Evaluation location: Washington, United States
S. bulbocastanum germplasm (all 53 populations available in 2012) were obtained from the US Potato Genebank in Sturgeon Bay, WI. Fifty plants, one from each germplasm population, were placed into each of two dome cages. Each cage was infested with 100 adult potato psyllids, which were evenly distributed throughout the cages. Twenty-one days after releasing adults, the numbers of eggs, early instars (first through third instars), and late instars (fourth and fifth instars) present on each plant were counted. The 21-day duration was chosen because the eggs oviposited on the day of adult releases should reach the fifth instar by this time, but not yet molt to adult (Tran et al. 2012). The experiment was repeated three times (trial) with different cohorts of plants and insects for a total of 6 cages. The large number of plant populations included in the assays prevented meaningful statistical comparisons in susceptibility to psyllids among germplasm populations. Instead, plant susceptibility to B. cockerelli 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 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 B. cockerelli.