Aphis fabae

Appearance

The black bean aphid is a small, soft-bodied insect that has specialised piercing and sucking mouthparts which are used to suck the juice from plants. This aphid is usually seen in large numbers and is a tiny, plump insect about two millimetres long with a small head and bulbous abdomen. The body is blackish or dark green in colour. Many adults are devoid of wings, a state known as aptery. Winged forms, known as alates, are longer and more slender than aptates and have shiny black heads and thoraxes. The membranous wings of the alates are held angled over the body. The antennae are less than two-thirds of the length of the body and both they and the legs are pale yellow in colour with black tips. The tibiae of the hind legs are swollen in egg-laying females. Near the rear of the abdomen is a pair of slender, elongated tubes known as cornicles or siphunculi. Their function is the production of a defensive waxy secretion. They are twice as long as the finger-like tail and both are brownish-black.

Distribution

The black bean aphid may have originated in Europe and Asia, but it is now one of the most widely distributed species of aphids. It is found throughout temperate areas of Western Europe, Asia, and North America and in the cooler parts of Africa, the Middle East, and South America. In the warmer parts of its range, apterous individuals can survive the winter and they may continue to reproduce asexually all year round. It is known to be migratory.

Behavior

The black bean aphid has both sexual and asexual generations in its life cycle. It also alternates hosts at different times of year. The primary host plants are woody shrubs, and eggs are laid on these by winged females in the autumn. The adults then die and the eggs overwinter. The aphids that hatch from these eggs in the spring are wingless females known as stem mothers. These are able to reproduce asexually, giving birth to live offspring, nymphs, through parthenogenesis. The lifespan of a parthenogenetic female is about 50 days and during this period, each can produce as many as 30 young. The offspring are also females and able to reproduce without mating, but further generations are usually winged forms. These migrate to their secondary host plants, completely different species that are typically herbaceous plants with soft, young growth.

Further parthenogenesis takes place on these new hosts on the undersides of leaves and on the growing tips. All the offspring are female at this time of year and large populations of aphids develop rapidly with both winged and wingless forms produced throughout the summer. Winged individuals develop as a response to overcrowding and they disperse to new host plants and other crops. By midsummer, the number of predators and parasites has built up and aphid populations cease to expand. As autumn approaches, the winged forms migrate back to the primary host plants. Here, both males and sexual females are produced parthogenetically, mating takes place, and these females lay eggs in crevices and under lichens to complete the lifecycle. Each female can lay six to ten black eggs which can survive temperatures as low as −32 °C . More than 40% of the eggs probably survive the winter, but some are eaten by birds or flower bugs, and others fail to hatch in the spring.

Habitat

Natural predators of black bean aphids include both adults and larvae of ladybirds and lacewings and the larvae of hoverflies. Certain species of tiny parasitic wasps lay their eggs inside aphids and the developing wasp larvae devour their hosts from inside. Members of the wasp genera ''Diaeretiella'' and ''Lysiphlebus'' behave in this way and may provide a measure of control of the aphids.

Ants climb the host plants and feed on the honeydew secreted by the aphids. Many species of ants have developed behaviours to enable them to protect and encourage their aphids. Black garden ants , for example, remove predators such as ladybirds from the vicinity of aphids, thus keeping their "milch cows" safe. On a test plot of field beans , plants without black bean aphids yielded an average of 56 seeds per plant, those with aphids and no ants yielded 17 seeds, and those with both ants and aphids averaged only 8 seeds per plant.