Despite being the dead of winter, some deciduous trees still have leaves on them in New England. Oak, Beech and Hornbeam often hold old, dry leaves left from the summer. These marcescent leaves die in the fall but they aren’t shed.
A layer of cells, called the abscission layer, develops at the stem base. The leaf falls off when this layer is completely formed. The separation layer doesn’t fully allow the leaves to detach. That’s why most of their dead leaves remain on the tree through much of winter until the wind rips them away
And it is this fact that allowed me to spot something I hadn’t noticed before, an oak tree full of see-through leaves. I held one of the old leaves up to the winter sun and it acted like a window. I tore the leaf in half and found that there were clear upper and lower leaf layers with an air pocket between them. There appeared to be some tiny little specks of black frass, also known as insect poop, in the space.
It turns out this oak tree was attacked by Leafminers during the growing season. These oaks are attacked by Solitary Oak Leafminer (Cameraria hamadryadella) and Gregarious Oak Leafminer (C. cincinnatiella), moths in the Gracillariidae family. As suggested by their common names, one feeds alone in a mine only joining when the mines come together on a crowded leaf, while the other feeds jointly inside a single, large mine. These leaves I found had large, open mines, suggesting the party atmosphere of the latter.
There are two generations each year. The second-generation larvae spend the winter in the leaves on the ground. Leaves are thin and the mines have low ceilings. The caterpillars have flattened heads and mouthparts specialized for feeding on the narrow area of spongy mesophyll, the photosynthetic cells found between the upper and lower epidermis layers. Infestations like the one I found do occur occasionally, but they tend to be short-lived and usually have very little impact on long-term tree health.
A chance encounter with a translucent leaf on a sunny, winter day opened a window for me to learn about the tiny Oak Leafminers