Cynipid galls on oak leaves are resilient to leaf vein disruption

Oaks serve as host plants for numerous insects, including those forming galls. Galls induced on oaks are completely dependent on leaf resources. Many other folivores damage veins of leaves, which may result in cutting galls off from sources of assimilates, nutrients and water. We hypothesised that the disruption of the continuity of leaf vascular tissues stops gall development, leading to the death of the larva. Leaves of sessile oak (Quercus petraea) with Cynips quercusfolii galls in the initial stage of development were marked. The diameter of the galls was measured, and the vein on which the gall was present was cut. Four experimental treatments were established: control – with no cutting, cutting the vein distal to the gall relative to the petiole, cutting the vein basal to the gall and cutting both sides. The average survival rate (live galls at the end of the experiment including healthy larvae, pupae or imagines inside) – was 28.9%. The rate varied depending on the treatment and was 13.6% in the treatment with the vein cut on both sides and about 30% in the remaining treatments. However, this difference was not statistically significant. The growth dynamics of galls are highly dependent on the experimental treatment. The largest galls grew in the control treatment, and the smallest galls were in the treatments with the veins cut on both sides. Unexpectedly, even cutting veins on both sides did not result in the immediate dieback of the galls. The results suggest that the galls are very strong nutrient and water sinks. The functions of the cut vein are likely taken over by other lower-order veins, allowing nourishment of the gall to complete larva development.