Bryan Jensen
UW Extension
After looking at a few alfalfa fields on the Arlington Agricultural Research Station and talking with PJ Liesch, UW Entomology, it appears that some aphid species have been doing quite well this spring.
Pea aphid populations in both the established stands and new seedings were elevated but not to the point where rescue treatments were needed. However, as pea aphid populations start to increase they become very conspicuous and can get people’s attention.
Pea aphids are an occasional insect pest on alfalfa and their threshold is high compared to potato leafhoppers. Although it is possible to have economic damage on established stands it is unusual. Many factors keep populations at sub-economic levels including intense cutting schedules, environmental conditions, pathogens, predators and parasitoids. Aphid populations can build to higher levels in new seedings but that is also rare. Do scout and monitor aphid populations just don’t get too excited because you see lots of insects. Unnecessary insecticide applications can, and have led to aphid resurgence. You kill most of the natural enemies but not all the aphids. Without this natural control aphid populations increase rapidly.
Pea aphids can be either green or rose colored, winged or unwinged and large or small depending whether they are nymphs or adults. You will also find dead individuals that are light tan and attached to leaves. Either this can be the result of a fungal pathogen or aphids kill by parasitoids.
The economic threshold is listed as a field average of 100 pea aphids/sweep. Under good growing conditions I doubt you would see an economic benefit to spraying, especially in established stands. But if the alfalfa is under stress then a foliar application may be needed. Unless you are close to cutting. Harvesting alfalfa is an excellent cultural control method and few aphids will survive. Essentially, winged pea aphids will have to recolonize the field.
So how might this weather relate to Soybean Aphids? While this type of weather can be conducive, it is much too early to tell for sure. Proper and timely scouting will give you the information needed.
