I received an email message this past weekend from Brad McBride (The Davey Tree Expert Co.) alerting me that Willow Pinecone Galls are reaching their full glory on their namesake host near the Everett Covered Bridge in the Cuyahoga Valley National Park. It’s certainly worth a visit to enjoy this picturesque wonder, and perhaps look at the bridge too.
The galls are induced by the so-called Willow Pinecone Gall Midge, Rabdophaga strobiloides (family Cecidomyiidae), to 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 pine cones, particularly when the galls darken during the winter.
I've only ever found willow pinecone galls in Ohio on black willow (Salix nigra). I believe they add ornamental value to this otherwise scrubby lackluster willow and the galls even provide winter interest. Of course, I’m a galloholic, so I may be biased.
Although I’ve only found the galls on black willow, the literature lists several willow hosts for R. strobiloides. Willow pinecone galls have also been reported over a wide geographical range from New England to California and north to Alaska, but many are produced by other Rabdophaga species on a variety of willow species.
Digging Deeper
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.
The midge galls arise from apical buds, so they are only found at the tips of branches. 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 their 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, proof that galls produced under the direction of an arthropod gall-maker consist entirely of plant tissue is provided in the image below. The willow pinecone gall shows unmistakable evidence of a leaf-infecting willow rust fungus (Melampsora spp.) which shouldn’t be surprising given the gall was formed from hi-jacked 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 observations in Ohio that the number of willow pinecone galls tends to rise and fall dramatically from year to year.
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 more than meets the eye.
Selected References (listed chronologically)
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), pp.430-433.
Weis, A.E., 1984. Apical dominance asserted over lateral buds by the gall of Rhabdophaga strobiloides (Diptera: Cecidomyiidae). The Canadian Entomologist, 116(9), pp.1277-1279.
Van Hezewijk, B.H. and Roland, J., 2003. Gall size determines the structure of the Rabdophaga strobiloides host–parasitoid community. Ecological Entomology, 28(5), pp.593-603.