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 a number of 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 cause no apparent harm to overall tree health. In fact, I believe they add ornamental value to their willow hosts.
Carefully slicing the galls open lengthwise at this time of the year will reveal a multi-layered structure surrounding an orange to orangish-yellow midge fly maggot nestled within a central elongated chamber. The layers are packed with dense down-like fibers presumably to serve as winter insulation.
The gall maggots overwinter in the final instar stage meaning that galls remain attached throughout the winter. Research published in 1987 in the journal Oikos showed the maggots survive winter deep-freezes by loading their bodies with "antifreeze" in the form of glycerol. The galls have been reported from New England to California and north to Alaska.
The maggots pupate in the spring with adults emerging just prior to bud break. What happens next applies to both midge fly as well as wasp gall-makers and is poorly understood. However, it's one of the most fascinating things you'll ever come across in nature.
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 look like anything else.
The galls at first appear as a dense cluster of curved, nascent leaves. They later pass through a "ball stage" before becoming elongated into a cone-like structure.
As with the vast majority of plant galls created under the direction of insect gall-makers, willow pinecone galls cause no appreciable harm to the overall health of their willow host. However, this does not mean they don't produce measurable effects beyond their strange appearance.
Research published in 1984 in Ecological Entomology showed that the willow pinecone gall midge manipulates their willow 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.