Pea Aphid

(View photos to identify aphids)

The pea aphid is a large green aphid with long legs, antennae, cornicles, and cauda. It is very similar in appearance to the blue alfalfa aphid, but they can be distinguished by examining the antennae. The antennae of the pea aphid have narrow dark bands on each segment, whereas those of the blue alfalfa aphid gradually darken to brown as you near the tip of the antennae.

A pink biotype of the pea aphid has been found in California. Except for its pink color, it is identical in appearance to the green biotype. The pink biotype causes similar damage to the green pea aphid and management practices are the same, but some studies have suggested it may be partially resistant to parasitization by Aphidius ervi and may also circumvent some of the pea aphid resistance bred into many alfalfa cultivars.

Both the blue alfalfa aphid and the two strains of the pea aphid prefer cooler temperatures (optimal temperature for development of both blue alfalfa and pea aphid is around 60°F) and reach damaging levels in spring. However blue alfalfa aphid is more tolerant than pea aphid of cool temperatures and appears earlier in the late winter and early spring. Pea aphid populations often reoccur in fall. In the spring, pea aphids are often present in alfalfa fields at the same time as the alfalfa weevils. Blue alfalfa aphid colonizes the plant terminals while pea aphid is usually more generally distributed. Both species prefer to feed on the stems rather than the leaves.

As aphids feed on alfalfa, they inject a toxin into the plant. This toxin can have damaging effects on plant growth. Of the four aphid species that infest alfalfa (pea aphid, blue alfalfa aphid, cowpea aphid and spotted alfalfa aphid), the pea aphid has the least damaging toxin to alfalfa. While the number of pea aphids feeding on the plants can cause damage by removing plant sap (photosynthates), the pea aphid toxin itself is not particularly damaging to alfalfa plants.

A black fungus, sooty mold, also grows on the honeydew excreted by the aphid. This reduces palatability to livestock. Damage is more severe on short alfalfa than taller alfalfa

Using resistant varieties of alfalfa and encouraging populations of natural enemies are important in managing pea aphid. It is important to distinguish between the pea and blue alfalfa aphids because blue alfalfa aphid causes more damage than pea aphid, and the two species have different treatment thresholds. Natural enemies, especially lady beetles, are monitored along with the aphids to determine the need for a pesticide application. Aphids frequently become problems when weevil sprays kill their natural enemies. Border harvesting or strip cutting can be important for preserving natural enemies.

Planting alfalfa varieties resistant to pea aphid has been the most effective means of controlling these aphids in alfalfa. Studies in the eastern U.S. have shown that the pink biotype of the pea aphid has overcome resistance in a number of cultivars with the exception of CUF 101. When selecting varieties, consult your farm advisor for information on resistant varieties suited to your area, or check a list of alfalfa varieties provided by the National Alfalfa & Forage Alliance. Additionally, a yearly alfalfa variety report can be found at http://alfalfa.ucdavis.edu.

(View photos of natural enemies)

The most significant aphid predators are several species of lady beetles, including Hippodamia convergens and Coccinella septempunctata that attack and consume both pea and blue alfalfa aphids; treatment thresholds for pea aphid are based on the number of lady beetle adults and larvae present. Green lacewings can also be important in regulating aphids and many other predators including bigeyed bugs (Geocoris spp.), damsel bugs (Nabis spp.), and syrphid fly larvae also play a role.

The major parasites of the pea aphid are Aphidius smithi and A. ervi. However, several studies have suggested that the pink biotype of pea aphid shows signs of partial resistance to A. ervi. Large golden-brown aphid mummies on the upper surfaces of leaves indicate parasitization. If parasites are present in high numbers, consider this when making your pesticide application decision since parasites frequently provide adequate control.

A naturally occurring fungal disease, which is most prevalent during cool, rainy, or foggy weather, may also control aphids.

Use border-strip cutting during harvest to help maintain populations of parasites and predators within the field. For more details, see BORDER-STRIP HARVESTING.

The use of resistant varieties, biological control, and cultural control are acceptable to use on an organically certified crop. Organically certified insecticides such as azadirachtin (Neemix), neem oil (Trilogy), and pyrethrin (PyGanic) are registered for use on alfalfa to control aphids. Studies conducted in California, however, have shown that at best they provide some suppression of aphids but do not control them.

Start to monitor fields in February for pea aphid and continue monitoring through spring. In fall, resume monitoring for pea and blue alfalfa aphids by combining with cowpea monitoring as described in APHID MONITORING.

If natural enemies fail to keep the aphids in check, an insecticide application may be necessary. Economic treatment thresholds for both pea and blue alfalfa aphids are as follows (if both species are present, use the blue alfalfa aphid treatment levels):