North America is a continent with many points of entry for invading plants and animals. Its climate zones go from Arctic to tropical so there is a comfortable zone somewhere for any invader. If the newcomer has left its enemies behind, a veritable breeding frenzy ensues. Once the continent has been breached, with the current state of biology, we’re stuck with the invader forever.
A relative newcomer is once again sweeping North America since its introduction (probably accidentally) from Europe where it is native. Dog strangling vine or swallow wort (Cynanchum rossicum) is related to milkweed and grows almost to 2m in dense clumps, hence its common name. Quite at home in southern Ontario, it has been around for decades and is regarded as a bigger menace than garlic mustard by many. In Toronto’s High Park, desperate measures (using Roundup) have been used after physical attempts to remove the plant failed.
The seeds look a little like milkweed seeds and in fact the plant can confuse Monarch butterflies into laying eggs on it. Given a choice, Monarchs will lay eggs on DSV 25% of the time. Monarch eggs laid on DSV will not survive.
This plant seems to have few redeeming features although cattle will eat it.
The answer to this problem (once again) seems to lie in biological control through careful importation of benign enemies. This takes time to ensure that adding another import to the long list doesn’t backfire.
One can only wonder which other invaders are waiting in the wings.