Green ash is threatened by the emerald ash borer, a beetle introduced accidentally from Asia. Asian ashes have a high tannin content in their leaves which makes them unpalatable to the beetle, while most American species (with the notable exception of blue ash) do not. A common garden experiment showed that green ash is killed readily when exposed to emerald ash borer, while the Asian species "F. mandschurica" shows resistance against emerald ash borer. The United States Forest Service has discovered small numbers of green ash in the wild that have remained healthy after emerald ash borer swept through the population. The possibility of these trees possessing genetic resistance to the beetle is currently being investigated with the hope that green ash could be restored using the surviving trees.