The Seattle Post Intelligencer is reporting that at least some of the milfoil (Myriophyllum spp.) taking over waters in northern Idaho is a hybrid between native Northern water milfoil (M. sibiricum) and Eurasian water milfoil (M. spicatum). Not only is the hybrid milfoil tougher than its parents, but scientists say it's also tough to determine whether you've got the hybrid unless you're studying it in the lab.
While the state of Idaho has been pouring money into non-specific herbicides to eradicate water milfoil from bodies of water, there is anecdotal evidence suggesting the hybrid water milfoil has greater resistance to chemical treatment. One environmental group is inquring about a biological control, but it sounds like it is going to be pretty hard to use it yet spare the native plants - the insect they typically use to control European water milfoil is a weevil (Euhrychiopsis lecontei) that was discovered on Northern water milfoil (read this article by Roley and Newman for more info).