2017

A Cone in the Oak Foliage

From June to September, "cones" may often be spotted in the oak foliage; sometimes they appear even in winters. Of course, they are not real cones, they are galls - pathological formations made by oak artichoke gall wasps (Andricus fecundatrix).

The peculiar formations on oak branches stand upright like candles. The galls are about 2 to 3 cm long and have an oval shape. The galls themselves, which are basically of light green colour, and the wasps residing therein cannot destroy the tree, but they can make the tree weak.

These galls consist of two layers - the external and the internal layer. The outer layer is thick, soft, on the surface generously covered with adhering scales, while the inner layer is much smaller, yet much denser and more compact.

A great part of gall wasps, also oak artichoke gall wasps, in their cycle of development experience two generations: same-sex generation formed by females able to multiply partenogenetically, and gametophyte consisting of males and females.

In spring, usually in April, the females able to multiply partenogenetically, break the cocoon and depart. If the weather conditions are appropriate and if the oak trees are in the respective vegetation phase, these insects lay their eggs in male flower buds. As a result, galls are formed on the pollen catkin stalks. These galls are oval, with pointed ends, just a few millimetres large and they serve as nursery for the next generations.

than a few millimetres. Gametophyte insects, of course, mate. After copulation males die, while females let ripen the eggs, and lay their eggs on the ends of oak branches and in side buds. The hatching and development of larvae begin already in June. The development of insects leads to the release of auxins that stimulate the formation of tannins. These substances contribute to the emergence of new galls that are already larger than the previous ones.

The parthenogenetic gall wasps in the stage of a larva develop in the small, rigid layer of the internal cone-shaped layer of the gall. They are fully developed in August or September. Then the inner layer of the gall together with its inhabitant simply falls out.  The external layer of the gall, which initially was green, is becoming more brown and its surface scalation loses its smoothness since the ends of the scales can recurve. Afterwards, only an empty distorted ball of leaves remains in the tree.