Linda Jerris, age 12, of Wichita, Kansas for her question:
What causes galls on leaves?
The plant world is attacked by a countless variety of galls. Some cause nodules on the roots underground, others cause hard or soft bumps to grow on stems, blossoms, leaves or twigs. These galls may be the work of insects, bacteria or molds. In any case they are parasites, because they feed on the juices of the host plant.
Each of these parasites attacks its own favorite plant and produces its own type of gall. One of the best known galls in the oak apple, which, of course, is no apple at all. It is a round, woody globe growing on the twigs and leaves of the sturdy oak tree. Actually there are a number of different oak apples, each caused by a different insect.
The insects which cause oak apples are cynipid flies. The adults are small creatures that flit about like midges. Mama selects an oak tree fox laying her eggs. For she knows that her youngsters will need the juices of the oak for food. She drills a hole, usually in a young twig, lays her egg in it and flies away.
The egg hatches into a tiny grub. By some mysterious process, this hungry grub forces the oak to grow a wad of wood tissues. Pretty soon the little creature has a round, woody home to protect him and a pantry full of plant juices to satisfy his enormous appetite.
The little fellow is completely sealed inside his oak apple. He eats until he reaches his full size. Then he becomes a pupa and goes into his sleeping stage. While he sleeps his body goes through the wonderful process of metamorphosis. He changes from a grub into a winged insect.
The inside of a woody oak apple is no proper home for a creature that can fly. So the tiny fly begins to eat his way out. He eats a small tunnel, crawls through it, spreads his tiny wings and takes to the air.
The cynipid flies are a pest to the plant world but they are of some use to us. The oak apple is rich in tannic acid and, before the age of chemistry, they were used to tan leather and make dyes. What’s more, the cynipids attack and destroy goldenrod and other plants which we rate as weeds.
Galls of various shapes occur on blackberries, sunflowers, roses and countless other plants. You may find 15 or so different shaped galls on the leaves of a hickory tree. Each is the nursery for a certain insect who enjoys a diet of hickory juice.
Mold and bacteria galls most often attack the roots of plants. Sometimes cabbage roots are attacked by bacteria which force them to grow all out of shape. This disease, called clubfoot, is treated with lime in the soil which destroys the bacteria.
Other gall makers are the nematodes, or roundworms. These microscopic, hair‑like creatures attack the roots of tomatoes, tobacco, cucumbers, lettuce, strawberries and cotton. They form small galls. Nematodes also attack the stems and leaves of clover and alfalfa. They also attack the flowers of wheat and give us tiny galls in place of grain.