Willow pinecone gall. Photo: Genevieve
By Larry Hodgson
Question: This shrub with huge buds appeared spontaneously in my yard in a place where the soil is moist, even flooded in the spring, but quite well drained in the summer. I’ve had no luck identifying it. Can you help me?
Answer: It’s a willow. Probably an American pussy willow (Salix discolor), but the exact identification of willows is complex, so that’s a bit of a guess. However, what’s confused you is clearly the “huge bud” you mention and show in your photo. After all, willow buds are tiny: they never produce buds 2 to 4 inches (5 to 10 cm) long on their own!
Often people mistake such a plant for a rhododendron, a magnolia, or other plant that produces large flower buds and so wait patiently for showy blooms the following spring. However, the blooms never appear.
Others take such growths for fruits, but that’s not the case either. The “fruit” of willows is a tiny capsule that releases itsy-bitsy seeds coated with white fuzz, not something anywhere near that massive.
Nor is the growth a pinecone, although it certainly does look like one. After all, pinecones grow on conifers with thin needles (pines, specifically), not on broadleaf shrubs like willows.
So, what’s confused you is in fact a gall caused by a tiny insect.
This curious growth is called the willow pinecone gall, because it looks a lot like a pinecone and is only found on willows (Salix spp.). In fact, it seems to be able to settle on almost any species of willow.
The gall is caused by the willow pinecone gall midge (Rabdophaga strobiloides), a tiny midge barely 3 mm in length found almost everywhere in the temperate northern hemisphere.
Typically, in late spring (late April or May), the female emerges from the gall produced the previous year, now dry and brown, and lays an egg in the terminal bud of a willow stem. When the larva, a small pink maggot, emerges a few days later, it emits a chemical that causes the normal extension of the stem to stop. Thus, the 30 to 60 leaves which were to supposed to form equally spaced all along the elongating stem are now bunched together and overlapping each other, forming a gall reminiscent of a scaly pinecone in which the gall midge larva can feed.
The gall is pale green in summer, but turns brown in fall and retains this color in winter.
There is only one gall midge larva per gall, but several other insects take advantage of the shelter provided by the gall and soon move in. Thus, if you cut a gall in half, you may find several insects at various stages, from adults to larvae. Some, like the gall midge larva, feed on willow tissues inside the gall, but others are predators. For example, one tiny wasp injects its egg into the midge larva and its pupa then eats the larva from the inside, then emerges from its dying body in the spring … yes, just like in the movie Aliens!
In the wild, parasite predation on willow pinecone gall midge larvae is fierce: rarely does more than one gall in four successfully produce an adult gall midge.
Other Willow Galls
Willows seem particularly prone to gall-forming insects and all sorts produce galls on them, both on leaves and, as in this case, stems.
Another gall midge (Radbophaga rosaria*, syn. Cecidomyia rosaria) provokes a stem gall that is quite similar, but where some leaves open partially, creating an effect like a green rose (brown rose in winter) at the end of a willow stem. Thus, it is called willow rosette gall. The life cycle of the willow rosette gall midge is similar to that of the willow pinecone gall midge.
*There are, in fact, several species of gall midge that cause willow rosette gall. R. rosaria is simply the best-known species.
Willow pinecone gall is more of a curiosity than a major problem. Even when a willow has several, it doesn’t really harm its health to any degree. Therefore, no pesticide treatment is really necessary. And besides, few pesticide treatments would work, because the larvae, housed safely with their gall, are well protected from insecticide sprays.
In the wild, simply leave the galls on the plant. In winter, birds, especially chickadees and tits, visit the galls and feed on the insects inside.
When willow pinecone galls show up in a garden and their appearance bothers you, simply remove them with pruning shears as soon as you notice them. If not, leave them all winter as decorations and bird fodder. If you remove them in early spring, before the adults emerge in late April or May, you can still break the local reproductive cycle and your willow will be gall-free the following summer … unless more midges arrive from elsewhere!