Species of the Month - December 2014

Giant Willow Aphid

Tuberolachnus salignus

Said by some to be the world's largest aphid, the Giant (or Large) Willow Aphid is also one of the least understood.  It's an enigmatic creature that breaks all the rules.  Normally a rare beast, with only 4 dots on the NBN map of Britain, it is having a bumper year with sightings reported from many parts of northern Scotland.

The aphid is found in large aggregations on willow branches.  It prefers branches of about 1-3 cm diameter, that are old but not covered with moss or lichen.  Groups of aphids can be seen from a distance as dark patches on the underside of branches.

When you find a willow, scan it for dark patches, and if you find any check that they are made up of aphids and not caused by something else.  The aphids are unmistakable.  The adults are up to 5 or 6 mm long and have a tubercle on their back like a shark's fin.  Aphids of all ages and sizes will be present, since they reproduce continually through the winter.  The younger ones do not have the tubercle, but they are still distinctive with the rows of black spots on a grey background.  Nothing else that is at all similar forms patches on willow in winter.
 

No male Giant Willow Aphid has ever been found.  The colonies are made up of females who give birth to live young who are genetically clones of their mother.  Some adult females grow wings, but most do not.
 

This picture shows the "shark's fin" on the back of an adult aphid.  This is easily seen if you view them from the side.  No other species of aphid has this feature, and its purpose is not known.

Unlike just about every other insect, the Giant Willow Aphid is most active during freezing weather, when they may leave the trees and walk about in a fast and purposeful manner.  In the summer, by contrast, they completely disappear.  No-one knows where they go between March and August.  Perhaps this year, when they are so much more plentiful than usual, someone will find out.

Here are some links to articles about this fascinating creature.  They are well worth reading.

• Highland Biological Recording Group
• Natural History Museum
• Influential Points
• Harper Adams Univeristy

The photos on this page are taken from the Influential Points site under a Creative Commons Attribution 3.0 Unported License, as I don't yet have any decent photos taken by me or other LNHG members.  So the next challenge after finding the aphids is to take some really good macro shots of them and also more distant shots showing the dark patches on the trees.  Then we can replace these photos with our own.