Decorated in a peppering of black spots, this dark-brown aphid has been cited as the largest in the world, measuring nearly 6mm long. Adults of the species can be readily identified by the thorn-like tubercle which protrudes from the aphids back. As with other species of aphid, some giant willow aphids are winged; an adaptation believed to emerge in reaction to environmental pressures. Giant willow aphids appear to reproduce entirely asexually, differentiating them from other aphids which are heterogamous, as no males of the species have been recorded. Found primarily in large colonies on the trunks and branches of willow and sallow trees, the aphids splay their back legs and kick in response to potential threats.