Blackfly

Some black aphid species, particularly the species known as the black bean aphid, are troublesome on some vegetables and flowers during the spring and summer.

Blackfly on broad bean

Quick facts

Common name Blackfly including the black bean aphid
Scientific name Aphis fabae species group
Plants affected A very wide range including broad, French and runner beans, nasturtium, dahlia, philadelphus, viburnum and euonymus
Main cause A black sap-sucking insect
Timing April-August

What are blackfly?

There are several species of black aphid that infest garden plants but the most common blackfly is a species called Aphis fabae, which is also known as the black bean aphid. This is a sap-sucking insect that forms dense colonies on the soft young growth of many plants.

Cherry trees are prone to a different aphid known as cherry blackfly.

Symptoms

The aphids are up to 2mm long and are mainly black but may have some white flecks on the upper surface of their bodies. Dense aggregations of the aphids 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. 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 will be poor if the plants become heavily infested. Flower formation on ornamental plants, such as dahlia, nasturtium and poppies, can be adversely affected 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 chemicals secreted into the foliage as the aphids feed.

Control

Non-chemical control

Aphids have many natural enemies, including ladybirds, hoverfly larvae, lacewing larvae and several parasitic wasps. Some of these are available for biological control of aphids in greenhouses (aphid predators). Unfortunately, damaging aphid infestations often build up on garden plants before natural enemies are active in sufficient numbers to achieve control. Use your finger and thumb to squash infestations on small plants. Picking off the tips of broad beans as soon as the blackfly are seen can reduce and delay infestation, and also improve the yield of beans

Chemical control

  • Check susceptible plants frequently from late spring onwards so action can be taken before a damaging infestation has developed
  • Organic sprays, such pyrethrum (e.g. BugClear Gun for Fruit & Veg, Defenders Bug Killer), fatty acids (e.g. Bayer Bug Free, Doff Greenfly & Blackfly Killer) or plant oils (e.g. Vitax Organic Pest & Disease Control, BugClear for Fruit & Veg Gun) can control blackfly. These pesticides have a very short persistence and so may require several thorough applications to keep blackfly numbers down
  • More persistent synthetic insecticides include lambda-cyhalothrin (e.g. Westland Resolva Pest Killer) or deltamethrin (e.g. Bayer Ultimate Fruit & Vegetable Pest Killer). These insecticides can be used on beans
  • For blackfly on ornamental plants, systemic insecticides, such as acetamiprid (e.g. Bug Clear Ultra) can also be used
  • When using pesticides on edible plants make sure the food plant is listed on the label and follow manufactures instructions on maximum number applications, spray interval and harvest interval
  • Plants in flower should not be sprayed due to the danger to bees and other pollinating insects

 

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Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biology

The 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.

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