The Pest of this season: Cowpea Aphid
What is Cowpea Aphid?
With scientific name Aphis craccivora, the Cowpea Aphid is readily distinguishable from other aphids inhabiting alfalfa because it is the only black aphid found infesting the crop. It is a relatively small aphid and the adult is usually shiny black while the nymph is slate gray. The appendages are usually whitish with blackish tips.
Cowpea Aphid numbers are highest from April to September; numbers peak from October to January in the desert.
This aphid has an extensive host range, including beans, cotton, and weeds.
Which damage can they do?
Cowpea Aphid injects a powerful toxin into the plant while feeding and, when their numbers are high, this can stunt or even kill plants. While feeding, this aphid produces a considerable amount of honeydew upon which sooty mold can grow. The black sooty mold reduces photosynthesis and may make leaves unpalatable to livestock. The honeydew also makes the alfalfa sticky, which causes problems with harvest.
How to manage this Pest?
There are no known varieties of alfalfa that are resistant to Cowpea Aphid and economic thresholds have not been developed specifically for this pest. Treatments may be necessary if large numbers of Cowpea Aphids are present. Border harvesting or strip cutting can be important for preserving natural enemies.
Two common aphid parasites, Lysiphlebus sp. and Diaeretiella sp., have been identified from the desert production areas. Although parasitism as high as 95% has been documented, aphid numbers can become so high that enough non-parasitized individuals remain to cause significant injury.
This aphid is also susceptible to the usual complement of aphid predators including lady beetles (convergent lady beetle, multicolored Asian lady beetle, twice stabbed lady beetle), lacewings, big eyed bugs, damsel bugs, and syrphid flies. Early in the season (February and early-March) many of these predators are generally not active, but in the low desert the seven spotted lady beetle, Coccinella septempunctata, is abundant and feeding on the aphid.
Use border-strip cutting during harvest to help maintain populations of parasites and predators within the field.
Organically Acceptable Methods
Use biological and cultural controls on organically certified crops. Organically certified insecticides such as Azadirachtin , Neem oil and Pyrethrin are also registered for use on alfalfa to control aphids.
Monitoring and Treatment Decisions
Cowpea aphid infestations are typically patchy in a field, especially early infestations. Stems on alfalfa plants in infested areas are often completely covered with aphids, whereas plants in other areas of the field may appear aphid-free. Because of the spotty distribution of cowpea aphid infestations, spot treatments may be feasible, especially if the infestation is on the field border.
No guidelines or economic threshold levels have been established for cowpea aphid in alfalfa. Use the following thresholds, which were developed for the blue alfalfa aphid:
Source: UC IPM Pest Management Guidelines: Alfalfa