Walking the paths through the woodlands it is usually the larger things that attract attention, trees, birds, occasional flowers, - the dog, the mobile phone - but a closer inspection of the plants beside the path may reveal some strange markings on some of the leaves.
No, they are not messages in some strange script from inter-planetary travelers, they are the tracks made by tiny insect larvae as they eat their way through the soft leaf tissues between the tougher surface layers. When you ‘get your eye in’ you can find several different examples and a couple of websites will help identify the species making these tracks [UKflymines and leafminers]. Most are made by the larvae of small flies and many of the species are specific to one or a group of closely related plant species. They are easier to see in late summer when the leaves are still functioning, but the signs of this activity can still be found even now on some of the leaves that remain. A selection of those seen recently are shown below
The larva of the fly making these mines in Red Campion can just be made out in one of the galleries.
Even the tough leaves of Holly are not immune from attack, but there is only one species, Phytomyza illicis, that can live in these leaves, though the plant can sometimes fight back. By forming thickened callous tissues where the leaf is damaged the fly larva can sometimes end up being crushed.
Against the dark green of the holly leaf these mines are quite conspicuous and those that escape crushing may be subject to predation by either Blue- or Great-tits. This may be the cause of the blotch part of this mine having had it's surface torn off.
While birds may manage to pluck larvae out of their mines, the main cause of mortality at this stage is said to be a range of tiny parasitic wasps that can penetrate the gallery tissue to lay an egg either inside or beside the larva, after it hatches, the wasp grub then eats the fly larva.
While some mines affect only a small part of the leaf, as in most of those shown above, some can almost completely destroy even a large leaf as in the mines in the Burdock leaf below. (Although the suggested species in this photo certainly fits the appearance of the mine, its distribution in the UK is said to be in the southern counties of England, though there is an accepted record from north of Inverness so it may be just poorly recorded.)
Other plants too can be seen to suffer significantly when there is a high infestation of leaf mines as in this Smooth Sow-thistle with mines of, most probably, Chromatomyia 'atricornis'.
The authors of these ‘leaf mines’, as they are collectively
known, will by now mostly have pupated, often in the leaf litter on the ground, where they will remain until next spring or summer.
Leaf mines can be found in some of the plants growing by the paths in all the woodlands and are a further indication of the wider diversity of mostly invisible insect life that the woodlands support.
Being inside the leaf may protect many of them while they are larvae but when they emerge as adults they are fair game for predators. After all, not all woodland birds benefit from garden bird feeding, some, like Wrens, Goldcrests, Long-tailed Tits and Warblers depend on a continuous supply of small insects over different periods of the year. Watching Pipistrelle bats recently on an October evening, as they circled and darted around over the Heritage Viewpoint area, it was clear that they were catching insects that were much too small to be easily seen. Adults of the leaf miners could well form part of this important, but little noticed, population of small insects.
It just goes to show that it is not just the large, colourful, and charismatic - bumblebees, butterflies and moths - that are important in woodland ecology, the small and rarely noticed also have a role. This group of insects will also have benefited from the expansion in the range and diversity of woodland edge plants following the Ivy clearing work of our Primary School children and adult volunteers - another benefit of our strategy of improving the woodlands for wildlife as well as people.