Strange “Flowers” on a Rosebush

These fuzzy red growths are not flowers, but galls. Photo: Peter O’Connor, www.flickr.com

Question: We have been growing a wild rose for ten years now, but this is the first time we’ve seen these wonderful flowers form. I say “form,” since strangely, they seem to grow as much on the leaves as on the rose’s branches. They look like pompoms or balls of fur. They were green at first, but have now turned beautiful shades of yellow, gold, pink and red. Is it possible that they are a parasite of some sort?

Simon Lanctot

Answer: Yes, they are indeed parasites and they’re not flowers, but rose galls, usually called mossy rose galls, rose bedeguars or Robin’s pincushions. Bedeguar traces back to Persian and means “brought by the wind,” although they’re actually “brought” by insects, while “Robin” is the name of a forest sprite. 

They’re caused by a small black wasp known as the mossy rose gall wasp (Diplolepis rosae). The adult is rarely seen, but it pierces a stem or leaf vein with its ovipositor and lays up to 60 eggs inside. This causes the formation of a gall of variable size (the more larvae there are inside, the larger it becomes) covered with a mass of sticky filaments, giving it its typical mossy appearance. The gall is green at first, but becomes various colors as fall sets in.

The larvae feed on tissues inside the gall. If you cut one in half, you will see that it contains holes that the larvae have dug as they fed. The larvae are even edible and considered a delicacy in some countries!

Certain wild roses such as Rosa arvensis, R. canina, R. rubiginosa, R. dumalis and R. glauca (R. rubrifolia) are more prone to mossy rose galls than cultivated roses.

A Fascinating Life Cycle

The life cycle of the insect begins with eggs being laid in May. The larvae grow all summer, passing through five instars, and form overwintering nymphs in late fall. In the spring, after a short pupation, a new generation of wasps emerges from the gall. They are almost all females, as this insect usually reproduces by parthenogenesis (without needing fecundation from a male). Then the cycle starts all over again.

But that’s the simple version. In fact, the larvae are often parasitized by other wasps and these are sometimes parasitized by yet other wasps. This is called hyperparasitism. Thus, there is often more than one species of larva inside the gall.

Mossy rose galls are most often found on somewhat stressed roses, for example those growing in very dry soil or very humid ones and also on heavily pruned plants.

The gall itself is essentially harmless and its presence does not seem to weaken the plant in any way, even when it bears many galls. Purists will tell you to remove the galls you find on your roses so that the gall population doesn’t increase next year, but since you find them attractive, I suggest you just leave them. After all, what’s the harm in allowing Mother Nature to continue doing her job?

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The mossy rose gall: it’s less a pest than a fascinating lesson about the complexity of nature!