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A member of the Ichneumonidae family this wasp is the main parasite of the Cynipidae wasp Diplolepis rosae. Other species targeted include D. spinosissimae, D. mayri and D. eglanteriae. It is an endoparasite and as such deposits its eggs directly into the larvae of its host before the bedeguar gall has begun development, thereby making all the larvae available, as opposed to just the outer ones of the already formed gall. Being an early stage parasite it may well then open itself for hyperparasitization at a later date by Eurytoma rosae, Pteromalus bedeguaris and Torymus bedeguaris.
The female wasp is long and slender in shape. Head and body measurements are 2.4-5mm, averaging to 4mm.
The wasp has a black head and thorax, with large black eyes and ocelli. It has long, slender, dark antennae that consist of 21 segments. The coxae are black, as is the hind femora, while the rest of the legs, including the trochantor, are brown with pale joints, although someitmes the rear leg may be darker or grey in colour. The gaster (abdomen) is testaceous with yellow beneath and brown markings on the side and top with a short ovipositor.
The male wasp, like the female, is long and slender in shape although slightly bigger, measuring 2.7-5.5mm with an average of 4.6mm.
It has a black head and thorax, with large black eyes and ocelli. It has long, slender, dark antennae that consist of only 20 segments. The coxae are black, as is the hind femora, while the rest of the legs, including the trochantor, are brown with pale joints, although someitmes the rear leg may be darker or grey in colour. The gaster (abdomen) is testaceous with yellow beneath and brown markings on the side and top.
As an internal parasite the wasp gives no indication of its presence until it is full grown, at the end of september, by which time it will have absorbed the entire contents of its host, excepting mouthparts. At this point the skin of the host is dull and ill fitting of the new wasp larva until after only a few days the host skin is shed and the O. mediator larva is revealed.
The larva shape is of even thickness and bluntly rounded at both ends and slightly curved. The body surface is glabrous and shining, and the segmental divisions are not deeply incised. It has a lateral beeding along the side of the abdomen. by the end of october the larva has started to change and develops a waist as the thoracic segments become more compact and the first 2 abdominal ones become slender. The eyes also start to form on the second segment. These changes progress through the winter and are very pronounced by april, when the larva begins to pupate.
After pupation the adult wasps tend to emerge at the end of april through to early june which corrisponds with the emergance of the remaining Diplolepis wasps and surprisingly along with the Pteromalus bedeguaris wasps aswell.
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