Why Are There Colonies of Blackfly on My Peas?

Dense black aphid colonies appearing on pea shoot tips, tender stems, and flower buds are caused by the black bean aphid (Aphis fabae) or the pea aphid (Acyrthosiphon pisum — which is typically pink-green rather than black). Both species cluster on soft, young growth at the tips of the plant where cell sap is most concentrated and easy to access. In large numbers they weaken the plant by extracting sap, distort shoot growth, contaminate developing pods with honeydew and sooty mould, and — critically — transmit viral diseases that can be more damaging than the aphids themselves.

When aphids peak

Aphid populations build rapidly in warm, dry weather from late April onward. The winged alate forms disperse from overwintering hosts (spindle tree and other woody plants) and colonise peas in late spring. A single aphid can give birth to multiple live young per day parthenogenetically (without mating), meaning a single aphid arriving on a plant in early May can become a colony of hundreds within two weeks. The worst infestations typically occur in May and June on early-sown peas. A wet May with cool temperatures significantly suppresses aphid population growth.

Physical and biological control

Rub aphid colonies off shoot tips by hand or squeeze them between finger and thumb — this is highly effective on small plantings. A jet of water from a hose dislodges colonies on larger plantings. Encourage natural predators by avoiding broad-spectrum pesticides: ladybirds (both adults and larvae), lacewing larvae, hoverfly larvae, and parasitic wasps all consume large numbers of aphids. A well-planted wildlife garden with flowering plants near the vegetable plot draws these predators in. Tolerate small aphid numbers — predators need an initial population to arrive and establish before they can make an impact.

Aphids as virus vectors

Pea aphids are efficient vectors of pea enation mosaic virus (PEMV) and other pea viruses, transmitting them in seconds of feeding even before colonies become obvious. Once a plant is infected with virus there is no cure — remove and bin it. The presence of aphids is therefore more concerning than their numbers suggest when they are present during the critical early flowering period. Early sowings that are past the vulnerable young-plant stage before aphid peak in late May escape the worst of virus transmission.