Identification of potato aphid and transmitted viruses

Potato aphid is usually the most common aphid pest found on potatoes and can transmit several viruses, including Potato leafroll virus (PLRV) and Potato virus Y (PVY). Although potato aphid may infest vegetable brassicas and lettuce, it is generally a contaminant, rather than a source of severe damage. This species can also transmit Beet yellows virus (BYV) and Beet mild yellowing virus (BMYV) to sugar beet.

• Damage in potatoes is greatest when aphid numbers peak after mid-July
• Risk of virus transmission in sugar beet is greater in crops that have fewer than 12 true leaves; older crops become unpalatable

Scientific name: Macrosiphum euphorbiae

Wingless adults are 2.5–4 mm long, with a pear–shaped green–to–pinkish-red body, red eyes and a dark stripe running down the back. They have long legs and antennae at least as long as the body.

Winged adults have a yellowish-brown head and green thorax.

Nov–Apr: Wingless adults and immature stages overwinter on weeds, potato sprouts, rose and protected lettuce.

May–Jun: Winged forms migrate to summer crops.

May–Oct: Wingless forms feed and reproduce on crops.

Jul: If heavy infestations occur, a further migration is common.

Sep: A small autumn migration may occur.

With heavy infestations on potato, aphid feeding can result in ‘false top roll’ symptoms (upper leaves roll). Unlike PLRV, these symptoms occur in distinct patches earlier in the summer, with aphids or their cast skins present.

Yield reduction is mainly associated with the transmission of viruses, such as Potato virus Y (PVY) and Potato leafroll virus (PLRV), but also sap-feeding, under heavy infestations.

PVY infections result in leaf drop streak, whereby the lower leaves develop black streaks on their underside veins and eventually collapse – hanging off the stem by a thread. Younger leaves may develop necrotic spotting.

Aphid-vectored PLRV infections result in the margins of young leaflets rolling upwards and inwards, particularly at the leaflet base. Infected leaves may develop a purple discolouration. Symptoms of secondary (tuberborne) infections of these viruses may differ from the primary infections described above.

Sugar beet does not usually suffer from direct feeding damage, but potato aphid can transmit Beet mild yellowing virus (BMYV) and Beet yellows virus (BYV), although less efficiently than peach–potato aphid.

In sugar beet, BYV symptoms are characterised by diffuse chlorotic patches on mature leaves, which expand and coalesce. Leaves may be subject to infection by secondary pathogens, such as Alternaria. Leaves also become thickened and brittle, snapping crisply when broken.

Non-chemical control

Natural enemies include parasitic wasps, ladybirds, predatory flies, spiders, ground beetles, rove beetles, lacewings, and insect-pathogenic fungi. Providing habitats that encourage the presence of these natural enemies may help control aphid numbers.

Natural enemies may not prevent virus transmission, as this can occur even at low aphid densities. Early sowing of sugar beet can make it less vulnerable because older leaves are less palatable to the aphids.

Monitoring

Thresholds

Potatoes: none established. Treat if aphid numbers start to increase rapidly on varieties known to be susceptible to ‘false top roll’.

Insecticide resistance

None known.