Willow Pinecone Galls are produced by the Willow Pinecone Gall Midge, Rabdophaga strobiloides (family Cecidomyiidae), to house, nourish, and protect a single fly larva (maggot) located deep within the gall. The elaborate structures bear a striking resemblance to a pine cone complete with faux seed scales.
The literature lists several willow hosts; however, I've only ever found them in Ohio on black willow (Salix nigra). The galls arise from apical buds, so they are only found at the tips of branches. The galls have been reported from New England to California and north to Alaska.
As with the vast majority of plant galls produced under the direction of insect gall-makers, willow pinecone galls cause no appreciable harm to the overall health of their willow host. In fact, I believe they add ornamental value to otherwise lackluster black willows and even provide winter interest. Of course, as a gall-maniac, I may be biased.
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 a modicum of protection against gall-maker enemies and to serve as winter insulation.
The gall maggots develop through three instar stages during the growing season with the galls gradually increasing in size. 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.
What happens once the adult midge flies emerge in the spring applies to both gall-making midge flies (family Cecidomyadae) as well as gall-wasps (family Cynipidae). It's one of the most fascinating things you'll ever come across in nature; however, it remains poorly understood.
New galls are initiated when females use their sharp ovipositors (= egg depositor) to insert an egg into an apical bud; the galls always appear at the tips of twigs. The females also introduce chemicals into the 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.
However, it is known that the chemicals 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. In this case, the improbable-looking willow pinecone galls are formed; they never develop into any other type of gall.
The galls at first appear as a dense cluster of curved, nascent leaves. They later pass through a "ball stage" before becoming elongated into the final cone-like structure.
In teaching about plant galls, I always stress that galls produced under the direction of an arthropod gall-maker consist entirely of plant tissue. Proof of this point is provided by 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.
Although willow pinecone galls cause no appreciable harm to the health of their willow hosts, it does not mean they don't produce measurable effects beyond their strange appearance. Research conducted in Michigan (Weis and Kapelinski 1984) showed that the willow pinecone gall midge manipulates their host's growth and development to funnel tree resources to their maggot progeny. Twigs with a gall at the tip become significantly larger in diameter compared to twigs without galls even when the foliage is stripped from both galled and non-galled twigs early in the gall development.
Populations of this gall-making midge fly can vary dramatically from year to year in Ohio. A four-year study conducted in Alberta, Canada, on the parasitoid and predator community associated with the willow pinecone gall-maker examined some of the factors driving the population dynamics (Hezewijk and Roland 2003).
The researchers observed a midge fly mortality that ranged from 51 to 78% over the course of the study. Midge mortality was dominated by bird predation and parasitism by the wasps Torymus cecidomyiae (Hymenoptera: Torymidae) and Gastrancistrus sp. (Hymenoptera: Pteromalidae). The birds selected the smallest-sized galls while the parasitoids focused their attention on medium-diameter galls. While the research may not apply to pinecone gall population dynamics in Ohio, it certainly demonstrates that life is not always easy for the midge fly maggots as they lounge about in their gall-homes.
Weis, A. E. and A. Kapelinski. 1984. Manipulation of host plant development by the gall-midge Rhabdophaga strobiloides. Ecol. Entomol. 9(4): 457-465. doi.org/10.1111/j.1365-2311.1984.tb00844.x
Van Hezewijk, B. H. and J. Roland, J. 2003. Gall size determines the structure of the Rabdophaga strobiloides host–parasitoid community. Ecol. Entomol. 28(5), 593-603. doi:10.1046/J.1365-2311.2003.00553.X