted honeydew. The accumulation of honeydew may result in reduced photosynthetic efficiency and hamper the harvest operation.
Cowpea aphids (courtesy of Kevin Black, Growmark Company).
According to information from the University of California, host plant resistance is not a management option due to the lack of resistant varieties (as of December 2004) for this insect pest, nor have economic thresholds been developed. However, consider the following input that I gleaned from Marlin Rice's newsletter article (July 14, 2003): "However, Blake Sanden, a farm advisor with the University of California system, states, 'The current best bet is to use the blue alfalfa aphid thresholds. Reduced yields will result if aphid numbers are 10-12 per stem on new regrowth just after cutting, or more than 60 per stem when the hay is 12 inches or taller.'" Natural enemies are likely to be very important in reducing aphid numbers; they include lady beetles, lacewings, damsel bugs, and syrphid fly larvae. Border harvesting or strip cutting may serve a very useful purpose in the establishment of natural enemy reservoirs.
Syrphid fly maggot predation (courtesy of Kevin Black, Growmark Company).
Very heavy pea aphid infestations also are being observed currently. Pea aphids are pea green and approximately 1/6 inch in length. These aphids may spend the winter as adults or in the egg stage. In mild winters, adult survival is enhanced and may lead to earlier-season injury. A complete life cycle requires about 12 days (temperature dependent), and many generations (7 to 20) occur over the course of a season. Like all aphids, the adults and nymphs suck plant juices from many portions of alfalfa plants, including the leaves, petioles, stems, and flower buds. Pea aphids commonly feed on new growth, and heavy infestations in the spring can damage the first cutting. If temperatures remain cool (50°F to 60°F) and humidities low, aphid densities may explode. The suggested economic threshold for pea aphids is as follows (University of Illinois Extension Field Crop Scouting Manual): "If pea aphids average more than 100 per sweep, and beneficial insects are not plentiful, economic damage may result." That's a lot of aphids! So make sure that you sweep several areas (at least five) within a field and give the natural enemies a chance.
Pea aphids--one sweep (courtesy of Kevin Black, Growmark Company).
Several insecticides can be considered for use in alfalfa to control aphids, including Baythroid 2 (suppression only), Lorsban 4E, Mustang Max, Proaxis, and Warrior. For additional discussion of this topic, please consult this Web site for an article by Wayne Bailey, extension entomologist, University of Missouri. --Mike Gray