Be on the lookout for cowpea aphids

Central Tablelands producers should be on the lookout for cowpea aphids (Aphis craccivora) this spring. These are a common and widespread pest, especially of lucerne crops. However, they can also be found on legume based pastures and other legume crops like field peas and lupins. Given that cowpea aphids are active in spring (and again in autumn), it is worthwhile monitoring pulse crops, lucerne and pasture paddocks for this pest.

Cowpea aphids are soft-bodied bugs. Mostly, they are in the nymph stage at the moment and appear as dull grey, sometimes seen with a light dusting of wax. The adult cowpea aphids are shiny black and have white and black legs. Once winged, it becomes harder to distinguish different types of aphids.

For further information on distinguishing between aphids, Cesar has a fantastic website to help with identification.

Cowpea aphids can cause plant damage directly by feeding on the plants, causing wilting of leaves and potential death. Indirect damage can come from the secretion of honeydew. This can inhibit photosynthesis and reduce plant growth by encouraging secondary fungal growth. Plants can suffer further indirect damage via the aphids spreading viruses. These viruses can include cucumber mosaic virus (CMV), bean yellow mosaic virus (BYMV), alfalfa mosaic virus (AMV) and pea seed-borne mosaic virus (PsbMV).

Photosensitisation has been diagnosed in sheep that have grazed lucerne crops infested with cowpea aphids, but it is not clear whether the aphids are the cause of the disease especially as lucerne  alone can cause photosensitisation. It is probably best to err on the side of caution and closely monitor sheep grazing infested lucerne. Seek veterinary care if you have concerns about photosensitisation in sheep.

Cowpea aphids can be found in the nymph phase at the moment. However, as the weather warms, they can mature from the nymph stage to the reproductive stage very quickly. Once daytime temperature reaches 30–35 degrees, aphid activity often declines before we see another resurgence of the pest again in the autumn.

As always, an integrated pest management plan should be considered when deciding how to manage this pest.

Firstly, producers should understand the benchmark or economic threshold for the particular crop or pasture. That is, assessing the numbers of cowpea aphids and other species present. Are there also beneficial insects present? Are there any ‘mummies’? What is the degree of plant damage? Can biological factors compete against the pest?

Secondly, are there any available physical or mechanical control measures? This may include reducing other hosts around the crop, reconsidering variety selection, sowing time (and hence flowering time) and sowing rate (a dense crop canopy can deter aphid landings).

Lastly, there are a number of chemical options, these include, for example seed treatments and selective insecticides. The decision for chemical options should always consider the amount and presence of ‘good’ or beneficial insects.

For further information on cowpea aphids, contact Clare Edwards or Phil Cranney. For information on animal health issues speak to your vet on diagnosis and livestock management.