Management of symptoms and viruses spread by the pea aphid

Pea aphid is a serious pest of peas and beans. As well as causing direct damage to combining peas, particularly once flowering commences, this aphid species can transmit over 30 plant viruses, including the Pea seed-borne mosaic virus (PSbMV), Pea enation mosaic virus (PEMV) and Bean leafroll virus (BLRV).

• Combining peas are at highest risk from direct damage once flowering has started. Control after the development of the fourth pod-bearing node will not normally increase yield
• Overwintering crops, such as clover and lucerne, in neighbouring fields can increase the risk of aphids migrating into the crop in May
• Crops are most at risk from PEMV if it is transmitted before flowering occurs
• PSbMV is most likely to become established via infected seed. The virus is transmitted by aphids in early spring

Scientific name: Acyrthosiphon pisum

The pea aphid is large, 2.5–5.0 mm long, pale green or pink with red eyes, with a pear-shaped body, long antennae, and long legs.

Oct–Jan: Eggs overwinter on forage crops, such as lucerne, trefoils and clover. Active stages may overwinter in mild years.

Feb–Apr: Eggs hatch and wingless generations reproduce on overwintering plants.

May: Winged forms migrate to pea and legume crops.

Jun–Aug: Aphids feed and reproduce rapidly on peas and legumes, with highest numbers usually seen between late June and early July.

Sep: Winged forms migrate to overwintering sites.

Pea aphid can cause flower abortion and misshapen pods, which fail to fill, and reduce yields. Additionally, the honeydew produced by the pest can cause contamination issues, leading to the growth of saprophytic fungi, and may increase the cost of cleaning vining machinery.

The pea aphid also transmits viruses, including Pea seed-borne mosaic virus (PSbMV), Pea enation mosaic virus (PEMV) and Bean leafroll virus (BLRV). PSbMV affects quality in vining peas and the maintenance of disease-free seed stocks. In severe cases, PEMV can cause large yield reductions.

PSbMV causes stunting, shortening and downward rolling of leaflets, vein clearing and apical malformation (rosetting). Flowers and pods may be distorted. A white blistering may also develop over the seed coats as the peas mature, giving them a ‘tennis-ball’ marking.

PEMV causes vein clearing and the formation of translucent spots, which are apparent when infected leaves are held up to the light. Development of stipules (the leaf-like structures at the base of leaves) is often retarded, while leaflets become crinkled and may contain necrotic spots. Plant tops often become yellow and mottled, with distorted leaves. Pods may be severely malformed and fail to fill. At an advanced stage of infection, scaly leaf-like structures (enations) may appear. These are small, irregular, protruding ridges of plant tissue found on pods and the underside of leaves. Further symptoms of an advanced infection are the cessation of terminal growth, the disappearance of axillary buds and impairment of flower set.

BLRV causes leaf yellowing, upward leaf rolling and a decrease in the number of pods in field beans.

Non-chemical control

Avoid growing peas or beans in fields with nearby concentrations of clover or lucerne.

Predators, such as ladybirds and hoverfly larvae, may help control pest populations, but the presence of hoverfly larvae can contaminate vining peas. Other natural enemies include spiders, fungal pathogens and parasitoids.

Ensure seed stock is free of PSbMV to minimise virus transmission risk.

Monitoring

Generally, during May to July, consider control if there is a light or general pea aphid distribution and humid weather, or if breeding colonies are evident.

Thresholds

• Combining peas: 20% or more of plants infested at early flowering
• Vining peas: 15% or more of plants infested
• Field beans: none established

Virus transmission can occur even with low aphid numbers. If crops are in high-risk virus areas, control aphids as soon as colonies are present, particularly if this occurs before flowering.

There is little benefit of treating combining peas infested after the development of the fourth pod-bearing node.

Insecticide resistance

None known.