Management of the black bean aphid in field beans and sugar beet

Damage caused by the black bean aphid mainly occurs from direct feeding, particularly if this occurs before flowering. Feeding damage causes retarded growth and deformed leaves. The black bean aphid can also transmit several damaging viruses.

• Spring crops, particularly late-sown crops, are at higher risk than winter crops, as flowering is more likely to coincide with aphid migration into the crop
• Aphid colonies can grow rapidly in the summer, especially if conditions are warm and humid
• There is a tendency for serious infestations to occur in alternate years, so severe attacks can be expected in a year following little or no infestation

Scientific name: Aphis fabae

Wingless adults are 1.5–3 mm long, black or olive green, often with distinct waxy stripes.

Winged adults are very dark, with faint black crossbars on the upper surface of the abdomen.

Oct–Jun: Eggs overwinter on spindle (Euonymus europaeus). In mild winters, active stages may overwinter on leguminous weeds or winter beans.

May–Jun: Winged forms migrate into a variety of summer crops.

Jun–Oct: Breeding continues throughout the summer, usually peaking in July/August. Colonies are often attended by ants and can become very large and dense, developing from the top of the plant downwards. In response to crowding, further winged forms are produced, which spread within the crop and to other crops.

Sep–Nov: Winged forms migrate back to spindle to lay eggs.

The black bean aphid can cause significant damage to faba bean crops, especially through direct feeding. Honeydew produced during feeding also encourages chocolate spot (caused by Botrytis spp.), which can reduce yields.

On spinach and sugar beet, populations can develop rapidly on the undersides of leaves, causing them to become chlorotic and crinkled.

In beans, direct feeding can damage flowers, retard or prevent pod development and cause plants to lose vigour and wilt in dry conditions. Direct feeding damage on sugar beet is rarely worth controlling.

The aphid can transmit viruses, such as Bean leafroll virus (BLRV), Pea enation mosaic virus (PEMV), Bean yellow mosaic virus (BYMV) and Beet yellows virus (BYV).

BLRV causes leaf yellowing, upward leaf-rolling and a decrease in pod numbers. Symptoms are often more obvious if aphids colonise before flowering.

PEMV in beans causes vein clearing and the formation of translucent spots on leaves. Upper leaves become pointed and crinkled and may contain necrotic spots. Symptoms of an advanced infection are the cessation of terminal growth, the disappearance of axillary buds and impairment of flower set.

Following BYMV infections, leaves become crinkled and occasionally pointed, the plant becomes stunted and vein clearing may occur. Symptoms can be mistaken for PEMV, but without the translucent spotting and streaking on the leaf surface.

Non-chemical control

Natural enemies include parasitic wasps, ladybirds, hoverflies, lacewings, and insect-pathogenic fungi. Although these may help control aphid numbers, they may not prevent virus transmission, as this can occur even at low aphid densities.

Monitoring

Inspect the crop regularly from early flowering until pod formation. Colonies tend to appear on the headlands first.

Thresholds

In beans, immediate treatment is justified when 10% of plants are infested. However, treatment when 5% of plants are infested can help prevent virus infection. In sugar beet, treatment is only justified on backward or stressed crops with more than 100 aphids per plant (averaged across the field and not just on headlands).

Insecticide resistance

None known.