A microscopic North American aphid that feeds on the roots and leaves of grapevines. Responsible for destroying most of Europe’s vineyards from about 1860-1900. North American vines and hybrids are mostly immune and are used as rootstock for virtually all grapevines today.
Before the days of quarantines and importation standards, humans trafficked freely in plants and animals, thereby transplanting a lot of invasive species that wrought destruction instead of the hoped for boon. When explorers touched down in the New World, they notice the grapes right away. Soon after came wine made from North America grapes. And what retched wine it was. So why not plant a bunch of European grapevines in Canada and the US. The foreign grapes flourished in this hospitable climate, but after a few years they all sickened and died, and for no known reason. Well, if that experiment wouldn’t work, maybe taking the prolific native vines back to Europe would bestow some of the classic European flavour to the wines. Worse than not a very good idea, the North American vines that were planted in France in the late 1850s and early 1860s carried an alien -- an alien that loved these defenseless grapevines. It took barely 20 years for the phylloxera aphid to destroy some 6 million acres of vineyards in France, Spain, Italy, and elsewhere.
North American vines had managed to co-exist semi-peacefully with the bug (it infects mainly the leaves), but the European vinifera grapevines had no such resistance. Many treatments were tried -- some of them irrational and even shocking -- with no success. Because North American vines were holding their own, vinticulturalist began to experiment with cross breeding. That resulted in a large number of “French hybrid” grapes, many of which are still in production in marginal wine-growing areas. But too often the hybrids fall short in terms of the wines they produce. Sometimes they're just rejected out of hand even when superior to comparable vinifera varieties. So the other, and final, solution was to graph European vines onto North American or hybrid rootstocks. That is how it’s now done in virtually all wine regions where phylloxera is an issue. There are some exceptions; Chile, for example, has very strict importation laws and has managed to keep the bug out. These days, research into rootstocks is as important as research in grape growing and winemaking.