Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 145-158). Also available on the Internet.

Both alfalfa weevil, <i>Hypera postica</i> (Gyllenhal), (Coleoptera: Curculionidae), and potato leafhopper, <i>Empoasca fabae</i> (Harris), (Homoptera: Cicadellidae), remain key pests of alfalfa in Virginia. Commercial varieties of potato leafhopper-resistant (or glandular-haired) alfalfa were released in the mid-1990s, but the impact of alfalfa weevil on these varieties has not been well documented. In 1999, two large-scale field experiments were initiated to compare the performance of a glandular-haired alfalfa variety against a standard, non-glandular-haired variety under both alfalfa weevil and potato leafhopper pest pressures in the southwestern and Piedmont regions of Virginia over a 3 year period. Results indicated that alfalfa weevil must be managed in potato leafhopper-resistant alfalfa to limit crop loss. Surprisingly, similar densities of potato leafhoppers were found in both the glandular-haired and standard varieties. Both varieties frequently had similar yields and forage quality. In general, the glandular-haired variety did not outperform the standard variety. Results also indicated that insecticide application did not always provide the expected benefits of higher yields and forage quality, despite reducing pest densities for 2-3 weeks after application. These data suggest that the economic thresholds for one or both of these pests in Virginia may require adjustment. The potential impact of glandular-haired alfalfa on the natural enemies of alfalfa weevil was examined as well. <i>Bathyplectes anurus</i> (Thompson) (Hymenoptera: Ichneumonidae) was the dominant parasitoid attacking weevil larvae at both locations. Parasitization of weevil larvae by <i>Bathyplectes</i> spp. did not appear to be adversely affected by the presence of glandular trichomes on the potato leafhopper-resistant variety. Glandular trichomes had little impact on the infection of weevil larvae by the fungus <i>Zoophthora phytonomi</i> as well, but this was not unexpected. The genetic variation of <i>B. anurus</i> was surveyed at both study sites using RAPD-PCR to establish or eliminate the possibility that differences in parasitization levels between the Piedmont and southwestern regions could be attributed to the presence of different parasitoid strains. Most of the detected phenotypic variation was attributed to within population variation, with very little variation occurring between the two populations. However, the between population variation was statistically significant in 2000, but not in 2001. / Ph. D.

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

Selected insecticides were evaluated to determine effects on the potato leafhopper and its predators. Three types of tests were conducted. First, the common predators of the potato leafhopper in alfalfa were examined to determine their response to the stages of the leafhopper. Second, under field conditions, insecticides were tested to detect effects on the populations of the insects. Finally, LC₅₀ toxicities of selected insecticides were investigated under laboratory conditions. In the predation tests, two studies were conducted, namely, predation on egg stage and predation on nymphal and adult stages of the potato leafhopper. An indirect method of determining egg predation was employed because potato leafhopper eggs are not visible, being concealed in the xylem and phloem tissues. Using two sets of treatments (with and without predators), the potato leafhopper females were allowed to oviposit in predation cages and the suspected predators introduced thereafter. Hatching nymphs were counted and used as indications of the number of eggs laid. Nabis americoferus and Orius insidiosis were found to be predaceous on eggs of the potato leafhopper. In the predation tests, using nymphal and adult stages of the potato leafhopper, the following adult and larval stages of insects were confirmed as predators: Hippodamia convergens, Coccinella novemnotata, and Chrysopa carnea. Two field tests, one in summer of 1977 and one in summer of 1978, were conducted to determine the influence of insecticides on populations of potato leafhoppers and its predators in alfalfa. Samples were taken using a 38 cm sweep net employing a 180° motion at 2 days, 1 week, and 2 weeks after treatment. In both tests, significant control was provided by the chemicals against potato leafhoppers 48 hours following application. A gradual population recovery occurred one week and two weeks after treatment. Most of the entomophagous insects sampled were adversely affected by insecticides. No statistical differences in yield and percent dry matter were obtained with various insecticidal treatments. The acute toxicities of selected insecticides on potato leafhopper, tarnished plant bug, and damsel bug were investigated. The bioassay method used was the immersion technique and toxicity was expressed in LC₅₀. The relative toxicities of Nabis americoferus to 50° Empoasca fabae and Lygus lineolaris were obtained by deriving the selectivity ratios. Selectivity ratios are calculated by dividing the LC₅₀ of the non-target organism divided by the LC₅₀ of the pest. The ratios indicated that most of the insecticides tested were highly selective. Among the organophosphates, azinphosmethyl was the most selective, while among the carbamates, carbaryl was the most selective. / Master of Science