Evaluation location: Illinois, United States
Ulmus taxa leaves for the no-choice (NC) laboratory feeding bioassays were randomly collected from ground level from all four cardinal directions and held in cold storage in plastic bags at 5 C (41 F) for a maximum of 2 d. Leaves collected from each test tree were combined for the NC laboratory feeding bioassays. Depending on availability, one to three individual trees of each Ulmus taxon were evaluated. Adult beetles used in the NC study were field-collected from plants at The Morton Arboretum, Lisle, IL and The Chicago Botanic Garden, Glencoe, IL. Upon collection, the adult beetles were held in clear plexiglass cages under a photoperiod of 16:8 (L:D) h at 25 C (77 F). While being held in the cages (no longer than 12 hours), the beetles were allowed to feed on fresh crabapple (Malus spp.) foliage to ensure predisposition to feeding. Prior to the beginning of the feeding trials, the Japanese beetles were sexed, and one adult male/female Japanese beetle pair was placed into each of 10 clear plastic petri dishes (10.0 cm diam by 0.6 cm depth) along with one leaf of the specific Ulmus taxon to be tested. Each beetle pair was used only once. There were 10 dishes (sub-replicates) for each tree for each taxon evaluated for a total of 10 to 30 male/female beetle pairs per taxon. Petri dishes were examined daily for beetle mortality and evidence of feeding. Foliage was replaced every 2 d. At the time of leaf removal, the leaves were visually rated (nearest 5%) for the percent of leaf tissue removed, by two independent estimators using a defoliation template. Petri dishes were placed in clear plastic bags to prevent drying of the foliage and were held in a growth chamber under a 16:8 (L:D) h photoperiod at ∼ 25 C (77 F). Condensation of water on the lid of the petri dish indicated a high relative humidity. At the end of seven days, the remaining foliage and fecal pellets were removed from each petri dish and the fecal pellets were oven dried and weighed (nearest mg). The NC feeding bioassays were terminated after 7 days. The measure of the susceptibility for each taxon was defined by mean percent leaf tissue removed and mean dried fecal pellet weights.