Aphids

The two most common aphids on potatoes are the green peach aphid and the potato aphid. Both species occur statewide. The green peach aphid is usually the most common and abundant species; infestations typically begin on the bottom most leaves of the plant. Potato aphid infestations are generally scattered over the plant.

Winged adults of the green peach aphid are pale or bright green and black, with a large, dusky blotch on the dorsum of the abdomen. The immature forms are yellow, pinkish, or pale green. The mature, wingless forms are pale or bright green.

Pink and green forms of the potato aphid are found in potatoes. This aphid is larger than the green peach aphid with longer cornicles and legs. Potato aphid colonies are made up of adults with offspring closely clustered together.

The two species can be most reliably distinguished by looking at the tubercles between the base of the antennae. The tubercles of the potato aphid slope outward and those of the green peach aphid converge.

Aphids damage potatoes primarily by spreading plant diseases. Occasionally, aphids become so abundant that their feeding weakens the plants. Potato leafroll virus is spread by both aphids, but green peach aphid is by far the more effective vector. Early season leafroll infection stunts the plant. Plants grown from infected seed potatoes will not produce marketable potatoes. An infected Russet Burbank potato often has phloem net necrosis, a brown discoloration inside the potato that reduces quality. The brown discoloration is most intense at the stem end but may extend well into the tuber. White Rose and red-skinned varieties do not develop net necrosis. Other viruses spread by aphids include cucumber mosaic and alfalfa mosaic (calico).

Management of green peach aphid and potato aphid involves an integrated program of reducing overwintering populations, controlling weeds in and around the field, and the use of foliar sprays. Monitor to schedule spray treatments.

Many parasites and predators attack aphids. Among the more common predators are lady beetles and their larvae, lacewing larvae, and syrphid fly larvae. Populations of green peach aphids are reduced in winter by a parasitic fungus, Entomophthora aphidis. Most materials available for aphid control are highly disruptive of natural enemy populations.

Weeds along ditch banks, roads, in farm yards, and other noncultivated areas contribute directly to the aphid problem. Malva is an important overwintering host in the Central Valley and southern potato production areas. In northern areas, tumble mustard (Sisymbrium altissimum), penny cress (Thlaspi arvense), and other mustards (Brassica spp.) serve as early season host plants where aphid populations increase before spreading to other host plants, including commercial potatoes.

It is also important to control nightshades and volunteer potatoes because these plants are reservoirs for potato leafroll virus. Rogue infected potato plants to reduce the incidence of infection and spread of the disease within a field. For maximum effectiveness remove the diseased plant, the three plants on each side of the diseased plant in the same row, and the three closest plants in adjacent rows. Rogueing is most important in seed fields.

Plant disease-free seed to reduce the incidence of potato leafroll virus.

Use biological and cultural controls on organically certified potatoes.

In Northern California, migration of winged green peach aphids from weed hosts usually begins in early June. Inspect fields weekly. Aphids are first found on those plants along the edge of the field toward the prevailing wind, usually the north or west edge of the field. If aphids are found on the edge, sample 100 leaves, taking 50 bottom leaves on a line from one corner of the field to the center of the field and another 50 bottom leaves on a line to the other corner of the field (example: 50 leaves from northwest corner to center; 50 leaves from center to northeast corner). Record your results (example form—PDF).

In other areas of California, growers should make general observations to determine if aphids are present. Sample weekly throughout the growing season and record your results (example form—PDF). Heavy populations normally occur late in the spring.

In northern areas, apply foliar insecticides when 5% of the leaves are infested. There are no established thresholds in other parts of California because aphids rarely cause economic damage to cultivars grown for commercial production in these areas.

In seed potato production, a preventive program using insecticide applications at 2-to-3 week intervals may be necessary.

• Potato aphid monitoring, Northern California (PDF)
• Surveying foliage pests form (PDF)