Co-extinction is the extinction of a species following the extinction of another species that it used as a resource, such as a food plant in the case of insect herbivores. The magnitude of the global co-extinction threat to two herbivorous insect taxa (aphids and scale insects) was estimated by compiling a list of species in these groups that are dependent on globally threatened trees. Eleven species of aphid (0.69%) and thirteen scale insects (1.15%) have a threatened tree as their sole host. This measure is comparable to recent estimates for insect herbivores, but far less than the published overall estimates of extinction risk for invertebrates, and highlights the dependence of insect herbivores on a wide range of habitat features.