Evaluation location: Illinois, United States
We used newly emerged and unfed elm leaf beetle (Xanthogaleruca luteola) larvae hatching from eggs laid on the foliage of Ulmus pumila seedlings. These seedlings were growing in 7.6-liter pots and covered with light screen mesh to prevent beetles from escaping. Seedlings were held in the laboratory at 25C (77F) and under 16:8 hr (L:D) photoperiod. Adult beetles were reared from late instar larvae and pupae collected from U. pumila trees at North Platte, NE, and shipped overnight to The Morton Arboretum, Lisle, IL. On arrival, these larvae and pupae were held in clear Plexiglas cages in an incubator at 25C (77F) and 16:8 (L:D) hr photoperiod. As adults emerged, they were released onto U. pumila seedlings and allowed to feed, mate, and oviposit. For the host suitability test, we used 24-hr-old X. luteola larvae. Neonates of X. luteola initially cluster at or near the egg mass before initiation of feeding. These were randomly selected from an egg mass and transferred to a single leaf of the test elm that was placed in a plastic petri dish (0.6 × 10.0 mm). Ten such petri dishes were used for each single tree replicate and there were three trees per elm accession. Petri dishes were placed into a clear plastic bag to retain moisture. The petri dishes were checked daily for larval mortality, evidence of feeding, prepupation, pupation, and adult emergence. Candidate elm accessions growing at The Morton Arboretum were approximately 2 m (6.6 ft) high and growing in 8 liter (2.1 gal) pots. Leaves for the bioassays were randomly collected from the trees from all four cardinal directions. The leaf samples included the terminal 15 cm (5.9 in) of elm branches. Only fully expanded leaves were used. Leaf samples were taken in this way to compensate for variation in leaf quality within trees. Leaf samples were held in cold storage in plastic bags at 5C (41F) for a maximum of 2 days. Leaves collected from each test tree for each biotype were combined for the bioassays. Larval bioassays ceased when larvae died or when adults emerged. Larval suitability for each biotype was defined as the mean development time from larva to adult, mean proportion of larvae pupating, mean percent adult emergence, and mean pupal weight.