Willow Pinecone Galls, with their faux seed scales, bear a striking resemblance to pine cones, particularly when the galls darken during the winter. 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. The midge galls arise from apical buds, so they are only found at the tips of branches.
I've only ever found willow pinecone galls in Ohio on black willow (Salix nigra). I believe the decorative galls add ornamental value to the otherwise, scrubby lackluster black willows and the galls even provide winter interest. Of course, I’m a galloholic, so I may be biased.
Although I’ve only found pinecone galls on black willows, 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. However, many are produced by other Rabdophaga species affecting a variety of willow species.
On the other hand, even though there are many Rabdophaga species that produce pinecone galls on willow, R. strobiloides is apparently the only gall-midge that induces a single pine cone-like gall at the tips of willow stems. Multiple pinecone galls produced by another Rabdophaga species are shown in the following picture taken by Jim Chatfield (OSU Extension, Emeritus) in the Grand Staircase–Escalante National Monument in Utah.
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 aren’t directly contributing photosynthetic resources to stem girth.
The processes behind plant gall development, whether it’s midge galls (family Cecidomyiidae) or wasp galls (family Cynipidae), is one of the most fascinating things you'll ever come across in nature. However, there’s much that we do not understand.
We know that the biological processes used in fabricating plant galls start with undifferentiated (= meristematic) plant tissue. Gall-makers can’t hijack plant cells once they’ve differentiated into final forms. For example, the final leaf cells can’t become gall cells. However, we can’t read the blueprints for exactly how plant galls are constructed.
We understand that new willow pinecone galls are initiated when females use their sharp ovipositor (= egg depositor) to insert an egg into an apical bud which is why the galls always appear at the tips of twigs. The females introduce chemicals into the oviposition wound. Whether the chemicals coat their ovipositors or are found in their saliva or both, is not well documented. The eggs may also exude gall growth-directing chemicals, but this is also poorly understood.
Although we may not understand the entire process, it's clear that the chemicals must turn plant genes on and off in the meristematic bud tissue at just the right time to direct gall formation. It is highly directed growth specific to the gall-maker. The willow pinecone gall-making midge flies only produce willow pinecone galls; they don’t produce any other type of gall.
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 clear 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.
Digging Deeper Into Willow Pinecone 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.
The galls change their size and appearance as the midge maggots develop through three instar stages. The galls first appear in the early spring as a dense cluster of curved, nascent leaves at the tips of the twigs.
The willow galls later expand into round ball-like structures. Most of the galls are currently in the “ball stage.”
Eventually, the galls elongate to form the final pine cone-like structure responsible for their common name. This transition will happen soon.
In the fall, the third instar maggots line their chamber with a thin layer of silk and spend the winter in stasis. Pupation occurs in the spring with adults emerging just before bud break.
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 the parasitoid and predator community associated with the willow galls observed a fly maggot mortality rating that ranged from 51 to 78% throughout the study.
Midge mortality was dominated by bird predation; 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 an important 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.