Species of the Month - December 2016

Willow Cabbage Gall

Rabdophaga rosaria
 

Willows shed all their leaves in autumn - well, not quite.  Sometimes in the depths of winter you come across a willow shoot tip with a cluster of leaves abnormally close together, and, even more abnormally, still on the tree.

The story begins in early summer when the tiny midge Rabdophaga rosaria lays an egg in a developing shoot tip.  From the egg emerges a larva which manipulates the host tissue so that the shoot doesn't elongate properly and its leaves remain bunched together.  This provides shelter for the larva as well as concentrating its food supply near at hand.

I don't have any photos of the fresh galls in summer but there are some good ones on NatureSpot.  The galls are much more conspicuous in winter when there are no other leaves to be seen.

In autumn the larva pupates and no longer needs food, but still wants shelter, and it somehow manages to prevent the tree from dropping the leaves that surround the gall, giving it a cosy home through the cold months and protection from predators.

Joensuu Molecular Ecology Group has a good photo of the overwintering pupa within the gall.  The page also shows several other galls that occur on willow.

In the spring an adult midge emerges and flies off to find a suitable willow to lay eggs in and start the process over again.
 

Two great photos by Jan Hamilton of the Willow Cabbage Gall on LNHG field trips

Many Willow species can be affected by this gall, but in our area it is most frequent on Eared Willow (Salix aurita), which is a common roadside and hillside bush or (in sheltered places) a small tree, with short very wrinkled leaves and widely diverging branches.

It is important to record the host tree where possible, as there is every possibility that R rosaria will be found to consist of more than one species, each galling a different willow species or group of species.  In that case, records without a named host will become records of a species aggregate, but records with a named host will continue to be records of a species.

Of the records in our database, 129 are on Eared Willow (Salix aurita), 4 are on Grey Willow (S cinerea), 3 are on the Eared x Grey Willow hybrid (S x multinervis), 1 is on Grey x Goat Willow (S x reichardtii) and one on White Willow (S alba).  15 are on unknown willow species.  Willows can be difficult to identify, especially in winter, however you can usually find fallen leaves underneath which will help with identification.

It is possible that the galls are easier to see on Eared Willow, whose shoot tips are usually not much higher than head height, than on taller and more upright willow species, so this may have influenced the above figures.

The photo below shows the Willow Cabbage Gall on Grey Willow, with leaves more elongated and smoother than those of Eared Willow.

Some older books use the name Rabdophaga strobilina for the gall-causer, and Mapmate and the NBN both still use this outdated name.  R strobilina does often occur in the galls but it is an inquiline.  Having no gall-causing abilities of its own, it simply gatecrashes the galls made by R rosaria, but without doing any harm to the latter.
 

Please send in your Willow Cabbage Gall sightings using the form below, or email sightings@lnhg.org.uk with the details if you prefer.  If you are not sure of the identity of your gall or of its host Willow tree, please send a photo to sightings@lnhg.org.uk, or put one on the LNHG Facebook page.