Early summer is not a time you would expect to see trees with dead leaves, but Eastern and Central Missourians are seeing white oak leaves turn brown due to a high population of the jumping oak gall wasp. While the tiny, stingless wasps can greatly affect the appearance of white oaks, generally the insects do not cause long-term damage to the trees.
"Jumping oak gall wasps are around all the time but in certain years we get very high populations in one or more parts of the state," said Missouri Department of Conservation Forest Entomologist Rob Lawrence. "There's nothing we can do to control the damage and it's not a serious threat to tree health. But a lot of leaves may fall in severe cases."
Damage to the leaves of white oaks is caused by pinhead-sized galls created when female wasps deposit eggs on young leaves in spring. Each round, button-like gall contains one larva. When the galls drop from leaves in early to mid-summer they leave brown pockmarks. Where many galls were present, leaves look scorched. In the most severe cases, leaves turn black, curl up and drop from trees. Fallen galls sometimes are observed to "jump" due to the vigorous movement of the larva inside the gall. The larval behavior allows jumping oak galls to sink deeper into grass and leaf litter, which helps shelter the larvae during the winter. The larvae emerge as adults the following spring.
Lawrence said a jumping oak gall outbreak can last for up to two years, then natural controls, such as weather and parasitism, will help lower the population numbers.
"With really large populations of jumping oak gall wasps you can easily pick out white oaks across a hillside due to their damaged leaves," said Lawrence. "If the trees lose most of their leaves, they will try to put out a second flush of leaves. Fortunately, we've had a lot of rain this year and that should help the trees recover."
Homeowners can help trees recover by practicing good tree care. A few tree-care basics include mulching around trees, avoiding root damage and damage to bark from lawn mowers and string trimmers. If we have drought conditions later in the summer, Lawrence said watering will be essential to help white oaks recover from jumping oak gall infestation.