Snow crunched under my tires and bare trees whizzed by as I coasted down the hill on my fatbike. Cold air stung my cheeks, but my hands were warm in brand-new, homemade pogies that wrapped around my hands and bike handlebars like old-fashioned muffs. At the bottom of the hill, we passed through a wetland with dried grasses and shrubs frozen into the ice.
In the slightly drier ground next to the graveled forest road grew sparsely elegant clumps of willows. This was an unusual willow, though, that looked like it had hybridized with a pine tree. At the tip of most of the twigs perched a compact little “pine cone,” about the size and shape of a young, tightly-closed cone from a red pine tree.
These cones don’t hold any tree seeds, however. Instead, they harbor the “seeds” of a gall-midge called Rhabdophaga strobiloides. These are willow pine cone galls.
Like most galls, these began in early spring during the active-growth period for the plant and the egg-laying season for the midge. The adult midge laid an egg on the tip of the twig, right where a single, dunce-cap-like bud scale protected baby leaves. The larva hatched in early May and started burrowing into the willow stem. Some combination of the chewing action and saliva of the newly hatched larva triggered an increase in plant growth hormones. Cells grew bigger and more plentiful, but the stem did not extend. Instead, leaves once destined to flutter along a twig now layered together in the cone-like structures that caught my eye.
What’s fascinating is just how much the larva can control the plant. One study found that the twigs hosting a gall were larger in diameter than twigs with no gall—even if the twig did not have leaves. Bigger stems were correlated with bigger galls, and bigger galls were correlated with bigger larvae. This confirms the hypothesis that the larva somehow draws in the products of photosynthesis from other (probably un-galled) twigs in order to spur the growth of the gall, the hosting twig, and the larva itself.
This type of control may seem creepy, but it is frighteningly common in the world of parasites. Parasitic cordyceps fungi force ants to climb a plant and attach there before they die, providing a breezy platform for dispersal of the fungal spores. A brain parasite causes rats to be attracted to cats so that the parasite can complete its life cycle in a feline host. Horsehair worms drive zombie crickets to a watery death. In comparison, a tiny larva causing a plant to grow some extra tissue is pretty tame.
It is still amazing. All summer, the larva—eating stolen nutrients from the plant—grew within the protective walls of the gall. Late last May, the larva would have expanded enough that it needed to shed its skin, thus entering its second instar developmental stage. By late July, the growing larva shed its skin once more. Its next task before winter was to construct a cocoon and leave it open at the top.
So, there we are. Inside each of these cone-like galls by the side of the road is a little larva in a cocoon sleeping bag, steeling itself against the cold. The loosely packed structure of the gall may provide some insulation, but it isn’t nearly as good as my homemade pogies. The dormant larva doesn’t have the benefit of my fiery metabolism fueled by regular eating to keep it warm. In these situations, some creatures allow themselves to freeze. Wood frogs are one incredible example.
These larvae take a more daring route. By concentrating glycerol (once used in cars as antifreeze) in their bodies, the larvae can supercool their liquids down below the freezing point of water without them becoming solid. In extreme examples, larvae have safely supercooled to negative 76 degrees Fahrenheit. This requires a delicate balance. A disturbance in the system could cause the larvae to freeze and die instantly.
If a larva survives the winter, it will pupate inside the pre-formed cocoon early next April, and the adult gall-midge will squeeze out between the layers of the cone and fly away.
Or not. Thirty-one other creatures might be illicit squatters in the loosely packed scales of the pine cone gall. These gall apartments are in such demand that during one study the scientist found 564 individual insects in just 23 galls. Most of the beetles, caterpillars, sawflies, and eggs of meadow grasshoppers are relatively harmless. The wasps, however, beat the gall-midges at their parasitic game by raising their wasp larva on the tender flesh of midge larva.