|
Oklahoma State University Stillwater, OK |
|
Oklahoma Alfalfa |
|
|
|
Oklahoma State University Stillwater, OK |
|
Oklahoma Alfalfa |
|
|
FIELD SCOUTING OF BLUE ALFALFA APHID AND PEA APHID |
|
| Most damage due to the blue aphid and the pea aphid occurs in March and early April. Infestations of blue aphids usually begin with arrival in February and early March of winged adults that have migrated northward from overwintering areas. These blue aphids become mixed with pea aphids that hatch from overwintering eggs. Although both species may be present in relatively large numbers, the greatest threat of damage exists with the virulent biotype of blue aphid called BAOK90, which has predominated in Oklahoma since 1991-92. This biotype may cause severe losses in yield and stand during growth of the first crop of alfalfa. It is critical that stands be sampled carefully to detect aphid infestations during late February and early March, even if there is little green foliage. Application of insecticide should be considered if numbers exceed 10/plant in new stands seeded the previous fall, and 30/crown in established stands, particularly if the infestation is comprised primarily of blue aphids. As growth of the first alfalfa crop begins, sampling may be conducted by estimating numbers of aphids in 30 stem samples collected when using the "shake-bucket" scouting method for alfalfa weevil larvae. Shake the stem sample over the bucket and brush the foliage to dislodge aphids along with the weevil larvae. After estimating the numbers of aphids in the bucket, divide by 30 to calculate the number per stem. If plants are over 6-8" in height, populations may also be estimated by use of a net to take samples of 20 sweeps. It is important to identify the species present and to remember that infestations made up mostly of blue aphids have the greatest potential for causing damage. The economic threshold for the blue aphid in alfalfa <10" tall is 10 per stem (100/sweep); while that for pea aphids is 40 per stem (300/sweep). Blue aphids are somewhat smaller than pea aphids and have darker green or blue-green coloration. A magnifying glass is needed to more accurately separate these species by examining the antennae. Antennae of the pea aphid are light in color with a dark ring at the end of each of four segments. The antennae of the blue aphid do not have dark rings, but gradually become black in color near the tips. Additional information on sampling and insecticides registered for aphid control can be found in OSU Extension Facts #7150 and #7184. |
|
| Richard Berberet,
Alfalfa Entomologist Department of Entomology and Plant Pathology Oklahoma State University  |
 |
