A newly found invasion of gall-making Hymenoptera, Leptocybe invasa Fisher & La Salle, that damages Eucalyptus trees is reported here. Since 2000, this wasp has spread throughout most countries of Africa and Europe, causing huge economic losses. In June 2010, this wasp was found on Eucalyptus trees. This pest alert describes the fundamental biological data of adult Leptocybe invasa, and presents its existing host plants and its distribution around the world. Also, we report the distribution of this wasp and its host plants in Taiwan. Because of the overwhelming invasion, this article alerts silviculture companies and pest inspection staff to allow them to reduce the impact of this pest.