Above is the case of a Phryganidae caddis fly larva that has been parasitised by Agriotypus sp., an Ichneumonid wasp. The adult wasps swarm over streams in spring and mate. The female wasp then crawls down a plant stem, into the water and down to the stream bed where she searches under stones for a Phryganidae caddis fly larva.
She lays an egg inside the case. When the wasp hatches from the egg it starts to eat the body of the caddis fly larva. However, it does not eat the nervous system, as it does not want to kill the caddis larva.
Only once the caddis larva has fixed its case to a stone to prepare for pupation will the wasp kill it by eating all the caddis larva. Then the wasp makes its own silk cocoon inside the caddis case and emerges as an adult the following spring. A parasitised case is easy to recognise as it has a long thread hanging from it.
In England in 1920 Encarsia formosa, a tiny parasitic wasp, was used to control whitefly in greenhouses where tomatoes were grown. Then, with the abundance and ease of the use of new pesticides made in the 30s and 40s bio-control died out. It was revived by the Dutch greenhouse tomato growers in the 70s when their whiteflies had developed resistance to the currently available pesticides.