Physiological responses of Eucalyptus trees to infestation of Leptocybe invasa Fisher & La Salle

The gall wasp Leptocybe invasa Fisher & La Salle is a significant pest to eucalyptus. The physiological responses of eucalyptus clones (DH201-2 and GL-UG9) infested by L. invasa were investigated using artificial inoculation at different wasp densities, microtome sectioning, microscopic observation, and chemical analysis. The results indicated that the stem and petiole cross-sections of DH201-2 and GL-UG9 had similar structures that were typical of dicotyledonous plants. The tissues were clearly divided into an outer epidermis, cortex, vascular bundles, and medullary structure, However, the petiole epidermal cells of DH201-2 had significantly more oil capsules than GL-UG9 (P=0.00209), and the DH201-2 petiole cutin layer was thicker than that of GL-UG9 (P=0.00001). However the petiole epidermal cells of GL-UG9 were significantly thicker than those of DH201-2 (P=0.015215), and GL-UG9 had significantly more vascular bundles than DH201-2 (P=0.002375). In addition, in DH201-2, the stem cortex parenchyma cells were thicker (P=0.04071), the oil capsule diameter was greater (P=000016), and there were more vascular bundles (P=0.00000) than in GL-UG9, while in GL-UG9, the stem corneous layer was thicker (P=0.00167) and there were more epidermal cells (P=0.00000) than in DH201-2. Each gall contained about 10 insect larvae. The anatomical structure of the gall tissue, from inside to outside, was: nutritive tissue, thin/sclerenchyma layer, vascular bundle layer, cortex, epidermis. When the adult wasps were not dense, they formed small galls only in petioles, while leaves occasionally had calluses. As adult wasp density increased, females laid eggs in the veins of stems to form stem-petiole-vein galls. Early in gall formation, the larvae had not hatched, but the plant tissue organization had begun to differentiate, the normal vascular structure had been destroyed, and the vascular bundle layer structure had deformed. As the eggs hatched into larvae, a cortex vascular bundle formed outside the approximately spherical internal space. Further structural analysis of the L. invasa gall structure indicated that the gall was divided into several distinct layers that could be roughly classified, from inside to outside, into: nutrition organization layer (21.84 plus or minus 0.42) mu m, parenchyma layer (13.34 plus or minus 0.19) mu m, sclerenchyma layer (18.68 plus or minus 1.13) mu m, vascular bundle layer, cortex, and epidermis. The development of the gall destroyed the origi