Black Bean Aphid Aphis fabae
Plant lice (or Aphids, or Green Flies) often become a pest in one's garden. And they do not only attack outdoor plantlife, but our indoor green as well. Usually you notice their presence by furniture or window sills being sticky. Especially in warm and damp summers they are numerous and everywhere. Even though Black Bean Aphids are very vurnerable, they often succeed to survive. That's because they have friends: ants. They exude a sweet liquid, much favoured by ants. In return the ants protect the lice against smaller enemies, such als ladybirds and their larvae. The Black Bean Aphid is one of the more common species and is also known as the Blackfly. It likes living in colonies. Such a colony is started at the top of a stem, usually just below a flower or bud. When more aphids join the colony it starts to expand downwards untill the whole stem is covered by aphids. This may damage the plant seriously. Flowers may refuse to bloom, the fruit will not grow at all.
We are not absolutely sure the animals in the picture actually are Black Bean Aphids. Aphids are notoriously difficult to identify and often there are very similar other species as well.