Idaho                              Aphid                                                        Flyer

PEA APHIDS
    Pea aphids have increased significantly in Moscow and Lewiston. According to Larry Smith, Nez Perce County extension agent, they have already begun spraying in the area. 

Description
    Adult pea aphids are soft-bodied, slow moving, and range in color from light to dark green. They are pear shaped, about 1/8 in. long and 1/16 in. wide, with long slim legs. The antennae of the pea aphid show narrow dark bands at the tip of each segment. The nymphs are smaller but closely resemble the adults.

Host Crops
    Field peas, alfalfa and clovers are the main hosts of the pea aphid. Other legumes, including vetches, sweet peas, trefoil and dry beans are also attacked.

Economic Thresholds
    Sampling to determine aphid density should be done when 50-70% of the pea plants are in flower. The threshold in Century peas is 2-3 aphids per 8 in. of plant tip at flowering. Trapper peas can tolerate higher levels. Plants infested before the flowers open recover without loss of yield. The threshold in alfalfa is 10 aphids per stem at 2 weeks before hay cutting.

Scouting Techniques
    The key to aphid management is early detection through careful periodic scouting.  A 10 or 20X hand lens is needed to identify aphids when scouting in the field.  The fields should be sampled weekly fro aphids, from the time of plant emergence to harvest.  Populations may increase 2-3 x on a biweekly basis.  Remember that you have to look over the entire field.  If the aphids fly in, they may land anywhere in the field. If they migrated in from an adjacent field, chances are that they will be found on the edge of the new field.  Counts should be at least 150 ft. apart and observations should be made well into the center of the field.  Carefully examine 20 stems in each of 5 areas of the field, examining at least 100 stems per field.  Look for symptoms of damage, including leaf discoloration in host plants.  Severe infestations may appear as large bronzy or brown patches in the field, often easily visible from the road.  Later in the season, these patches may appear as thin stands.  In younger plants    , aphids may cluster near the base of the plant, on the upper sides of the leaves.  In older plants, check inside leaf whorls for the aphids.  Some agronomists routinely take stem samples from aphid counts.  Cut the stem close to the ground and ten hit it sharply against a stiff piece of white paper or into a white pan.  This dislodges the aphids so they can be counted.

  
                                                                     Funded by the University of Idaho Agricultural Experiment Stations and the Wheat & Barley Commissions.

Samples collected June 30, 2000