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Robin's Pincushion

Robin's Pincushion ~ http://www.flickr.com/photos/qwertyqwertyqwerty/

The Rose bedeguar gall, Robin's pincushion gall, or Moss gall develops as a chemically induced distortion of an unopened leaf axillary or terminal buds mostly on Field Rose (Rosa arvensis) or Dog rose (Rosa canina) shrubs, caused by the parthenogenetic hymenopteran gall wasp (Diplolepis rosae (Linnaeus, 1758)), previous synonyms are D. bedeguaris, Rhodites rosae or Cynips rosae.

Diplolepis lay up to sixty eggs within each leaf buds using their ovipositor. The asexual wasp emerges in spring; less than one percent are males.

A similar gall is caused by Diplolepis mayri, but this is much less common.

The physical appearance of Diplolepis rosae[]

The females insects are about 4mm / 0.2in long; parts of their abdomens and legs are yellow-red, while the rest of the body is black. The male is black and lacks the hypopygium structure which clearly identifies the species in the female. Its legs are bi-coloured yellow and has a body length of about 3mm.

The physical appearance of the Rose bedeguar gall[]

The bedeguar gall is surrounded by a dense mass of sticky branched filaments. This structure gives the appearance of a ball of moss and its filaments are often brightly coloured, being at their best in around September; starting off green and then passing through pink and crimson to reddish brown. A large specimen can be up to 10 cm in width. The larvae develop and then over winter as pupae in the now brown and dry looking structure, emerging in May.The unilarval chambers are set in a woody core which persists after the filaments have worn off.

RobinPgall

Robin's Pincushion Gall - WWC Archives

The bedeguar may also develop on Rosa rubiginosa, R. dumalis, or R. rubrifolia.

The gall induced by D. mayri differs in being more sparsely covered in short, unbranched filaments and the galls usually develop on the twigs.

Life-cycle[]

A week after the egg has been laid, the larva hatches and begins to feed on the leaf bud tissue. This activity stimulates (in a way not yet understood) the development by the host plant of enlarged 'nutritive' cells in the area around the growing larva. These are fed on by the larva and are continually replaced by new cells. Further concentric layers of tissues develop around the core nutritive tissue and come to form the structure known as the bedeguar, complete with the outermost and characteristic fibrous outgrowths that give the gall its alternative name of Robin's Pin Cushion. As the larva feeds and grows within this gall, it probably undergoes five larval instar stages (the growth stages between moults. The final instar stage is reached by late October. The larva ceases feeding. It now passes into the prepupal stage, in which form it overwinters inside the gall. In the following February or March the prepupa undergoes a final moult and becomes a pupa. Through the thin transparent skin of the pupa it is possible to see the fully formed antennae, legs, wings and body segments of the adult wasp. As stated earlier, the adult wasps which start to emerge from the Rose bedeguar will be mostly female, and these females will go on to lay eggs through parthenogenesis. The adults begin to emerge from the old galls, which are still attached to the rose, in May. Emergences may continues through to August. No alternation of generation exists in this species.

As stated, males are known, but are very rare. One possible reason for this scarcity is the presence of a bacterium named Wolbachia which is endosymbiotic in the females gametes. A female infected with Wolbachia produces only diploid eggs, when in the cells of the ovaries presumably cause the fusion of the pronuclei, which leads to entirely female progeny. When the females were treated with antibiotics they were then able to produce normal male and female eggs.

Gall forming insects[]

Some herbivorous insects therefore create their own microhabitats by forming, in this case, a highly distinctive plant structure called a gall, made up of plant tissue, but controlled by the insect. Galls act as both the habitat, and food sources for the maker of the gall. The interior of a bedeguar gall is formed from the bud, and is composed of edible nutritious and structural tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect with some physical protection from predators.

Predators, inquilines, parasites, hyperparasites and fungi[]

Mature galls are sometimes broken open by vertebrate predators to recover the larvae or pupae. The large size of the emergence holes of the individual cells sometimes suggests that predation by birds or small mammals has taken place (See photo).

The bedeguar is a good example of a complex community of insects. The cynipid wasp Periclistus brandtii is an inquiline that lives harmlessly within the bedeguar gall and like Diplolepis rosae itself, is often parasitised by insects referred to as parasitoids or even by hyperparasitoids in some cases.

The gall-wasp Periclistus brandtii causes no gall itself, but deposits its eggs in the bedeguar tissues on which the larvae feed. In turn these larvae may be parasitised by a chalcid wasp, Eurytoma rosae, which works its way from one inquiline's cell to the next. The parasitoid ichneumon Orthopelma mediator lays its eggs directly into larvae of D. rosae, killing them. The chalcid wasps Eurytoma rosae and Glyphomerus stigma can attack both the larvae of D. rosae and of the inquiline P. brandtii. These parasitoids may in turn be attacked by hyperparasitoids such as the chalcids Caenacis inflexa and Pteromalus bedeguaris. It is clear that the mossy and sticky filaments of the gall are ineffective against preventing the entry of inquilines, predators, parasitoids and hyperparasitoids.

The tissues of the bedeguar gall are frequently attacked by the parasitic fungus Phragmidium subcorticum, more so than the other parts of the host rose plant.

Infestations of Rose bedeguar galls[]

The galls occur more commonly on plants that are under stress, i.e. very dry conditions, waterlogging or hedge cutting, whereas vigorously growing plants are less commonly found to have galls. Whether the vigorous plant suppresses gall formation or is avoided by the wasp in favour of easier targets is unknown. Young and damaged plants tend to produce larger and more numerous than old and intact ones. Many eggs are laid but the number of galls formed is relatively few.

The relative number of parasitoids decreases with increasing gall volume. The vertical distance between gall and soil affects the total number of emerged specimens. The distances from margins of shrubs does not affect nor the parasitoid ratio of galls, neither the volume of the galls. It seems better for a female D. rosae, to induce larger galls on the lower branches of the shrubs in order to increase the survival probability of the offsprings.

Removing and destroying galls before they dry and the wasps emerge may help to reduce the infestation. While fairly large, and sometimes present in quite large numbers on scrub specimens, they cause no measurable harm.

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