(H. Loew, 1850)
Willows are extremely susceptible to gall induction and growth manipulation and Salix is one of the plant genera with the highest known numbers of associated galler species.
The gall consists of from 30 to 60 leaves, shortened and crowded together in rosettes on white willow (S. alba), crack willow (S. fragilis), goat willow (S. caprea) and purple willow (S. purpurea) willows as well as the eared sallow (S. aurita) and grey sallow (S. cinerea). The oviposition of this species results in a chemical interaction that halts the lengthwise growth of infected willow shoots, the leaves however continue to develop and thus the characteristic "rose" forms at the tip of the affected shoot.
R. rosaria is found on willow branches and the gall diameter depends upon the species, being larger on Salix caprea at 8 centimetres (3.1 in) than on Salix alba at 3 cm (1.2 in). The development in the United Kingdom begins in May / June with leaf expansion and reaches maturity in August / September whilst the leaves are still green. The gall becomes brown and remains on the tree over winter until it is forced off by new growth in spring.
Causer, inquiline and associations
Each rosette contains a single pinkish-coloured larva which, unusually, pupates within the gall. A common inquiline is another gall midge, Perrisia iteophila. The gall mite Eriophyes marginatus often appears on Camellia galls. R. salicis and R. heterobia also form galls on willows.
- Darlington, Arnold (1975) The Pocket Encyclopaedia of Plant Galls in Colour. Pub. Blandford Press. Poole. ISBN 0-7137-0748-8.
- Stubbs, F. B. Edit. (1986) Provisional Keys to British Plant Galls. Pub. Brit Plant Gall Soc. ISBN 0-9511582-0-1.