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The rose pea gall is caused by either the cynipid wasp Diplolepis nervosa, D. eglanteriae or rarely by D. centifoliae on the leaves of the Dog rose (Rosa canina). The Pea gall comes in two forms. There is the smooth spherical form , which can house D. nervosa and D. eglanteriae, and the spikey form, which can have 2 to 6 spikes and houses D. nervosa or the third form which usually has papillae but not always and houses the rare D. centifoliae. The causer wasps need to be reared and keyed out to species to be certain of which one you are dealing with due to the fact that they all can occupy smooth galls. The galls shape instead appears to be more related to its position on the plant than the species creating it. Smooth galls tend to appear on the top smooth part of the leaf, whereas the spikey galls tend to be formed on the underside of the leaf. the galls may be enlarged or distorted due to the presence of inquilines and a guide to Identification can be found in Cynipoidea-key to families and subfamilies and Cynipidae (Cynipinae). R.D. Eady & J. Quinlan. 1963 .
Each gall is, in effect, a hollow, fleshy nursery chamber for a small white grub, the larva of the Diplolepis nervosa or D. eglanteriae gall wasp, which feeds on the chamber wall. The galls appear to detach from the leaves before leaf fall and will lie in the leaf litter until the grub pupates and emerges as the small adult wasp, only about 4mm long. If the gall is on the underside of the leaf or otherwise shaded from the sun then the gall of either form will remain greeny white with maybe a slight flush of pink, however if the gall is exposed to the sun then it will turn a deep red colour before it matures.
The inquiline Periclistus brandtii (Ratzeburg) sometimes attacks the galls, resulting sometimes in a larger or mishapen gall. P. brandtii is a harmless inquiline cynipid and lays its eggs on the ready-made gall, just like the cuckoo uses other ready-made nests for its eggs. Inquiline larvae cause the gall to be hypertrophied and increasing number of inquilines causes increased growth of host gall tissues and wall thickness. The P. brandtii larvae subdivide a single host larval chamber into many inquiline chambers and this fact leads to the prediction that inquiline presence will modify the gall size and the number of emerging specimens. Inquilines, particularly if they are many in a single chamber, sometimes kill the larva of the gall inducer.
Parasitoids associated with D. nervosa and D. eglanteriae include; Orthopelma mediator, Aprostocetus aurantiacus, A. cyniphidum, A. eurytoma, Elachertus inunctus, Eupelmus atropurpureus, E. muellneri E. urozonus, E. vesicularis, Eurytoma caninae, E. rosae, Caenacis inflexa, Pteromalus bedeguaris, Glyphomerus stigma, Glyphomerus tibialis, Torymus eglanteriae, T. flavipes, T. geranii, T. laetus, T. microstigma and T. rubi.
In Diplolepis nervosa the black shiny head is short and transverse with small eyes. The large ocelli form a depressed triangle in the crown. The antennae have 14 segments in the females. In the males there are 15 segments and are longer than the body.
The thorax has a very short collar. The wings are longer than the body with a large subtrigonate marginal cell. The discoidal cells are incomplete but occasionally with a triangular areolet. The legs os the female are pale ochre to intermediate brown with the base of the femora being slightly darker. The tibia have minute spurs at the apex and the long, slender tarsi consist of 5 segments. The claws and pulvilli are short.
The gaster (abdomen) is smaller than the thorax, especially in the males. Attached by a thick petoile it is ovate, compressed and trunkated obliquely. It is very smooth and ochreous with the lower sheath being brown. The ovipositor is slender and curved and attached above the near apex, emergin through a sheath below.