Bumper acorn crops warn of Lyme tick surges two years later, a 35-year study found

Oak trees in New York’s Hudson Valley have been telling ecologists what is coming for decades. A heavy acorn crop reliably predicts more nymphal blacklegged ticks two years later, according to more than 35 years of monitoring at a forested research property in Dutchess County. The finding comes from the Cary Institute of Ecosystem Studies and was published in the Proceedings of the National Academy of Sciences.

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Nymphs matter because they are the stage most likely to bite a person. They are tiny, hard to spot, and active mainly from May to July. Nearly 500,000 Americans are diagnosed with Lyme disease each year, and the disease is the most frequently reported tick-borne illness in the country, so a signal visible on the forest floor a full two seasons ahead is unusually valuable.

From acorns to mice to nymphs: the two-year chain

Disease ecologist Richard Ostfeld, the lead author, and Shannon LaDeau run the long-running program on a 2,000-acre property that has been monitored since 1991. Their ScienceDaily-distributed summary of the paper describes the chain in plain terms. A mast year, when oaks drop an exceptional number of acorns, feeds a boom in white-footed mice. Mice are efficient carriers of the Lyme bacterium, Borrelia burgdorferi, and they host many larval ticks without suffering much for it.

Years with especially large mouse populations were followed by about 40% more nymphal ticks the next year, the summary reports. Put the acorn step in front of the mouse step and the warning arrives two years ahead of the nymphs. The Cary Institute’s own tick-borne disease program page spells out the mechanism: heavy acorn crops let more mice survive the winter, more larval ticks then feed on infected mice the following spring, and the result is more infected nymphs.

Deer numbers failed to track tick numbers

Many people assume deer drive Lyme risk, since adult ticks feed on them. Across the entire dataset there was no statistically significant link between deer abundance and nymphal tick abundance. Ostfeld said the whole data set in its entirety shows no such thing, and noted that a weak positive relationship appeared only when short subsets of years were examined. A separate write-up by CIDRAP adds that deer abundance swung more than sixfold during the study without any matching change in nymph density.

The practical consequence is that culling or fencing deer would not be expected to lower nymph numbers in this kind of forest, whereas watching oak masting could help predict the next bad year. That is the argument the authors make: nymph density is more predictable from white-footed mouse abundance than from white-tailed deer abundance, and CIDRAP says this is relevant to managing Lyme risk through host populations rather than through deer alone. Where large red oaks are abundant, acorns serve as both a driver and an early indicator of the risk that follows.

Warm years, infection rates and unresolved puzzles

Not everything in the 35-year record was tidy. Mouse abundance did not consistently predict the share of nymphs carrying the bacterium. Skunks, squirrels and opossums are poor transmitters, and the authors suggest that a wider mix of hosts can dilute infection prevalence. Ostfeld said the infection-rate puzzle kept him up at night, and LaDeau said studies of this depth and length are incredibly rare.

Weather produced a further surprise. Laboratory work had suggested that extreme heat or cold would kill blacklegged ticks, yet in the wild they appear resilient, probably by retreating into deeper soil or leaf litter. Warmer years, though, tended to be followed by fewer nymphs even though warmth favors mice. CIDRAP reports roughly 45% fewer nymphal ticks in the warmest years than in colder ones. The full synthesis appears in the paper “The ecology of Lyme disease: Long-term data, surprises, and a synthesis,” in PNAS volume 123, issue 34.

The same Cary program page lists other forces that shape risk around the Hudson Valley. Forest fragments tend to carry a higher share of infected ticks than intact forest, because fragments hold fewer kinds of animals, fewer predators and more mice. Opossums are singled out as notable tick killers, removing more than 90% of the ticks that try to feed on them, and foxes and bobcats generally reduce risk by keeping mouse numbers in check. The page also warns that a bite in the region can expose a person to Lyme, babesiosis and anaplasmosis at once, that Lyme costs an estimated $712 million to $1.3 billion a year, and that no vaccine exists for the infections that blacklegged ticks spread.

Researchers there are also watching the lone star tick and the longhorned tick, two species expanding into the Northeast that the acorn rule was never calibrated for.

The research first drew coverage in mid-August, when News-Medical reported the long-term tick findings, and the Cary Institute’s release circulated again in early November. The acorn rule applies where large oaks dominate the canopy, so a forest of a different composition could behave differently, and the authors have not claimed it holds everywhere.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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