Cary Institute predicts 2017 Lyme disease spike to follow 2015’s bumper acorn crop

Have you noticed more acorns underfoot this fall than usual? It’s not your imagination. According to Michael Fargione, manager of field research and outdoor programs at the Cary Institute of Ecosystem Studies in Millbrook, we’re experiencing a “mast year” for acorns in our region, meaning that the crop is much more abundant than usual.

“Mast” refers to the seeds of woody plants that are eaten by wildlife, he explains. Acorns and hickory nuts, for example, are classified as “hard mast,” reflecting their hard-shelled, protective outer covering, while “soft mast” has seeds surrounded by fleshy pulp: Think berries, stone fruits and apples. And while white and black oaks and hickories produced only modest amounts of seeds this year, the red oaks in our region were prolific, producing huge quantities of acorns. Many trees and shrubs in our area also had greater-than-usual soft-mast crops in 2015.

For local wildlife, this means an abundance of food. A mast year helps wildlife fuel their southern migrations or stock their winter larders.

For us, it means not only that the going might be difficult on some hikes – Fargione says that some of the forested sites at the Cary Institute have been like “walking on marbles” due to the volume of fallen acorns – but it could also mean an increased chance of getting Lyme disease two years from now.

But that’s not something to panic over, he adds. “You should be careful whenever you’re out in areas where there are ticks, whether it’s a high year or a low year. It doesn’t matter whether you get one or a hundred; the potential is always there.”

The reason there’s believed to be an increase in Lyme disease two years following a mast year is because animals that feed on seeds do better with an increased supply of food, so the summer following a mast year will bring an increase in seed-eater populations as well as the predators that feed on them, including ticks. For example, the summer after a big acorn crop, the white-footed mouse population explodes, says Fargione, increasing the likelihood that newly hatched black-legged ticks will take their first blood meal from a mouse.

“Mice are very efficient at infecting ticks with the bacterium that causes Lyme disease, so with the ticks that are out there having an easier time finding a mouse to feed on, there will be more ticks getting the Lyme spirochete. Then the following year – two years after the mast year – those ticks will produce their own offspring. And more ticks means the probability of a human encountering a tick is going to be higher.”

The theory is still speculative at this point, he adds, but persuasive research backing up an increase in Lyme disease two years after a mast year was developed by Cary Institute disease ecologist Rick Ostfeld, who specializes in study of the West Nile virus and Lyme disease. Ostfeld is one of a number of scientists at the Cary Institute who conduct research in the ecological field.

The property in Millbrook is open to the public from April 1 through the end of October with several nature trails suitable for hiking, but research and education are at the core of the organization’s mission. Structured educational programs examining an aspect of ecology are offered year-round; winter lectures are held in the auditorium on-site and the outdoor programs that Fargione coordinates the rest of the year are held out on the grounds.

Nobody really knows the reason why there are cycles in acorn production, he says, but it may be linked to warmer spring temperatures during oak flowering. “We haven’t identified all the factors that are involved in making a mast year, but certainly there’s a close relationship between weather conditions when the trees are blooming and whether or not we have a bumper crop of acorns. It’s not unlike a lot of other trees and plants that bear fruit: Weather conditions are a factor.”

And it’s not that a mast year is such an unusual thing; it’s just that it’s an irregular event and difficult to predict. “We may have one good year, then two or three years where [acorn production] is moderate or even poor,” says Fargione.

There is some speculation that the sporadic nature of mast years is actually an adaptation of the plant, he says, at least when it comes to hard mast. “If the trees produce irregular crops of acorns, the things that eat the acorns can’t track it and build their populations up. And unlike soft mast, hard mast can’t germinate into a viable plant once it’s consumed, so an increase in animals that eat all the acorns means a decline in hard-mast trees. Boom-and-bust crop cycles may be one way hard-mast trees cope with being the target of so many hungry critters.”


Cary Institute of Ecosystem Studies, 2801 Sharon Turnpike (Route 44), Millbrook; (845) 677-5343,

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