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Periclistus species all depend upon species of Diplolepis, and P. brandtii is an inquiline of several species, commonly found in Bedeguar galls, caused by Diplolepis rosae and Smooth Pea galls, caused by Diplolepis eglanteriae. As well as galls caused by D. mayri.
This wasp is a member of the family Cynipidae but is not a gall causing wasp, like many others. In fact it is incapable of forming a gall of its own so acts as an inquiline. This is basically a wasp that lays its egg into an existing gall at an early stage to share the same food store as the gall causer. Although the causer is not directly targeted, the fight for food resource can sometimes cause the death of the gall causer, especially if the inquiline is sharing the same cavity as the causer. Normally the chambers are seperate and the difference between D. rosae cells and P. brandtii cells are easily noticed. Those of D. rosae are large, round and not much coalesced, as opposed to those of P. brandtii which are smaller, thick walled and frequently fused and appear irregular in shape, sometimes forming a solid mass. Blair (1945) noted that the small or medium sized galls infested by P. brandtii where harder to cut into with a knife than the unifested galls.
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. Gall size increased significantly, but its magnitude was smaller than in unilocular cases. Number of emerged individuals and diversity was significantly higher. For inquiline-free galls, a highly significant positive correlation was found between gall size and number of emerged individuals. But the presence of inquiline entirely annihilated the correlation between gall size and number of emerged individuals, although this relationship is a basic truism for unilocular galls. Regarding to the complexity of the effect caused by the inquiline multilocular galls are more than just a multiple of unilocular ones. The final conclusion is that in multilocular galls the presence of inquiline species changes significantly the parameters that can contribute to the survival of the gall inducer. Thus, the presence of inquiline may affect the relation between host plant and gall inducer.
Periclistus brandtii is commonly attacked by three parasitoids; Glyphomerus stigma, Eurytoma rosae and Caenacis inflexa and thought to be targeted by Eupelmus urozonus. Sometimes by others such as Torymus flavipes.
Adults of inquilines emerge from galls soon after gall inducers and oviposit in freshly initiated galls. The adult P. brandtii wasps emerge from the galls from early june through to late july in the UK.
The female wasp has a dark brown to black head and body with testaceous gold legs and dark slender antennae consisting of 12 segments. The gaster has a ventral spine but not extended enough for a ploughshare to be present. The whole wasp measures 2.4-3.3mm averaging at 2.7mm.
The male is deep bodied with a black head thorax and gaster. The legs, like the female are testaceous gold however, unlike the female, the antennae are also testaceous gold and consisting of 14 segments, of which the third segment is curved.
The females carry on average 73 eggs (range 50-98). The egg is banana shaped and distinclty divided into an egg body and a long narrow peduncle. The egg body is three times longer than wide, and the peduncle can be twice as long as the egg. The exochoroin is divided into two parts. The outer part contains closely packed crystalline rods arranged in rows which are perpendicular to the egg surface. The inner part is less electron dense and also contains irregularly spaced crystalline elements. The endochoroin is electron tranluscent and uniform in structure, and about ten times thicker than the exochorion. (2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 139, 247–260). Blair (1945) made observations of P. brandtii eggs found in Smooth pea galls, which were opened in mid august. A number of elongate, clear, almost colourless eggs were found lying on the wall of the central chamber. Each egg had a stalk at one end that was the length of the egg itself. The other end of the stalk was attached to the wall of the chamber. The larvae when hatched were also colourless and clear with large knobbly looking heads that they repeatedly jerked up and down. When full grown, the larva is stout and hairless with very little or no tapering at either end. The intersegmental divisions are not as obvious as in D. rosae larvae, and the tail end is bluntly rounded with no terminal narrow segment. At each side of the thoracic segments there is a slight dorso-lateral swelling, which creates the appearance of the dorsum being flattened. Another difference from D. rosae is their jaws which have a more pronounced apex but which the teeth less deveoloped.
More detailed descriptions and identification keys are available from Robin Williams at the British Plant Gall Society.
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