These fusiform galls are formed by Cecidothyrus pexa sic. They are about 80 mm long and about 20 mm broad at the thickest point, and are always monothalamous. The larvae excavate the gall so that only the shell is left when the moth emerges. Just before pupating, the mature final-instar larva chews an escape hole for the adult at the base of the gall. The accumulated faeces are pushed through the hole, and pupation then takes place. Leaf growth on the gall does not appear to be affected until the adult emerges, after which the leaves begin to die off. Vacated galls are easy to distinguish from occupied ones, not only by the large escape hole but also by the fact that vacated galls dry up shortly after the emergence of the adult, and change from green to brown.

Cecidothyrus pexa [sic] has two generations a year at Nylsvley. First-generation adults emerged in about the middle of September and larvae were present from about the middle of October until about the middle of January. Second-generation adults were active in February, and larvae were present from early March until June. Pupation took place inside the gall, and pupae remained in diapause until September.