As I was driving along a dirt road in Southwestern Colorado’s plateau country in August of 2023, I noticed what looked like dense clusters of cotton balls on the roadside Saltbushes. Sometimes a single ‘cotton ball’ was on a Saltbush branch, looking like an out-of-place Christmas ornament. But often the cotton balls were packed along the branches in tight linear clusters, almost like snow on the Saltbush. Each individual ‘ball’ was about 1/2″ to 3/4″ in diameter.
These fluffy ‘cotton balls’ were actually galls – growths of the Saltbush tissues that were stimulated by another organism, such as an insect, a mite, or a fungus. In this case, the gall-forming organism was most likely a small fly, in the genus Asphondylia – likely Asphondylia neomexicana.
Asphondylia is a Cecidomyid Gall Midge. The adult midges are small – just 3 or 4 millimeters in length. They look like spindly little mosquitoes, and I would be hard-pressed to pick an adult Cecidomyid out of a police line-up. Which is a ironic, given how visible their galls are.
There are photographs of the adult Midges here, on Bugguide.net.
The Cecidomyid Midge adult lays its eggs on or in the plant tissue, the eggs hatch, and the Midge larvae somehow ‘tell’ the Saltbush to build them homes – the galls.
The larvae live and grow inside the gall-house that the plant has built for them. From what I’ve read, some species of Cecidomyid larvae eat the plant tissues lining the inside the gall. Some other species of Cecidomyid ‘convince’ the cells forming the interior wall of the gall to secrete a nutritive fluid which the larvae eat.
In still other species, the larvae eat a fungus that grows on the inside of the walls of the gall. I’m not sure which strategy Asphondylia neomexicana employs.
Below are a couple of cross-sections of one of an Asphondylia gall, showing the fly larva and the inside walls of the gall. I was amazed by how moist and bright-green the inside of the gall was compared with the faded pastel appearance of the the Saltbush’s normal leaves.
One of the fascinating things about these galls is how similar they appear to another type of cottony/woolly gall, this one on Rabbitbrush. The Cotton-ball Tephritid galls on Rabbit Brush are formed by Tephritid Fruit Flies – I’ve got an older post about the Cotton-ball Tephritid galls here. Why would two different taxa of gall-forming flies create similar-looking white fuzzy galls like this…on two entirely different plant taxa? Did they stumble upon these gall-formation strategies independently? Or are they tapping into the same physiological processes on the different plants?
Yet another thing that fascinates me about galls is the complexity of the ecological communities that they sometimes create. From what I’ve read in Ronald Russo’s book Plant Galls of the Western United States, some types galls will attract entire communities of organisms beyond the insect which initially formed the gall. For example, imagine there’s a fly larva that creates a gall. In some cases there are species of parasitoid wasp that specialize on parasitizing that species of gall-forming fly larva. And then there are other species of wasp that specialize on parasitizing the original parasitoid wasp, the one that was parasitizing the fly larva. If you ever want to get your mind blown, spend time reading about hyperparasitoids.
Also, there are beetles, mites, fungi, flies, and wasps that require galls to survive – but don’t create galls themselves. They live inside pre-existing galls, sometimes at the same time as the original gall-former. These guests are called ‘inquilines’. And they may have their own set of parasitoids, etc.
I hope to spend a lot more time studying these galls.
Sources:
BugGuide.net’s page on Asphondylia neomexicana. What a spectacular resource this website is.
Gallformer.org’s page on Asphondylia neomexicana.
Russo, Ronald A. 2021. Plant Galls of the Western United States (Princeton Field Guides). Princeton University Press; Illustrated edition (April 20, 2021). 400 pages. ISBN-10 0691205760. ISBN-13 978-0691205762.
