Evaluation location: Washington, United States
S. verrucosum germplasm from all 41 populations that were available in 2012 was obtained from the US Potato Genebank in Sturgeon Bay, WI. One plant from each germplasm population was placed into each of 6 dome cages (60×60×60 cm; MegaView Science Co. Taichung Taiwan) so that each cage contained a total of 39 plants. One hundred adult potato psyllids were evenly distributed throughout each cage. After 21 days, the numbers of eggs, early instars (first through third), and late instars (fourth and fifth) present on each plant were counted. Plant susceptibility to potato psyllid was ranked based on the mean numbers of eggs, early instars, and late instars observed on the plants at the end of the experiment. The lower and upper quartiles associated with the mean numbers of each insect age class, regardless of germplasm population, were calculated using SAS 9.3 (SAS Institute 2012). The least susceptible populations were identified as those with mean numbers of eggs, early instars, and late instars each below the lower quartile of their respective insect age class. The most susceptible populations were identified as those with mean numbers of eggs, early instars, and late instars each above the upper quartile of their respective age class. Germplasm that supported numbers of at least one insect age class between the lower and upper quartiles were ranked as having intermediate susceptibility to potato psyllid.