Holly leaf gall

See also: Holly; gall

Holly leaf galls are a feature anyone walking past a holly tree can spot at any time of year, and yet are created by an insect so tiny that most of us would never bother to look for it.

Holly trees are a prominent sight throughout the year with their glossy evergreen leaves, cheery red berries and distinctive shape. Take a moment to peer more closely at the leaves however, and instead of lustrous green many of them will be spoiled by brown discolouration. This marring is caused by the egg laying of the minute holly leaf gall fly, a tiny creature only 3mm long, but which is so prevalent that you will find very few trees that do not have galls present.

Find an affected leaf and turn it over, and on the pale smooth side there will be a dark brown spot with a tiny hole in, surrounded by an area of browny green discolouration. The fly lays its eggs on the underside of the leaf and the hatching larvae munch their way into the nutritious central vein of the leaf and along the palisade parenchyma cells. (These cells create the overall inner structure and support for the leaf and are stuffed full of chloroplasts - complex sugars which the larvae feed upon.)

You can trace the steps of the gall larvae as is moves into the outer parts of the leaf under the waxy surface by its discoloured trail, ending in an exit point where the larvae pupates into the adult gall fly.

The adult gall fly is only 3mm in length, small and black with clear wings. You are unlikely to distinguish it from the myriad of other small, black, undistinguishable flies without a good hand lens or a microscope.

The gall fly emerges from its leafy home in May. This coincides with a time of year when the holly is producing new growth, and unlike the tough, waxy older leaves, these are soft and pliable. The flies are so small that they are unable to eat the tough older leaves but can munch on the softer new ones. This is also where they oviposit, or lay their eggs, setting out next years generation of gall flies.

Usually only one gall fly egg is laid per leaf. The egg hatches after about ten days and the larvae begin to burrow into their new home. Gall fly larvae spend ten months of the year within their leaf and although they are tiny, two larvae on a leaf can mean serious food problems. The larvae won't cross into another's mine, and they won't go backwards, so where two occur one can sometimes eat their way into a corner and be unable to retreat, eventually starving to death!

Wasps and other carnivorous insects will tear open leaves and pluck out the defenceless larvae within, as will birds such as blue tits and bush tits. The flies will be eaten by birds, mammals and carnivorous insects which is why they are so prevalent, as they are food for so many!

Holly trees are pretty much unaffected by the activities of the gall fly. They have a tendency to drop a few leaves in the spring regardless and studies have shown there is often a higher number of galls on the fallen leaves than are left on the tree. It is also thought that due to the stress on the leaf, larvae infested ones will age and drop faster than normal, healthy leaves. The only real problem is for tidy minded gardeners who hate to see disfigurement to their trees!

Sources:
British Wildlife - Collins Wild Guide
http://crawford.tardigrade.net/bugs/BugofMonth10.html