American Grapevines Are Resistant to Invasive Insects

Q: Do you know what these three hard “knots” are on this muscadine leaf? Since this is the only leaf I’ve noticed that has them, I’m not worried — just curious. 

A: These knots are caused by phylloxera, a tiny insect that damages roots as well as leaves on muscadine vines. It sucks sap out of a susceptible grape vine and weakens it by perforating the roots and infecting them with a poisonous secretion that stops them from healing. This poison eventually kills the vine. In the late 1800s, European grapes suffered a devastating invasion of phylloxera insects, which are native to North America. Fortunately, American grapes had evolved to be resistant to phylloxera. The roots of American grapes produce a sticky substance that repels the nymphs. If the nymph is successful in wounding the root, American vines generate a layer of tissue to seal the wound.

European winemakers grafted European grape vines to American grape rootstock, thus making these vines resistant to phylloxera. There is really no need to control them on your muscadines since they can resist phylloxera on their own.