I. Scanning and transmission electron microscopy were used to investigate damage to apple leaves caused by the white apple leafhopper (WALH), Typhlocyba pomaria McAtee. Feeding holes created by WALH were found in the lower epidermis, with no visual exterior evidence of cell injury to the upper epidermis. The feeding holes were located in areas of the leaf with high stomatal density, and were therefore in close proximity to stomata. Groups of cells in the palisade layers were empty, or contained coagulated cell contents; adjacent palisade cells were apparently uninjured. These uninjured cells contained an abundance of starch granules, however, possibly indicating that they were not functioning properly. Spongy mesophyll cells dorsal to the feeding area were left intact as were the epidermal cells ventral to the feeding area. There was no evidence of cell wall collapse from the external view of either epidermis, or from internal leaf views. II. Scanning and transmission electron microscopy were used to investigate damage to apple leaves caused by the white apple leafhopper (WALH), Typhlocyba pomaria McAtee. Feeding holes created by WALH were found in the lower epidermis, with no visual exterior evidence of cell injury to the upper epidermis. The feeding holes were located in areas of the leaf with high stomatal density, and were therefore in close proximity to stomata. Groups of cells in the palisade layers were empty, or contained coagulated cell contents; adjacent palisade cells were apparently uninjured. These uninjured cells contained an abundance of starch granules, however, possibly indicating that they were not functioning properly. Spongy mesophyll cells dorsal to the feeding area were left intact as were the epidermal cells ventral to the feeding area. There was no evidence of cell wall collapse from the external view of either epidermis, or from internal leaf views. III. Two nine-year-old "Campbell Redchief Delicious’/MM.111 apple trees growing on the Virginia Tech Horticulture Research Farm were used to evaluate gas exchange characteristics of leaves injured by the white apple leafhopper (WALH). Two groups of leaves were used for gas exchange measurements. One group of 42 leaves had three leaves each with 0, 1, 2, 3, 5, 7, and 10 WALH per leaf on each of the two trees. This injury corresponded to 0, 12, 21, 28, 53, 70, and 100 cumulative WALH days, respectively. Gas exchange was measured both in the field and in the lab. A second set of leaves was selected which were visually rated as having zero, low, medium or high amounts of WALH damage, corresponding to approximately 0, 3, 7, and more than 10 WALH per leaf. Gas exchange measurements on the selected leaves were made in the field periodically through an entire day. When measured in the lab, photosynthesis (Pn) was reduced and transpiration (Tr) was increased by WALH feeding. Field gas exchange measurements indicated that WALH feeding reduced Pn and increased Tr before 1200 hrs, but no differences in Pn or Tr were measured after 1200 hrs. No explanation for the conflicting results is offered, and clear conclusions about feeding damage could not be made. / Master of Science