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Integrated Pest & Crop Management Newsletter
University of Missouri Vol. 16, No. 7 Article 4 of 6 April 28, 2006
The pea aphid, Acyrthosiphon pisum (Harris), is the most common of several aphid species found in Missouri alfalfa. Pea aphids are light green in color, about 1/8th inch in length, with long legs, a pair of antennae with dark bands located at the tip of each antennal segment, two cornicles or "tailpipes" projecting from the rear of the abdomen, and green to pink eyes. This aphid possesses piercing-sucking mouthparts used to remove sap from stems and terminal leaflets of alfalfa. Heavy infestations of pea aphid on alfalfa may result in wilted plants, deformed leaflets, and a shiny coating of sugary aphid waste called "honeydew". This waste material often attracts other insects such as ants, bees, or flies, which use the honeydew as a food source. The honeydew may support the growth of molds resulting in the plants taking on a dark color, whereas cast skins produced when aphids grow in size may give plants a white coloration. Weather and beneficial insects are major factors affecting pea aphid populations in this state. Cool springs with moderate to high relative humidity often favor pea aphid population growth, whereas, beneficial insects effectively reduce pea aphid numbers in most years. Common natural enemies of pea aphid include several species of ladybird beetles, lacewings, damsel bugs, syrphid flies, bigeyed bugs, and species of parasitic wasps. Management of pea aphid may be needed on seedling alfalfa when 5 or more aphids are present per stem and alfalfa is less than 3-inches in height. In established alfalfa the economic threshold for pea aphid is 50 or more per stem on alfalfa less than 10-inches in height. In alfalfa 10-inches or greater in height the threshold increases to 75 or more aphids per stem. If alfalfa is under drought stress, it is appropriate to reduce these thresholds as the alfalfa is less able to withstand aphid infestations. Early harvest of infested alfalfa (7-10 days prior to 1/10th bloom) is an effective management techniques for this aphid. In more western states, pea aphid outbreaks often occur following insecticide applications for alfalfa weevil, which reduce beneficial insect numbers and allow for a surge in pea aphid populations. A common question asked by alfalfa producers this past week was whether it is necessary to manage pea aphid infestations at this time in the season. To determine whether to treat with an insecticide, early harvest, or to do nothing really depends on the unique condition in each alfalfa field. In several fields I scouted this past week, pea aphids were present but in numbers approaching or below the economic threshold. Beneficial insect numbers were relatively high in number, alfalfa weevil larvae were low in number due to the presence of the fungal pathogen, and most alfalfa stems were 10-inches or greater in height. Given these factors, an insecticide application for control of pea aphid would not be justified. An exception might be if the alfalfa was under severe drought stress or exhibiting damage symptoms associated with aphid feeding. Early harvest of alfalfa may be an appropriate management strategy for some fields, but early harvest reduces beneficial insect populations along with pea aphid numbers. Each producer must assess their field conditions and determine an appropriate management strategy for this occasional pest of alfalfa. Other aphids commonly collected from Missouri alfalfa fields include the cowpea aphid, spotted alfalfa aphid, and to a lesser extent, the blue alfalfa aphid.
Wayne Bailey
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