Galls are growths on plants, produced in response to irritation or chemicals secreted by insects laying eggs. Galls can form in stems, leaves, flowers, fruits, roots, or any other part of the plant. The insects that cause them are mainly aphids and sawflies, but also include some moths and beetles, nematodes, psyllids, and true bugs. Each kind of gall maker makes a distinctive gall in a specific location on only one kind of plant.

Stem galls usually look like a swelling. Leaf galls often resemble “warts.” But many galls have a complex shape — a pine cone, fur, fingers. They seldom harm the host plant.

Here are some of the galls we’ve found at the Fletcher Wildife Garden.

Goldenrod Gall

These swellings in goldenrod stems are common at the FWG. Goldenrod galls are caused by a small fly (Eurosta solidaginis). After her eggs are fertilized the femaleEurosta will choose a goldenrod and insert an egg into its bud. A few weeks later, a bulge begins to appear in the goldenrod stem as the plant produces extra tissue in response to chemicals secreted by the fly larva. The gall has a complex structure with a hard corky exterior and an interior of specialized nutritive tissues. These tissues are the gall fly’s only food, as it does not feed as an adult.

As the larva grows, it eats the nutritive cells making a chamber inside the gall. In the fall, it makes an exit hole, then hibernates until spring. In spring, the larva pupates and the adult emerges from the gall to live only a few weeks before starting the cycle again.

When you see holes in these with jagged edges (as in the photo above left), they were likely made by a bird looking for a meal.

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Goldenrod Bunch Gall

Bunch galls are formed when a midge, Rhopalomyia solidaginis, lays its egg in a leaf bud. When the egg hatches, the larva secretes a chemical that keeps the goldenrod stem from growing even though it continues to produce leaves. The results is a tight, flower-like cluster of leaves, usually at the top of the main, but we’ve also seen them among flower stalks.

Rose Gall

Also called a bedeguar gall or Robin’s pincushion, this gall is formed by a Cynipid wasp, Diplolepis rosae, that overwinters in the gall in the larval stage, then completes its life cycle in the spring. There are usually several larvae in one gall, each in its own chamber.

Ash Flower Gall

This gall is caused by tiny mites, Eriophyes fraxinivorus that are only about half a millimetre long. They feed on the male flower clusters on ash trees early in the season, transforming them into irregular, fringed masses. The masses are green at first, but turn brown or even black as they dry. They can remain on the tree for several years, becoming more noticeable when the leaves fall in autumn.

The mites mature in only 2 weeks. Ash flower gall does not affect tree health; no control measures are necessary.

Willow Pinecone Gall

These galls, which look like small pine cones on willow trees, are made by the larva of a small mosquito-like midge (Rhabdophaga strobiloides). The larva contains a powerful growth substance that it releases to prevent the willow stem from elongating and causes the leaves to mature into what look like the scales on a cone.The larva lives in a small chamber in the centre of the gall. After eating the cells on the inside of the gall and the gall is full size, it hibernates for the winter. In spring when the temperature rises, it metamorphose into a gnat.

Hackberry Leaf Gall

These galls, which resemble pale green peas attached to the underside of Hackberry leaves, are caused by tiny insects known as psyllids. There may be several on one leaf.

Adult psyllids are tiny and look like miniature cicadas. In early spring, they lay eggs in leaf buds of Hackberry trees. In 7-10 days, the eggs hatch into tiny nymphs that begin to feed on the leaves. This stimulates the tree to produce bead-like galls in which the nymph lives and feeds for the rest of the summer. They pupate in the fall and adults emerge in September.

Infested Hackberry trees are not harmed by the galls, although leaves with many galls may fall prematurely.

Jewelweed Leaf Gall

Jewelweed Gall is caused by the Jewelweed Leaf Gall Midge (Schizomyia impatientis).

Oak Apple Gall

Several species of wasps cause oak apples. Oak apple galls are round (2-4 cm in diameter) and smooth. The outside is green and darkens to brown with age. The inside is filled with a juicy, white, spongy substance with a small, hard centre where the larval wasp is located. After the wasp leaves, the gall dries and the inside becomes a mass of fibres.

Cecidomyiid Gall

This very tiny gall is on a Grass-leaved Goldenrod (Euthamnia graminifolia) leaf. It is caused by a Cecidomyiid midge. The photo which was taken through a microscope set at a magnification of ×10. You can just see the hole made by the emerging insect.

Maple Spindle Gall

Maple leaves are susceptible to several gall-forming insects. These spindle galls were found late in the year on Sugar Maples leaves near the Old Woodlot. These small galls, which are pointed at both ends (like spindles) are caused by a mite, Vasates aceriscrumena.

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This page was revised 14 February 2018
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