Evaluation location: Maryland, United States
Thirty-six ramets were planted in a completely randomized block design, with two blocks and three trees per clone per block, in a field plot at the USDA, USNA, Agricultural Research Service (ARS), station near Glenn Dale, MD. A laboratory colony of the potato leafhopper (Empoasca fabae (Harris)) was started in the summer of 1995 and invigorized yearly with adults collected in a potato field at the Beltsville Agricultural Research Center, ARS-USDA, Beltsville, MD. Insects were reared on a mixture of Henderson's bush lima bean (Phaseolus lunatus L.) and fava bean (Vicia faba L.) kept in clear plastic cages (60 by 60 by 30 cm, or 60 by 60 by 60 cm). Cages were maintained in a walk-in chamber at 25.5°C, 60-80% RH, and a photoperiod of 16:8 (L:D) h. For seven consecutive weeks during the 1998 season, one branch on each of two trees per clone per block was randomly selected. Potato leafhopper females, about 1 wk old, were collected from the laboratory colony in groups of five, and released into organza sleeves (30.5 by 38.1 cm) tied around branches as previously described (Bentz and Townsend 1999). Females were allowed to oviposit for 6 d. At this time, sleeves enclosing the branches with leaves were cut off the trees, brought to the laboratory and stored in a freezer at -20°C to kill the ovipositing females. The leaves within sleeves were collected and wrapped. Staining and counting of leafhopper eggs within leaves was done as previously described (Bentz and Townsend 1999). At each of the seven weeks during the 1998 season, a shoot from each of two trees per block of the six clones was randomly chosen, inspected for the presence of leafhoppers, and tagged if it had no evidence of leafhopper presence. Each tagged shoot was covered with a white organza sleeve (30.5 by 38.1 cm) as described in Bentz and Townsend (1999). Second instars were collected from the laboratory colony and released in groups of two into each organza sleeve. Nymphs were left in the sleeves until becoming adults. At this time, sleeves enclosing the shoots were cut off, brought to the laboratory and stored in a freezer at -20°C. Later, leaves were removed from the sleeves and adults counted.