Girl power

I was hauling logs from under the willows where they'd been left to dry out. Patches of dark scaly substance on the branches caught my eye. Black aphids, clustered tightly together so as to coat the surface. Like you get on broad bean plants, as every gardener knows. But I went to check anyway - black aphids on willow - and found that they are something else entirely: the giant willow aphid Tuberolachnus salignus. And so interesting!

First of all they are huge, at around 5mm long the adults are the largest of all aphids. On closer scrutiny they are not black but have a sort of metallic sheen something in between lead and bronze, with rows of black dots across the surface. But there's also a projecting spike shaped like a shark's fin or a rose thorn rising up from the aphid's back - you can see this in profile on my inset photo. No one knows exactly what it's for.

These aphids can occur in vast numbers and damage willows by sucking sap. They make honeydew (there's a droplet visible in the smaller image, looking like a pearl of toffee syrup) and this may harvested by ants. Or kicked away to be collected by wasps. Both the ants and the wasps deter aphid predators of which the much maligned harlequin ladybird is one.

Not all ladybirds are ladies -- but willow aphids are.No males have ever been found. The female of the species IS the species: reproduction is exclusively parthenogenetic, the aphids giving birth to live offspring which are clones of themselves. The large wingless adults seen here are the mothers of the smaller nymphs which will grow wings and fly off to found another colony. Stay-home mothers and fly-away daughters alternate in the cycle and there's never a mister involved.

But there is a mystery: in February each year the aphid colonies vanish completely for a period of about 4 months. No one knows where they go and what they do during that time.

Better pictures and further info here and here.
(In Texas they come in giant size of course - ride'em cowgirl!)