Galls: Growth on Plants

by Pat Jaquith

Winter is a good time to spot unusual things when the leaves are off and grasses are dry and beaten down by snow. Sometimes it just takes a splash of color that you wouldn’t expect to draw your attention to an unusual growth. The galls in this article all are caused by different insects but are benign, that is, they usually don’t harm the plant they are on.

Spiny Leaf Gall on wild rose leaf

A Cynipid wasp of the genus Diplolepis is a tiny insect about 1/16″ in length whose life is centered around the host plant. In early summer, it lays eggs on these leaves; the larva that hatches starts to eat the leaf. Many insects eat and eat and when they exhaust the food on one leaf, move to another. This one causes the plant to change its normal growth pattern so a little prickly ball develops which becomes the food supply and home for the wasp larva. In winter, the larva starts to evolve into the adult wasp. In spring, it eats its way out of the gall and almost immediately starts to hunt for a mate. . .and the cycle repeats itself with the wasp never traveling very far. There are 8 species of Diplolepis that lay eggs on rose leaves, each producing a gall with a different appearance.

Pineapple Spruce Gall 12.12.20

This one is named for its appearance: similar to a pineapple fruit. Nothing tropical about this one! It grows on spruce trees in Montana!

Willow Pine Cone Gall on willow shrub growing on the banks of Beaver Creek in Kuhns Wildlife Management Area 10.07.20

This one is also named for its appearance. The gall gnat midge Rabdophaga strobiloides lays an egg on the tip of a growing stem; the larva’s feeding on that tip stimulates the plant to transform leaf structures into this home and food source for the larva. Its life cycle is similar to that described of the Rose gall wasp.

Gall on Canada Thistle (Cirsium arvense) near Pothole #3 West Spring Creek Road 12.08.20

The pinhole on the side of the gall could be the exit hole of the mature midge or it might be a predator’s entrance hole. Larvae are fed on by a number of organisms.

Goldenrod gall in December

Chickadees also predate on larvae inside of galls. They are not subtle about drilling into the gall, but it did open it up so we can see what it looks like!

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