Have you ever seen pinecones on the end of willow branches? They aren’t really pinecones, they are protective homes for the Pinecone Willow Gall-midge (Rhabdophaga strobiloides), a type of fly that relies on willows for its home.
The adult gall-midge lays its egg on the tip of a willow branch as the terminal buds begin to swell in the early spring. The egg, and larva that hatches, release a chemical that tricks the growing willow leaves into forming a structure that looks superficially like a pinecone made of overlapping leaves.
As the larva feeds the bud ceases to develop, but the plant still directs nutrients to the tissues. Biologists working at the University of Michigan Biological Station at Pellston, Michigan found that somehow the gall-midge manipulates the willow to provide resources from other places in the plant to the gall for them to continue to feed and survive. They found that galled twigs compared with normal twigs had greater growth in twig girth than when no gall is present and twigs with galls grew equally well with or without leaves.
The bud continues to swell as the larva feeds and grows. When winter sets in, encased in the cone structure, the larva is protected from predators, but not from the cold.
The larvae aren’t freeze tolerant. Instead, they rely on extreme supercooling, the process of lowering liquid temperatures to below the freezing point without becoming a solid. How do they do this? Overwintering willow gall larvae can contain as much as fifty percent glycerol, historically used in cars as anti-freeze. Some individuals in Alaska were found to have extreme supercooling that allowed them to survive down to -76 degrees Fahrenheit.
At some point in the spring, the larva will pupate and the adult gall-midge will emerge. They don’t have mouthparts to chew their way out of the gall. Instead, they simply push and squeeze between the overlapping leaves of the gall and fly away.