When Oak Trees Go ‘Nuts,' This U.S. Disease Follows — A Biologist Explains Why

Forbes

05-04-2025

Oak trees produce acorns in abundance every few years in a process known as mast seeding. Two years ... More later, Lyme disease follows like clockwork.
As hikers return to lush trails each summer, ready to take on the great outdoors, another presence awakens — one not nearly as welcome.
Lyme disease, a stealthy illness transmitted by ticks, affects nearly half a million people in the U.S. every year. The symptoms often begin with a simple red rash, but if untreated, the disease can infiltrate joints, nerves and even the heart.
Every few years, the disease seems to surge. And if you're looking in the right place, you would see it coming at least two years in advance. This unique connection offers a chilling look at how forest rhythms echo through ecosystems, all the way down to your bloodstream.
The white-footed mouse, or Peromyscus leucopus, lies at the heart of the Lyme disease lifecycle. Together with ticks, these industrious rodents help spread a disease that affects hundreds of thousands every year.
Ticks don't hatch carrying Lyme disease. Instead, newly emerged tick larvae, which are barely visible to the naked eye, seek a blood meal from these mice. The rodents act as reservoir hosts for Borrelia burgdorferi — the bacterium ultimately responsible for causing Lyme disease. The bacterium thrives in mice without making them sick, giving it ample time to be passed on to the next hungry tick. Once infected, the tick matures into its nymphal stage, where it becomes far more likely to latch onto humans.
Nymphs are the most dangerous stage of the tick life cycle.
They're about the size of a poppy seed, hard to spot and active during peak hiking and gardening season. Although adult ticks are larger, they're easier to detect and tend to bite larger animals like deer, which don't actually carry Borrelia. The mice, not the deer, are the real culprits. And since mice can reproduce rapidly — especially when food is abundant — they serve as a biological amplifier for the entire tick-borne disease cycle.
So, no infected mice means few infected ticks — and without those ticks, the risk of Lyme disease plummets. But the number of mice in a forest doesn't just fluctuate at random. Their populations rise and fall in rhythm with larger ecological forces, shaped not by predators or pathogens, but by oak trees.
Every few years, oak trees seem to go into overdrive. Branches bend with the weight of their seeds, and forest floors vanish beneath a carpet of acorns. This is a phenomenon known as mast seeding, and it unfolds with remarkable synchronicity across entire regions.
Rather than producing a steady supply of acorns year after year, oak trees operate on boom-and-bust cycles, much like certain species of bamboo which flower every 48 to 50 years. In the case of oak, however, the cycle is much smaller and mast seeding tends to occur once every two to five years.
In a mast year, nearly every tree in the area releases a glut of seeds at once, overwhelming squirrels, deer and other seed-eaters. Some of those acorns are inevitably left uneaten, giving the next generation of oaks a better shot at survival.
Researchers are still piecing together exactly how trees coordinate this behavior. Weather cues like cool and wet climatic conditions seem to play a role, according to a June 2021 study published in Philosophical Transactions of the Royal Society B.
But for white-footed mice, a mast year is a reproductive jackpot — and it's where the Lyme disease story begins to take a more alarming turn.
When oaks unleash a bumper crop of nuts in the fall, white-footed mice respond with a population boom the following summer. After all, more food means better winter survival and more energy for reproduction.
By summer, the stage is set. Larval black-legged ticks, newly hatched and free of infection, begin questing for their first blood meal. The now-abundant mice serve as the perfect hosts, both for nourishment and disease transmission. As the mice go about their business, they unknowingly pass on Borrelia burgdorferi, the bacterium behind Lyme disease.
Fast forward another year.
The larvae that fed on those infected mice have molted into nymphs — tiny, stealthy and now potentially infectious. It's these nymphs, active in summer and easy to overlook, that pose the highest risk to humans.
This cycle is so apparent, that the growing population of infected ticks can be predicted from the production of acorn 1.75 years before, according to a February 2001 study published in Vector-Borne and Zoonotic Diseases.
It's a timeline that connects trees to ticks through the quiet influence of mice — an ecological chain reaction that plays out over seasons, not seconds.
The cascading effect brought on by acorns shows us the true scale of nature's operations and where we fit in. How do you feel about the delicate relationship we share with nature? Find out where you stand on the Connectedness to Nature Scale.