Life-cycle and Arrival in Britain
Originally these wasps were introduced to Devon from the Middle East in the 1830s.
The marble gall has alternating sexual and asexual generations, often taking two years to complete, especially in the north of Britain. The familiar summer gall develops from eggs laid by a sexual female in the developing buds of our two native oaks in May or June; the host trees often being immature or retarded, scrub-oak, specimens; they are rarer on older healthy trees.
The developing spherical galls are green at first, brown later, and mature in August. Each gall contains a central chamber, with a single female wasp larva of the asexual generation, which emerges through a 'woodworm-like' hole as an adult winged gall-wasp in September. These asexual (agamic) females lay unfertilized eggs in the embryonic bud leaves of the Turkey oak, with galls slowly developing during winter, and are visible in March and April as small oval structures between the bud scales, looking like ant's eggs or pupae. The emerging adult gall-wasps in spring are the sexual generation, producing both males and females, which fly to the common oaks to initiate the formation of the summer marble gall.
The abnormal buds develop during summer and the bud is wholly replaced by the gall growth. Marble galls may remain attached to the tree for several years. The level of attack by the insect varies greatly from year to year.
Read more about this topic: Andricus Kollari
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