NIW Insect of the day: Large willow aphid

Posted 23 June 2014

This Insect of the Day feature has been produced as part of Harper Adams University's National Insect Week celebrations, June 23-29.

The large willow aphid by Professor of Entomology, Professor Simon Leather

The large (giant) willow aphid, Tuberlolachnus salignus, is, in my opinion, one of the world’s greatest unsolved mysteries. 

This aphid is sometimes regarded as being the largest aphid in the world. It can reach a length of 5mm, can weigh up to 13mg as an adult, and the new-born nymphs weigh about 0.25mg. You can get an idea of how big it is from the photograph (see right).
 
This is pretty big for an aphid, although not quite as big as one of my former PhD students, Dr Tilly Collins, liked to pretend! The photograph (see right) used to appear on her website and was the envy of a number of Texan entomologists.

Tuberlolachnus salignus, unlike most aphids, does not feed on leaves, but on stems and twigs and thus feeds through the bark. Its mouthparts, known as stylets, can measure up to 1.8mm, more than a third of it’s body length.
 
A great unsolved mystery about this aphid is the function of the dorsal tubercle, which so closely resembles a rose thorn, or to me, a shark’s fin. Nobody knows. Is it defensive? Unlikely, since T. salignus being a willow feeder is stuffed full of nasty chemicals and very few predators seem to want, or be able to feed on it. 

They feed in large aggregations on the stems of their willow tree hosts and can have serious effects on tree growth. The aphids produce a lot of honeydew, and are often ant-attended and these also deter potential predators. 

In fact, the aphid colonies produce so much honeydew in the summer that they attract huge numbers of vespid wasps that are in search of energy-rich sugar sources at that time of year. These too are likely to make potential predators and parasitoids think twice about approaching the aphids.

In addition, if you crush one of the aphids you will discover that it stains your fingers bright orange and that this stain will last several days if you don’t try too hard to wash it off. If you get this aphid ‘blood’ on your clothes they will be permanently stained.

The second mystery is that every year, in about February, it does a disappearing act and for about four months its whereabouts remain a mystery. 

We have an aphid that spends a substantial period of the year feeding on willow trees without leaves and then in the spring when most aphids are hatching from their eggs to take advantage of the spring flush, T. salignus disappears! Does it go underground? If so, what plants is it feeding on and why leave the willows when their sap is rising and soluble nitrogen is readily available?

So here is a challenge for all entomological detectives out there. What is the function of the dorsal tubercle and where does T. salignus go for the spring break? Truly a remarkable aphid and two mysteries that I would dearly love to know the answers to and yet another reason why I love aphids so much.