Seeing a lot of acorns on the ground already? Tiny wasps are to blame

Tiny cynipid wasps have been having a banner year

A picture of an acorn with eggs from cynipid wasps.
The Minnesota Department of Natural Resources says cynipid wasps have been laying eggs in acorns in unusual numbers this year. The larvae kill the acorn, and oak trees have been dropping them in flurries.
Courtesy of Minnesota Department of Natural Resources

It’s a very, very good time to be a squirrel in Minnesota.

A bumper crop of acorns have fallen this year, starting as early as June. They were damaged by tiny wasps.

“We’re talking about super tiny. We’re not talking about wasps that sting you or even look like wasps that would sting,” said Val Cervenka, forest health program consultant with the Minnesota Department of Natural Resources. She said they resemble gnats.

Cynipid wasps lay eggs in acorns, between the cap and nut. The larvae grow into what Cervenka describes as “fat little grubs,” and produce a protrusion, known as a “gall,” on the acorns. That’s why the insects are sometimes called “gall wasps.” The tree recognizes the acorns aren’t viable and drops them, leaving viable acorns to grow.

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“Where there are acorns, there are these wasps. We just don’t notice them. This year was apparently an awesome year for them,” Cervenka said. She said oak observers started calling in August to ask about the recent intense acorn drop.

The Wisconsin DNR has photos of affected acorns here.

It isn’t clear exactly why it happened this year, although climate, precipitation and the life cycle of oaks might have something to do with it. Oaks themselves also have variable cycles of acorn production.

“The reason this wasp could have been more bountiful this year was because of a boom in the acorn cycle last year,” Cervenka said. “Another theory, because we don’t really know why insect populations come and go sometimes, but it’s been a perfect storm if you will, of weather events that have allowed this little wasps to proliferate.”

She said the wasps are endemic to Minnesota and are found pretty much anywhere there are oak trees growing acorns. She said they don’t damage the trees, although people have been calling since August, concerned about the acorns falling. The DNR addressed the issue in its new issue of its Forest Insect and Disease Newsletter.

“Oak trees wouldn’t have survived this long if this wasp was actually harmful to it,” Cervenka said. “There’s still so many acorns left.”