Twig galls
6:16 am
Sun July 27, 2014

Blame Wasps And Squirrels For All The Oak Leaves On Your Lawn

If you have an oak tree in your yard, you may have found yourself picking up more leaves and branches than expected this summer. 

A gouty horned gall on a pin oak. The growth is caused by a small wasp, and will eventually grow large enough to cut off nutrients to the leaf.
A gouty horned gall on a pin oak. The growth is caused by a small wasp, and will eventually grow large enough to cut off nutrients to the leaf.
Credit (Courtesy of the Missouri Botanical Garden)

According to Missouri Botanical Garden horticulturist Chip Tynan, there are two culprits.

The first are tiny wasps that cause a growth, known as a gall, to form on twigs and small branches of oak trees. The second offender are squirrels, who think the galls make a tasty snack.

"Basically, you’ve got a meat and vegetable meal for a foraging squirrel," Tynan said. "Much like a squirrel will go ahead and take two or three bites out of your tomato and then toss your tomato away, basically the squirrels take a couple of bites [of the gall] and then go ahead and toss the twig on the ground."

Eventually, Tynan said, galls can grow big enough to choke off nutrients to the oak leaves. That's not a problem as long as trees can grow enough new leaves. But many trees in the state are still feeling the effects of droughts in 2011, 2012 and 2013.

"The drought really just predisposes the tree to more stress. Any kind of insect activity in the tree has a larger impact," said John Beckman, an arborist with Ray's Tree Service in Kirkwood.

The galls appear at regular intervals every two or three years, but without a natural predator, the infestations grow every year. Beckman said this year, the galls are among the biggest he's seen, and he expects that to continue.

Follow Rachel Lippmann on Twitter: @rlippmann