It hasn’t gone unnoticed that in some areas there are an awful lot of acorns this year, especially if you have an oak tree dropping them all over your lawn. If you have an oak tree with branches that reach over the roof of your house, you’ve probably gotten used to (well, maybe not) the regular sound of the acorns hitting your house as they fall. The acorns can be a nuisance but they are an important food source for many animals and the supply of acorns from one year to the next has an effect on not just the animals that eat acorns, but on what eats them, and even on their parasites.
Oak trees produce large seed crops at irregular intervals with lighter production during the years in between. Oaks over a large geographical area often do this as a population in a phenomenon known as masting. This has not yet been explained scientifically or from an evolutionary perspective. The effects of this inconsistent acorn production on some wildlife species and their populations have been studied. This year, I have actually noticed some prolific trees and some with not so many acorns, so it doesn’t seem like a true mast year.
Mammals, birds and insects all feed on acorns. Mammals include white tailed deer, raccoons, white-footed mice, squirrels, and chipmunks. Some of the birds that eat acorns are blue jays, nuthatches, woodpeckers, brown thrashers, grackles, quail, cardinals, wood ducks, flickers, grosbeaks, chickadees, titmice, and towhees. The most common insect that feeds on acorns is the acorn weevil. There are actually two kinds, the long-snouted acorn weevil and the short-snouted acorn weevil. You can take a look at them in the photos. The long-snouted weevil can drill a hole through the acorn shell and feed on the meat inside while the short-snouted weevil feeds on meats exposed by cracks in the shells, mostly on fallen acorns. Both types lay their eggs inside and the larvae hatch and feed on the acorn meat during the fall. They leave the acorns to pupate in the soil over the winter.
The effect of acorn crop fluctuation on an animal’s population has been found to depend at least in part on how specialized a diet the animal has. If the animal relies on acorns and nuts as their primary food source, populations are more likely to decline following years of low nut crops. More generalized feeders like raccoons and crows are not significantly affected because they have many other food source options. Birds are also less likely to be affected as a group, because their greater mobility allows them to go farther in search of alternate foods.
Some interesting work has been done studying the effects of acorn crop size on the population of the white-footed mouse. In northern climates, mouse populations increased the year following a large acorn crop. The following year, the population of mouse predators such as foxes, hawks and owls is typically higher. Their population fluctuates in response to the mouse population which in turn is affected by the size of the acorn crop. Insectivores that feed on acorn weevils also respond to the population ups and downs of their prey.
White-footed mice are of interest because they are hosts to the ticks that carry Lyme disease. A high mouse population influences the tick population. Also, white-footed mice eat the pupae of the gypsy moth, helping to control the population of this damaging leaf-eating caterpillar. When there are a lot of mice, there are fewer gypsy moths, and fewer mice mean more gypsy moths laying more eggs leading to more caterpillars and more defoliation.
Photo: University of Michigan website
Squirrels have a more mutually beneficial relationship with oak trees, because they not only eat the acorns, they also bury or cache them for future use, and in the process plant acorns that are sometimes left in ideal germination sites and will grow into a new tree. So of course, a mast year of acorns helps the oaks reproduce too! Squirrels and chipmunks do love acorns. When there are plenty, fewer overwintering tulip bulbs disappear from my landscape. If there aren’t too many acorns around, the spring garden is sparser. We also see a lot more deer browsing damage in winters following a light acorn year.
As mentioned in the first paragraph, a bumper crop of acorns can make a bit of an unwelcome mess in lawn and garden areas. We’ve received inquiries at the Home & Garden Education Center this year looking for tips on how to clean them up other than raking. Well, there aren’t a lot of options. I found one unofficial mention of someone successfully cleaning up their acorns with a strong shop vacuum. A very strong backpack style blower will move them pretty well. Mowers don’t generally pick them up. I hope that when you clean up your acorns, you’ll place them or dispose of them in an area where the local wildlife will be able to find and eat them. You might even want to plant a few!
A fun fact I found on the Farmers’ Almanac website is that a big acorn crop has historically been thought to predict a rough winter. This brings to mind (mine anyway) the traditional association of the width of the woolly bear caterpillar’s stripe with the severity of the upcoming winter. Folklore has it that a narrow band means a severe winter and a wide band means a mild winter. No scientific support for this, but fun to watch for! An article by C. Pollack of Ohio State says the width of the band is actually related to the age of the caterpillar. Apparently, the reddish band gets wider with each molt of the caterpillar, so it gets wider as the caterpillar ages.
Let’s see what this winter brings. We have a lot of acorns this year. Keep your eye on those woolly bears.