Two introduced insects have come to dominate Siberian elm leaf infestation in recent years. By far the most damaging in every corner of the state is European elm flea weevil. It was only recognized as present in 2008 and is rapidly expanding its range. Adults of this insect punch shothole wounds in emerging leaves. They will mate and females lay eggs along interior leaf veins. The larvae develop as leafminers and create serpentine patterns before pupating and emerging as adults in late May and June. Adults are present through the end of summer and their feeding produces a lacey appearance in leaves. Only one generation is produced annually.
The second most dominant along the Front Range is elm leafminer, also from Europe. Sawfly adults are present in the early spring and lay eggs as leaves emerge. Larvae mine leaves for only 3 to 4 weeks before dropping to the ground to tunnel in until adults emerge in late winter. A single generation is produced each year.