Ichneumon eumerus is a species of parasitic wasp belonging to the family Ichneumonidae, subfamily Ichneumoninae.

It is a specialist parasite of the larva of the mountain Alcon blue butterfly (Phengaris rebeli).

This gives the wasp the chance to search out the butterfly larvae and lay an egg in each one.

Afterwards the ants continue to feed and look after the butterfly larvae which in due course pupate.

[3] The mountain Alcon blue butterfly is a rare species which is known from the northern part of Portugal, the western Pyrenees, France, Switzerland, part of Germany and Greece at altitudes between 1,000 and 2,000 m (3,300 and 6,600 ft).