Robin’s Pincushion Diplolepsis rosae – a closer look
Robin’s Pincushion or the Bedeguar gall is induced by the cynapid ( gall ) wasp Diplolepsis rosae. In Germany it is known as the”sleep apple” as it is said to aid sleep if placed under one’s pillow. This strikingly beautiful gall, which incidentally is the emblem of the British Plant Gall Society, is found on species of wild rose, mainly Rosa canina. This photograph, taken in late July, shows a gall at its most attractive stage. The mass of reddish branched filamentous growths contain vascular strands which are continuous with those of the plant’s vascular system. (S. Randolph).
the bud to locate the most suitable position for insertion of eggs. Once in position she will insert her ovipositor with remarkable precision under the edge of an outer bud scale and remain motionless for 1-2 hours, during which she can lay 30- 100 eggs in the same bud. Each egg is glued to a single epidermal cell. Cell proliferation in the area of oviposition begins before the egg hatches in order to create a cavity into which the larva crawls once the egg has hatched, which will be within a week of oviposition. The eggs are produced by a method of reproduction called parthenogenesis which means that reproduction occurs without the necessity for fertilisation. It is likely that a single
female will lay eggs in more than one bud. Synchronisation between the emergence of the egg laying females and the expansion of the buds on the host plant is essential. Initiation of the gall itself arises as the larva starts to feed, causing mechanical damage to the cells around it with its mandibles. Several other factors are thought also to be responsible, including the wounding of tissue by oviposition. The larva becomes surrounded by an inner chamber around which layers of nutritive cells develop. These cells become layers which are continuously replaced as the larva feeds and are found nowhere else in the plant, only as part of the structure of the gall. Through the cells the gall causer stimulates the plant to send vast amounts of nutrients from other parts of the plant through a network of vascular bundles. Around the inner chamber a layer of thick walled cells develop protecting the inner chamber. The fully developed Robin’s Pincushion consists of many chambers each housing an inducer larva, with the outer thick walled layers coalescing and surrounded by a mass of branched sticky filaments.These hairy filaments are initially green-pink but turn brown to black in Autumn/Winter. A gall with many chambers like this is an example of a multiocular gall. During the period of rapid development of the gall, (the gall tissue is fully formed and reaches maturity within four to eight weeks ) the larva feeds only slowly and undergoes a series of moults until it is fully developed in late October. It then ceases feeding and enters the prepupal stage during which it overwinters. Pupation occurs inside the chambers the following March or April and
the final development of the adult takes only a short time. The adult cynapid wasps emerge in April using their strong mandibles to chew exit holes in the gall. The exit holes can be seen more easily when the filamentous branches fall away. The majority are females capable of laying fertile eggs without mating.
The following photograph, taken in August, shows the outside of a fully formed gall.
The one known inquiline which uses Robin’s Pincushion is another cynapid wasp called Periclistus brandtii. An assortment of parasitoids are known to feed on the host larvae. Since their action always results in the death of the gall causer, they are parasitic. Together with the host
inducer they form a community. During the winter the galls lose most of their hairs and so once the chambers are vacated the holes are more obvious and a variety of creatures take advantage of the gall as shelter. These, known as successori, use the old empty cells as a place to rest or hibernate and include moths, earwigs bugs etc.They may also feed on fungi and bacteria that colonise the spaces.
Britain’s Plant Galls by Michael Chinery.
The Natural History of the Rose Bedeguar Gall and its insect community by S. Randolph.
The Natural History of the Rose Bedeguar Gall part 2 by Robin Williams
Plant Galls by Margaret Redfern
Crown gall causing bacteria Agrobacterium humefaciens infect plants by inserting some of their own DNA into plant cells through a wound. This alters their function causing them to produce unusual amounts of plant hormones which in turn stimulate the plant cells to multiply and enlarge leading to the tumour formation. The bacterium forces the plant to produce opines which it uses for energy. Even if the bacterial infection is cleared gall growth continues.
These galls are quite common on Silver Birch tree trunks. Both photographs below taken in Park Wood, Rossington. The first by Nora Boyle and the second by Caroline Barrass.