Mystery Caterpillars by John Agar

Below FoCM Treasurer, John Agar describes some mysterious caterpillars that he spotted on a Chorlton postbox:

On the 10th of June, driving along Ryebank Road, Chorlton, I was puzzled to see that the normally red postbox appeared to be green. On closer inspection I was surprised to see it was covered in hundreds of green caterpillars. I had no idea of what the species might be, however, since moth species greatly outnumber those of butterflies I assumed they must be moth caterpillars.

The postbox was situated approximately three meters from an ash tree growing in a neighbouring garden and behind a privet hedge, clearly one or both must be the host plant. I checked several books, but given that there are in the region of 2,500 species of moth, and many green caterpillars, I was unable to identify the species.

Fortunately help was at hand in the person of, fellow FoCM member, Ben Smart. Imagine my surprise when Ben informed me that the caterpillars were not that of a moth but of an Ash Sawfly, the shape of the head and the five or six pairs of abdominal prolegs being the key to identification. Ben further informs me that there are seven British species that feed on ash (http://tech.groups.yahoo.com/group/sawfly/) .

On checking the photos he`s sure it is the Ash Sawfly (Tomostethus migritus) a species that can experience population explosions leading it to devastate foliage. Ben`s view is that the larvae had descended from the ash in search of further food plant.

The tree had been severely cut back in recent times and the foliage present, although limited, showed no sign of damage. Is it possible that leaf litter from the tree could have been blown into the box and the larvae were emerging from within?
It seems improbable but perhaps not impossible. Why if the larvae were seeking further food plant would they be concentrated in such large numbers on a bright red postbox? I assume that, in common with other flies, they have compound eyes and see colour differently to us, perhaps that answers the question. If anyone can enlighten me please do so.

Ben visited the site but could only find one larva, which he took to try and rear through to absolutely confirm ID.

I am indebted to Ben for his help and input.

John Agar, July 2010

Note:

The Sawflies: Suborder Symphyta

The insects in this suborder have no obvious `waist `between the thorax and the abdomen .They get their name because most females have a saw-like ovipositor, although in some species it works more like a drill. The eggs are nearly always laid inside plant tissue and the larvae are all vegetarian. There are over 400 British species.

Reference:

Collins Complete British Insects by Michael Chinery