Abundance of acorns: mice and ticks to follow

By Louis Bettcher | Nov 16, 2016
Source: Metro Creative

Walking around this fall, acorns are everywhere — they're poking into the soles of your feet from the grass, shooting out of lawnmowers, and careening you down driveways and sidewalks like miniature roller skates.

This year acorns are particularly prevalent as a result of an agricultural mast year, and the tree fruit is a harbinger of increased rodent and insect populations to come.

"The large quantities of acorns allow mice and other small mammals to be better fed, breed and have more offspring," said Griffin Dill, pest management specialist at the University of Maine Cooperative Extension. Mice serve as ideal hosts for larval ticks because they reproduce at a high rate and can live in much smaller spaces than larger mammals such as deer.

The Society of American Foresters defines "mast year" as "a year in which there is an abundant production of mast," and refers to a boom in a tree's fruit or seedling crop.

For oak trees, this crop is acorns.

"Typically a 'mast year' is in reference to a hard mast, which is the fruit and nuts of oak, beech, walnuts, etc.," said Kenneth Laustsen, a biometrician at the Maine Forest Service. Depending on the variety of oak, these acorns have begun to grow on the trees up to two years before falling to the ground. Although oak trees produce acorns each year, mast years only occur sporadically, he said.

So abundant are crops of acorns this fall that Penn State ecology and physiology professor Marc David Abrams called this a "super mast year" in a Nov. 4 Wall Street Journal article. Abrams said that in 30 years spent observing trees, he had never seen such quantities.

Factors such as temperature and rainfall invariably affect a crop's yield. In years such as this one, some scientists believe the oak trees are overcompensating their acorn production and flooding the proverbial market [or forest floor] in order to ensure the growth of future generations.

Most of these are acorns are consumed by mice, squirrels and other small mammals each fall. If there is a small yield of acorns, they are eaten by these creatures, and the number of viable seeds left to produce saplings is small. Conversely, with an overabundance of acorns on the ground, more new trees are likely to grow, and rodents thrive.

With a more-than-adequate food source, animals such as mice are better able to avoid predators and reproduce. It is these animals which, having fed on the acorns, provide sustenance to future ticks. Ticks in their larval form use small mammals such as mice and squirrels to feed off of.

Mice serve as the carriers for the bacteria Borrelia burgdorferi, which becomes Lyme when transferred to the ticks. Dill said that a "vector bridge" is necessary for the bacteria to become Lyme in a human who is bitten by an infected tick. Anthropod-borne viruses are transmitted to humans by a bite from a cold-blooded vector such mosquitoes, ticks or black flies.

Dr. Richard Ostfeld of the Cary Institute of Ecosystem Studies in Millbrook, N.Y. spent several years researching the correlation between acorns, mice and ticks, and conducted experiments to predict Lyme disease risks. In one such study, Ostfeld observed several plots of land in New England over the course of 13 years, and considered temperature, rainfall, and the presence of acorns, two species of deer, chipmunks and mice as variables in predicting the prevalence of Lyme after two years.

Ticks will develop over the course of two years from their larval state to nymph form, at which point they are able to move freely and bite animals. Ostfeld concluded that temperature, rainfall and the presence of deer did little to help predict the entomological risk:

"In no case did inclusion of deer or climate variables improve the predictive power of models based on rodents, acorns, or both. We conclude that interannual variation in entomological risk of exposure to Lyme disease is correlated positively with prior abundance of key hosts for the immature stages of the tick vector and with critical food resources for those hosts," Otsfeld noted in the study.

Although a pile of acorns in the yard may now seem an ominous portent of things to come, Dill said that raking-up 100 acorns in your yard and disposing of them would hardly be worth the effort. A particularly harsh winter may kill off a few ticks here or there. But we should prepare ourselves to be especially prudent checking our clothing and bodies for ticks in the summer of 2017 and 2018.

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