Willow Pinecone Galls are the work of the so-called Willow Pinecone Gall Midge, Rabdophaga strobiloides (family Cecidomyiidae). They house, nourish, and protect a single fly larva (maggot) located deep within the gall.
With their faux seed scales, the galls bear a striking resemblance to a female pine cone (female strobilus), particularly when the galls darken during the winter. Indeed, the specific epithet, strobiloides, references the strobili of Pinus spp.
Over the years, I’ve observed these odd galls throughout Ohio and posted numerous BYGL Alerts. I’ve only found them on black willow (Salix nigra), although the literature lists several willow hosts for R. strobiloides.
The willow pinecone galls I’ve observed and the over 100 pictures that I’ve taken all show the galls occurring singly, rising from the tips of the willow stems. They are consistent with the literature regarding the gall’s location and development. For example, Weis & Kapelinski (1984) state in their abstract: “The gall-midge Rhabdophaga strobiloides (O.S.) forms a gall in the apical bud of actively growing willow twigs.”
However, last week, I came across a heavily galled willow in a park in southwest Ohio that had clusters of multiple galls near the stem tips that appeared to arise from both the apical bud and lateral buds. There were also single galls arising from lateral buds on the stems.
The willow was growing near an ephemeral stream and did not appear to be planted. It had the attributes of a black willow, but I need to confirm that.
I had never observed clusters of willow pinecone galls before in Ohio; only single galls growing from apical buds. However, Jim Chatfield (OSU Extension Emeritus) took a picture of a similar arrangement of willow pinecone galls in 2017 on a willow growing near Calf Creek Falls in the Grand Staircase–Escalante National Monument in central Garfield County, Utah. At the time, we assumed it was a different species of gall-maker.
The image below shows the observations posted to iNaturalist under "Willow Pinecone Gall Midge, Rabdophaga strobiloides," as of July 11, 2026. The map was produced using the following filters: research grade, verifiable, and added on any date. As you can see, the midge fly appears to have a wide geographical distribution.
To learn how often willow pinecone galls stray from what I once considered “typical,” I did a quick review of the images posted with the iNaturalist observations. There were 12,979 observations posted; however, applying the same filters used to produce the map reduced the number to 10,190. Google displayed 96 images per page, meaning there were 106 pages of images. I viewed a subset by randomly selecting 21 pages, which displayed a total of 2,016 images, or about 20% of the 10,190 images posted.
I found that only 50 images (2.5%) of the 2,016 images that I reviewed showed galls attributed to R. strobiloides that were different from the expected. The images showed clusters of multiple galls that appeared to arise from the apical bud, lateral buds, or both.
There was also an interesting distribution pattern. Ten (20%) of the observations showing images of unusual willow pinecone galls came from Greater Cincinnati. The locations did not include the site where I took the pictures included in this Alert. Oddly, the rest of the observations came from locations west of the Mississippi, with only one observation from a Midwest location in St. Peters, Missouri. Interestingly, there was one observation from Garfield County, UT, in the Grand Staircase–Escalante National Monument, where Jim Chatfield took the picture in 2017 that I've posted.
In the interest of time and having a life outside of BYGL Alerts, my methodology relied on a subset. However, I believe the results still reflect the rarity of finding willow pinecone galls attributed to R. strobiloides that do not meet the expectation that the midge galls only form from apical buds.
The iNaturalist observations rarely included a willow species. However, the literature notes that the midge fly will utilize multiple willow species to form a gall from apical buds. I’ve never found a reference that notes different types of willow pinecone galls occur on different willow species.
There are many ways to interpret the data that I’ve presented. I don’t want to go too far with rampant speculation such as the possibility that so-called willow pinecone galls are the work of more than one midge fly species, although the large range map for such a small midge fly begs the question. Or perhaps we’re dealing with a host-related issue after all.
On the other hand, there is the distinct possibility that while R. strobiloides most often induces the formation of a single gall from an apical bud, it may also hijack lateral buds to produce single or multiple galls further down the stem. The bottom line is that there could be more than meets the eye with this cool midge fly gall-maker.
Digging Deeper
Based on the literature and personal observations, here’s what we find if we focus on the “normal” willow pinecone galls, and perhaps this also applies to the oddball galls. I haven’t returned to the park to investigate further, and in the interest of not wanting to reduce the population, I didn’t slice open any of the multiple galls.
