Field and garden experiments were performed to investigate if pathogens, generalist herbivores, or a specialist herbivore (the tortricid moth Pristerognatha fuligana) have the potential to control the invasive Himalayan balsam. Impacts of generalist herbivores and pathogenic fungi were excluded by using thiacloprid and trifloxystrobin. Results show no effect of any antagonist in the field experiment, though we found a modest impact of the combined influence of generalist herbivores and pathogenic fungi in the garden experiment, i.e., under suboptimal growing conditions. Limited information from the native area (the Himalayas) suggests a strong impact of specialist herbivores, thus confirming the enemy release hypothesis. We predict that impact studies of native specialized herbivores will clearly indicate antagonists from the native area with a strong impact on Himalayan balsam in its invaded area.

Nomenclature: Thiacloprid; trifloxystrobin; Himalayan balsam, Impatiens glandulifera J.F. Royle