Galls More Visible in Fall

by Kevin Strauss

Anyone who ventures into a meadow or forest edge this time of year is likely to run into plants with what appear to be tumors or other strange growths. While these growths have been getting bigger throughout the year, it is only now as many trees drop their leaves and plants turn yellow and brown that the growths become so obvious. But the growths aren't the result of a disease infection, they form on leaves or stems that have an insect "infection." In most cases, a small wasp or midge lays its eggs on a specific species of plant. When the eggs hatch, the insect larvae burrows into the plant. The plant often reacts with rapid growth at the point of "infection." This forms what scientists call a "gall."

Galls form the perfect winter home for several species of insect larvae. The insulating plant material helps keep the larvae warm and dry and when hungry, the larvae can just eat part of their houses. While the larvae remain and grow in most galls throughout the winter, some larvae will enter a kind of hibernation when the temperature drops too low.
Galls come in a variety of shapes and sizes. Most people are familiar with the round "ball galls" found on goldenrod stems. But there are other plants like oak trees and willow trees that form "finger galls" or "pine cone" galls when infected. It seems that the gall insects need to find a specific plant host. Just any plant won't do.

Historically, ice fishermen would gather goldenrod galls and cut them open to use the white "wax worm" larva for bait. With the development of artificial freeze dried baits, fewer and fewer anglers use this bait source. But that doesn't mean that all the larvae in these galls grow to become adult insects. I have watched the small downy woodpeckers flit form gall to gall in a field, pecking into galls to eat the larvae inside.

Quick Guide to Galls:

Goldenrod ball galls-A small fly grows inside these galls. After the larvae burrows into stem, the stem balloons up into a round gall.

Goldenrod leafy bunch gall-If you see what looks like a misshapen clump of leaves at the end of a stalk that almost look like an additional flower, it is probably a leafy bunch gall. Usually these galls contain midges, but they will have pupated and left the gall before winter arrives.

Goldenrod elliptical galls-These gall are more oblong than the ball galls and are generally home to larvae of the ichneumon wasp, an important predator of caterpillars and other insect pests.

Pine cone willow gall-The first time I saw this gall, I thought I was looking at a pine cone growing on a willow tree. While this gall looks like a closed pine cone, but it will never contain seeds. The gall looks green in the spring before changing to yellow and then brown and gray. A fly makes this gall on the willows.

The Ely Timberjay