Carefully slicing the galls open lengthwise at this time of the year will reveal a multi-layered structure surrounding a single orange to orangish-yellow midge fly maggot nestled within a central elongated chamber. The layers are packed with dense down-like fibers presumably to provide protection against gall-maker enemies and to serve as winter insulation.
Gall development is initiated in the spring when female midge flies deposit a single egg in a nascent bud. The willow galls change their size and appearance as the midge maggots develop through three instar stages. Early galls appear as a dense cluster of curved, nascent leaves at the tips of the twigs.
The galls later develop into round, ball-like structures (“ball stage”). The fake pinecone seed scales become obvious during this phase of gall development. Eventually, the galls elongate to acquire a pinecone shape.
In the fall, the third instar maggots line their chamber with a thin layer of silk and spend the winter in stasis. During the winter, the galls turn brown and truly mimic pinecones. They also add winter interest to black willow.
Pupation occurs in the spring, with adults emerging just before bud break and the females initiating the next crop of galls. There is one generation per season.
The willow pinecone galls cause no appreciable harm to the health of their willow hosts. However, it doesn’t mean they don't produce measurable effects beyond their strange appearance.
Research has shown that the willow pinecone gall midge manipulates its host's growth and development to funnel tree resources to its maggot progeny. Stems with a gall at the tip become significantly larger in diameter compared to twigs without galls. The increased diameter even occurs if the foliage is stripped, so the leaves on the affected stems aren’t directly contributing photosynthetic resources to stem girth.
On a side note, it’s important to understand that galls produced under the direction of an arthropod (insects and mites) are composed entirely of plant tissue. No part of the gall includes animal tissue. This demonstrates the fascinating nature of gall development, with the gall-maker manipulating plant tissue to grow the gall.
The images below provides proof that willow pine cone galls arise from plant tissue. The gall shows unmistakable evidence of a leaf-infecting willow rust fungus (Melampsora spp.), which shouldn’t be surprising given the gall was formed from hijacked leaf tissue.
Enemies at the Gate
Life isn’t always easy for the willow pinecone gall midge maggots. Researchers conducting a four-year study in Alberta, Canada, on willow pinecone galls observed a fly maggot mortality rate that ranged from 51 to 78% throughout the study. Birds were the dominant nemesis of the maggots; however, wasp parasitoids also got in on insecticidal acts.
Birds selected the smallest-sized galls while the parasitoids focused their attention on medium-diameter galls. Heavy losses to predation and parasitism may help to explain my past observations in Ohio that the number of willow pinecone galls tends to rise and fall dramatically from year to year. Indeed, if you look closely at the image below, you’ll see evidence that two of the galls appear damaged, perhaps by a bird.
Of course, what’s bad for the gall-maker maggots is good for others. Some ecologists consider Rabdophaga gall-maker midge flies to be keystone species for a wide range of predators, parasitoids, and hyperparasitoids. Indeed, where willow galls are common, the occasional maggot meal may play a key role in the winter survival of several birds belonging to the songbird family Paridae, including chickadees and titmice. Thus, the function of willow pinecone galls in the grand ecological scheme of things is also more than meets the eye.
Selected References
Van Hezewijk, B. H., & Roland, J. (2003). Gall size determines the structure of the Rabdophaga strobiloides host–parasitoid community. Ecological Entomology, 28(5), 593-603.
https://doi.org/10.1046/j.1365-2311.2003.00553.xDigital
Weis, A. E. (1984). Apical dominance asserted over lateral buds by the gall of Rhabdophaga strobiloides (Diptera: Cecidomyiidae). The Canadian Entomologist, 116(9), 1277-1279.
https://doi.org/10.4039/Ent1161277-9
Weis, A. E., & Kapelinski, A. (1984). Manipulation of host plant development by the gall‐midge Rhabdophaga strobiloides. Ecological Entomology, 9(4), 457-465.
https://doi.org/10.4039/Ent1161277-9
Wilson, L. F. (1968). Life history and habits of the pine cone willow gall midge, Rhabdophaga strobiloides (Diptera: Cecidomyiidae), in Michigan. The Canadian Entomologist, 100(4), 430-433.
https://doi.org/10.4039/Ent100430-4
