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Some black aphid species, particularly the black bean aphid, are troublesome on some vegetables and other plants during spring and summer.
Blackfly on broad bean
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.
Cherry trees are prone to a different aphid known as cherry blackfly.
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 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 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 the aphids feeding.
Check susceptible plants frequently from spring onwards so action can be taken before a damaging infestation has developed.
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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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