Wednesday, 5 October 2016

Leaf Mines

A reminder (if one is still necessary!) that this is the best time of year to find the early stages of some of our smallest moths.  Not only does it give you something 'mothy' to do this month while the contents of your trap get ever fewer, but you can add, perhaps significantly, to your garden list and you'll be producing much-needed records of some species which are often difficult to identify at the adult stage.

Many leaf-miners are specific to a particular food-plant so if you've got, say, some hawthorn on your boundary fence it could easily produce four or five species straight away.  There are excellent ID resources available on line, for example here, and the only real pre-requisite is that you can correctly identify the host plant.  A combination of that along with the characters of the mine (type, shape), the egg (if present, side and position on leaf), the larva (shape, colour) and its frass (colour, dispersal) can be used to obtain a positive ID in the majority of cases.  Quite a few vacated mines can also be identified with certainty.  A hand lens (x10) is usually sufficient to enable you to check any or all of these features.  You need to be aware that the larvae of some beetles, flies and wasps can also mine leaves but it doesn't take long to get used to their different characteristics.

There are two basic types of mine, those that create a corridor and those that create a blotch (or sometimes there can be a combination of both).  A few examples from my garden over the last week are illustrated below:

Corridor, Phyllocnistis saligna on willow

Corridor then blotch, Stigmella obliquella on willow

Contorted corridor which from a distance can look like a
blotch, Stigmella viscerella on elm

Initially a blotch then moving to a folded leaf edge,
Parornix devoniella on hazel
Blotch, Phyllonorycter corylifoliella on hawthorn

For mines that I'm unsure of I find it easier to photograph the leaf and then look at it closely on the computer screen.  For this a light source behind the leaf is needed, as in the two Stigmella pictures above, which will then give you a clear view of the interior contents of the mine (larva and frass).  Some computer printer/photocopiers can do this quite well, but I use the simple option of photographing the leaf without flash looking out through my study window, having taped it to the glass.

Dave Wilton
Westcott, Bucks

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