Eupelmella vesicularis Retz. was found in certain parts of Hungary infesting cocoons of Diprion sertifer Geoff. already parasitized by a primary parasite, Microplectron fuscipennis Zett. It was never found in the absence of this chalcid.

Experiments showed that Eupelmella confined itself almost exclusively to Microplectron as a host, easily detecting the presence of larvae or pupae of this parasite within the cocoons of the saw-fly host, but neglecting eggs.

A single egg is laid per cocoon and the larval Eupelmella eats from 10 to 20 Microplectron larvae or pupae, and finally kills all the remaining living parasites within the cocoon before pupating.

The egg and larva show interesting structural adaptations for this mode of life.

Reproduction is parthenogenetic and no males are known.

Several factors combine to make this insect a dangerous enemy of Microplectron capable of seriously impairing its efficiency. These are: its selective ability, its destructive powers, its rapid rate of increase due to the parthenogenetic habit, and its ability to establish itself and survive on small numbers of the prey, for the same reason.