What's Hot at the PPDL

November 6, 2015

Fuzzy Oak Galls Carpeting Lawns Are No Cause To Worry

Cliff Sadof, Extension Specialist, Purdue University

As the end of autumn approaches and only the oak leaves remain to be collected, some Indiana residents living under oak trees have been greeted with a fuzzy surprise.  Lawns appear to be covered with thousands of fuzzy round ball like structures called galls. Oaks produce these galls after a very tiny wasp in the family Cynipidae lays eggs in oak leaves in the early spring, They are so small that even when they are present in large numbers you are unlikely to notice them. The wasp grows inside these galls during the summer and in the fall after the wasp has completed development, galls detach from the leaves and drop to the ground.  Fortunately these galls pose no real health problem to the oaks. Indeed, in the war between insects and plants galls like these represent a truce.  The oak cuts its losses by growing a tumor (gall) and that restricts feeding to just a small area.  

Probably the most fascinating part of this story is that the gall maker gets plants to make the same kind of gall every time it attacks.  This has a downside for the gall makers because the galls are easy to find by their many parasites and predators.  For homeowners and trees this is good news.  Outbreaks of galls that produce enough of them to carpet the lawn is a rare event.  It does not usually last more than a year or two because the gall makers are consumed by their parasites.  So rather than struggling to get insecticide to the tree canopy during the small window of opportunity when it does any good, it would probably be better to enjoy them while they are in your trees and wait for the natural predators to do ‘their thing’. For more information about gall makers and other creatures inhabiting your landscape visit PurduePlantDoctor.com as well as our Purdue publication  “Galls on shade Trees and Shrubs”, http://extension.entm.purdue.edu/publications/E-56.pdf .