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

Report (rather late!) on the Birdwatching For Beginners walk on 6th June 2010

12 hardy people attended this event, taking place on a drizzly Sunday around Sale Water Park and Chorlton Meadows. The walk encompassed a variety of habitats around this diverse part of the Mersey Valley.

FoCM member John Agar had arranged for the feeders outside the visitors’ centre at Sale to be topped up with food, and, sure enough, his foresight paid dividends as, waiting for everyone to arrive, we were lucky enough to watch a Greater Spotted Woodpecker repeatedly come to take food. This gave us all a chance to test out our binoculars, many of which loaned to us by the Mersey Valley and Countryside Warden Service. Thanks again to them for their assistance on the day.

We set off towards the dipping pond adjacent to the visitors’ centre, pausing along the platform to take in the terrain over the marsh. As if on cue, soon after our arrival, we were treated to the sight of a Kestrel appearing overhead and taking up its distinctive hunting behaviour by hovering above the marsh, only a few feet above us. Even better, we witnessed it make a successful kill, a few of the mammal knowledgeable among us even able to identify the prey dangling from its talons as a short-tailed field vole, which the Kestrel took to a nearby tree to eat. This was a wonderful piece of luck (for us, not the vole!), and goes to show you never know what you’ll see, even on what at first seemed like a wet day, low on promise.

Our walk continued along the brook towards Sale Water Park, from the banks of which we added to our list a Sedge Warbler, betrayed to us by its messy trilling call. Our patience paid off as we were also able to catch glimpses of this elusive bird as it volleyed in and out of cover.

Around the lake we added many more species, including Lapwings, Little Grebes and a family of Mute Swans by the bird hide at Broad Ees Dole.

Our final stretch took us through Chorlton Ees, some of the eagle-eyed among us picking out from nearby woodland a number of Herons hidden among the branches. This was the Heronry, a place where Herons nest and breed, and one of Chorlton Meadow’s ornithological jewels. The more we looked, the more we saw, including one posing at the top of a Larch, silhouetted against the sky analogous more to a mango grove than a beauty spot 2 miles outside the city centre!


Our final ‘star’ species was the Reed Warbler, singing constantly from cover in the reed beds at the heart of the meadows. Hearing this bird’s repetitive and chirring song, as opposed to the more variable song of the Sedge Warbler, enabled us to put our ears to the test and appreciate that for many bird species it’s the aural realm where they’re best admired. A few of us saw the bird occasionally flit through the reeds, but it largely remained in cover, brilliantly camouflaged against last year’s dead reed stems.

For more information on the birds in and around Manchester, pay a visit to the Manchester Birding website, where, among many gems, you can find up-to-date records for what’s been seen in and around Chorlton Meadows:

http://www.manchesterbirding.com/

A complete list of birds we saw on the day.

Blackbird - Sale Visitor Centre
Black-headed Gull - Broad Ees Dole
Blue Tit - Sale Visitor Centre
Canada Goose - Broad Ees Dole
Carrion Crow - various locations
Chaffinch - Cow Lane
Chiffchaff - Broad Ees Dole/River Mersey
Coot - various locations
Cormorant - flying east to west towards Sale
Goldfinch - Stretford Ees/Turn Moss
Great Spotted Woodpecker - Sale Visitor Centre
Great Tit - Sale Visitor Centre
Greenfinch - Hardy Farm
Herons and Heronry - Broad Ees Dole, Chorlton Ees
House Martin - Stretford Ees/Turn Moss
Kestrel (successful kill) - Sale Visitor Centre
Lapwing - Broad Ees Dole and Stretford Ees
Little Grebe - Broad Ees Dole
Long-tailed Tit - Broad Ees Dole/River Mersey
Magpie - various locations
Mallard - Sale Water Park
Moorhen - Various locations
Reed Bunting - Sale Water Park and Chorlton Ees
Reed Warbler - Chorlton Ees
Robin - Cow Lane
Sedge Warbler - Sale Visitor Centre
Song Thrush (star singer!) - Stretford Ees/Turn Moss
Swallow - Mersey overflow
Swan with 6 signets - Sale Water Park
Swift - Stretford Ees/Turn Moss

Julian Robinson, August 2010

Small Ermine Moths by Ben Smart

Many people have remarked, this summer, on the webs smothering certain local trees. Our local Moth expert, Ben Smart, explains all below:

You may have noticed large grey webs coating the leaves of a variety of trees this year. These will usually be made by caterpillars of different species of Small Ermine Moths, and these seem to have appeared in large numbers this year. The particular species can usually be identified on the basis of the tree on which it is feeding. Each web contains large numbers of caterpillars (approx 20-50) feeding gregariously in May and June. Once fully fed the caterpillars move to a nearby branch, form a fresh web, and enter the chrysalis stage of their life-cycle. Again, they do this gregariously, and if you look inside one of these pupal webs, you may see large numbers of chrysalises all lined up together each in its individual white silken cocoon. The adults tend to emerge in June and don’t move far from the foodplant so you may see the adult moth sat on a leaf or on the trunk of the tree. All have small black spots on a white background and are about 1 cm long. The different species are so similar it is often easier to identify the moth by its foodplant.

There are five species that may be found in Chorlton.

Bird-cherry Ermine Moth (Yponomeuta evonymella) – This is the commonest of the family and feeds gregariously on Bird-cherry. It may defoliate the foodplant so much that webs may be formed on neighbouring plants even if unsuitable, in a desperate bid to get the nourishment required for successful development of the caterpillar. This moth can be differentiated from the others as its black spots are smaller and more numerous than the other related species.

Orchard Ermine (Yponomeuta padella) – Caterpillars form webs on hawthorn and blackthorn. The chrysalis has a greenish body with black wing cases. The moth, which emerges from the chrysalis after about two weeks, has a slightly greyer tinge to the forewing than its close relatives.

Apple Ermine (Yponomeuta malinellus) – Webs of these species can be found on the apple trees at Chorlton Water Park. Each contains lots of dense silk, half-chewed foodplant, lots of droppings, and tens of caterpillars. The adult moths are white with grey tips at the end of the forewings, and a small number of black spots.

Spindle Ermine (Yponomeuta cagnagella) – Quite an unusual moth in Chorlton, due to the relative scarcity of its foodplant, Spindle. As with all of these species, with the exception of Y.rorrella, each chrysalis is protected by a white, silken cocoon. The adult moth has a white forewing with a small number of black spots.

Willow Ermine (Yponomeuta rorrella) – A few webs have been found this year on White Willow close to Jackson’s Bridge on the north side of the Mersey. The caterpillar is typical of the Small Ermine moths in that it is grey with a black head and black spots. The adult has a greyish patch on the forewing and grey tips to the wings. The black dots are smaller in size than for most of this family.

Confusion species – There is a moth called the Thistle Ermine (Myelois circumvoluta) that looks similar to these, but is larger, about 1 ½ cm long, and has larger black spots. The caterpillars of this species do not form webs but feed over the winter in the stems of Spear Thistle.

Ben Smart, July 2010