Wednesday, 7 July 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

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