High phenolic content fails to deter mesograzer consumption of Myriophyllum spicatum (Eurasian watermilfoil) in New England

Eurasian watermilfoil (Myriophyllum spicatum) is often considered one of the most aggressive macrophyte invaders in freshwater habitats throughout the USA. However, conditions leading to successful milfoil invasions are not well understood. This study sought to illuminate the role of herbivores in determining milfoil invasion success via the potential mechanisms of enemy release and biotic resistance. We determined feeding preferences of three herbivores native to the northeastern United States and measured macrophyte phenolic content, which may act as an herbivore feeding deterrent. We found that phenolic content in milfoil was more than two times higher than in the most abundant native macrophytes at our study sites, consistent with enemy release. However, laboratory feeding experiments demonstrated that milfoil phenolics did not deter amphipod (Hyalella azteca), snail (Physella sp.), or weevil (Euhrychiopsis lecontei) herbivory. Furthermore, amphipod consumption rates in our study were an order of magnitude higher than amphipod consumption rates reported in milfoil’s native range, contrary to the predictions of enemy release. Amphipods and snails from habitats invaded by milfoil consumed similar quantities of both milfoil and the low-phenolic native plant Elodea canadensis. In contrast, weevils consumed milfoil but not E. canadensis in choice experiments. Amphipods collected from milfoil-free habitats also readily consumed milfoil, and they consumed 2.5 times more milfoil than E. canadensis in a choice feeding trial. These results suggest that high phenolic levels do not prevent native herbivores from consuming invasive milfoil. Instead, native generalist grazers like amphipods and snails may limit milfoil proliferation and provide a measure of biotic resistance.