Parasitoid wasps lay their eggs on or in other insects. Their life cycle is gruesome - the egg hatches and the parasitoid larva eats the host alive before emerging as an adult. These insects have an important role to play in all terrestrial ecosystems, including your garden habitats. In spite of this critical role, few people are familiar with parasitoids, unaware that they kill enormous numbers of garden pests, such as aphids. They are visible as adults from late spring until early autumn. Parasitoid wasps are produced on an industrial scale by several firms, for release in fields and greenhouses. You can even order smaller numbers for use in your greenhouse from many garden centres. For example, Aphidius ervi can be used to control a number of pests, including the pea aphid and the potato aphid.
Female parasitoid wasps of many species can choose the sex of their offspring whether the eggs is fertilised (female) or not (male).
If a female finds a good quality host, providing lots of food for her offspring, then she will often place a female egg in it. Female parasitoids benefit more than males from having extra food.
Aphids containing a parasitoid pupa can be recognised by their brown, papery appearance and are often referred to as mummies.