What are blackfly?

There are several species of black aphid that can infest garden plants, a very common blackfly is the group of species known as the black bean aphid, Aphis fabae group. These is a sap-sucking insects can form dense colonies on the soft young growth of many plants.

There are many other black aphid species for example cherry trees are prone to a different aphid known as cherry blackfly.

Symptoms

Black bean aphid are up to 2mm long and are mainly black but may have some white flecks on the upper surface of their bodies. Dense aggregations can rapidly develop on soft shoot tips, flower stems and on the underside of the younger leaves. The aphids are often attended by ants, which collect the sugary honeydew that aphids excrete the ants will also remove aphid predators such as ladybird larvae. Whitish cast skins of aphids often accumulate on infested plants.

Heavy infestations weaken the host plants and can result in stunted growth. On broad beans, pod formation can be poor if the plants become heavily affected. Flower formation on ornamental plants, such as dahlia, nasturtium and poppies, can damaged when blackfly are feeding on the developing flowers. The winter-spring host plants, such as Philadelphus, Viburnum and common spindle (Euonymus europaeus) often develop curled foliage in response to the aphids feeding.

Control

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by starting with the methods in the non-pesticide control section. If this is not sufficient to reduce damage to acceptable levels then you may choose to use pesticides. Within this group the shorter persistence pesticides (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action.

Non-pesticide control

• Where possible tolerate populations of aphids
• Use finger and thumb to squash aphid colonies where practical
• Encourage ‘aphid predators’ in the garden, such as ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs. Be aware that in spring aphid populations often build up before natural enemies are active in sufficient numbers and then give good control

Pesticide control

• Organic sprays, such as natural pyrethrum (e.g. Bug Clear Gun for Fruit & Veg, Neudorff Bug Free Bug and Larvae Killer), fatty acids (e.g. Solabiol Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Plant Guard Pest & Disease Control, Bug Clear for Fruit and Veg) can give good control of aphids. These pesticides have a very short persistence and so may require reapplication to keep aphid numbers in check. Plant oil and fatty acid products are less likely to affect larger insects such as ladybird adults
• Plant invigorators combine nutrients to stimulate plant growth with surfactants or fatty acids that have a physical mode of action against aphids (e.g. Ecofective Bug Control, RHS Bug and Mildew Control, SB Plant Invigorator and Westland Resolva Natural Power Bug & Mildew). These products contain some synthetic ingredients and so are not considered organic.
• More persistent contact-action insecticides include the synthetic pyrethroids lambda-cyhalothrin (e.g. Westland Resolva Bug Killer), deltamethrin (e.g. Provanto Ultimate Fruit & Vegetable Bug Killer, Provanto Sprayday Greenfly Killer) and cypermethrin (e.g. Py Bug Killer)
• The systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra) is also available

Download

Pesticides for gardeners (pdf document)

Biology

Blackfly Aphis fabae overwinters as eggs on shrubs such as common spindle (Euonymus europaeus), Viburnum and Philadelphus. Egg hatch occurs in spring as the winter host plant is coming into new leaf. Several generations of wingless aphids, which are all female and produce live young rather than eggs, develop on the spring foliage. By May, the foliage on the winter host plants has become older and tougher, which along with increasing day length induces a change in the aphids. A generation of female winged aphids develops that fly away in search of suitable summer host plants, such as beans, nasturtium, poppies and dahlia. At that time of year male aphids are not required for reproduction, so a single winged aphid arriving on a broad bean can quickly establish a new colony. Blackfly infestations on the winter-spring host plants die out during May-June.

At the end of summer, another winged generation of aphids is produced that migrates back to the winter host plants. At that time there are males and females that will mate before the females deposit eggs around buds and in crevices on the stems.