Blue Alfalfa Aphid

(View photos to identify aphids)

The blue alfalfa aphid is a large blue-green aphid with long legs, antennae, cornicles, and cauda. It is similar in appearance to the pea aphid but 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.

Both the blue alfalfa aphid and the pea aphid prefer cooler temperatures (optimal temperature for development of both blue alfalfa and pea aphid is around 60°F); however, the blue alfalfa aphid is more tolerant than the pea aphid of cool temperatures and appears earlier in growing season. The biology and population levels of the blue alfalfa aphid has changed in recent years. Damaging numbers are most common in the mid- to late winter in the desert production areas, in the late winter to early spring in the Central Valley, and the spring to early summer in the intermountain areas. However, blue alfalfa aphids may also be found in the fall in many areas.

The blue alfalfa aphid and the pea aphid may occur in mixed populations. Historically, both species were present in alfalfa fields at the same time as the alfalfa weevils. However, now both blue alfalfa and pea aphids can be present in the fall, winter, and spring. The 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.

While feeding on alfalfa, the blue alfalfa injects a toxin that retards growth, reduces yield, and may even kill plants. The toxin injected by the blue alfalfa aphid is more potent than that of the pea aphid (pea aphid toxin itself is not particularly damaging to alfalfa plants). Toxin that remains in the stems and crown after harvest of the upper plant material may continue to retard stem growth and elongation and may carry over to the next cutting, or even the subsequent two cuttings.

Damage can also reduce the alfalfa's feeding value. A black fungus, sooty mold, grows on the honeydew excreted by the aphid and reduces palatability to livestock. Damage is more severe on short growth alfalfa than on taller alfalfa for both species.

Using resistant varieties of alfalfa and encouraging populations of natural enemies are very important in managing blue alfalfa aphid. It is critical to distinguish between the blue alfalfa and pea 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 may become a more severe problem when weevil sprays reduce the numbers of natural enemies. Border harvesting or strip cutting can be a useful tool for preserving natural enemies.

Planting alfalfa varieties resistant to blue alfalfa aphid has been the most effective means of controlling aphids in alfalfa. Prolonged periods of below-normal temperatures, however, may lower plant resistance to blue alfalfa aphid when it is most needed and some crop injury may occur. At around 70°F, the host plant resistance should be fully expressed.

When selecting varieties, consult your cooperative extension 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 performance 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 the blue alfalfa and pea aphid species. 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.

Aphidius smithi and A. ervi are parasites of alfalfa aphids. Large golden-brown aphid mummies on the upper surfaces of leaves indicate parasitization. When a high level of parasitization is present, carefully consider the need to apply insecticides for aphids. 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 slight suppression of aphids but do not control them.

Start to monitor fields as soon as the first aphids are observed. Use 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 blue alfalfa and pea aphids are as follows (if both species are present, use the blue alfalfa aphid treatment levels):