Jumping Oak Galls Can Cause Brown Leaves and “Fall-Out”

Curling and premature drop of white oak leaves in early summer along with a “fall-out” of tiny balls (Figure 1) onto surfaces below indicate that jumping oak galls are abundant. These leaf galls are the handiwork of a tiny non-stinging wasp. Fallen galls spontaneously jump with flips of the wasp larvae inside. This random movement allows them to settle into ground cracks where they will spend the winter. Small bumps appear on upper leaf surfaces (Figure 2).

Figure 1. Jumping oak leaf galls that have fallen from an infested tree. (Photo: Lee Townsend, UK)

Figure 1. Jumping oak leaf galls that have fallen from an infested tree. (Photo: Lee Townsend, UK)

Figure 2. Small bumps surrounded by yellow halos are visible on the top view of an infested leaf. (Photo: Lee Townsend, UK)

Figure 2. Small bumps surrounded by yellow halos are visible on the top view of an infested leaf. (Photo: Lee Townsend, UK)

Look for tiny button-shaped galls on lower leaf surfaces (Figure 3).

Figure 3. Galls on underside of heavily infested leaf. Light galls are a few days away from maturity and exit of the insect, while dark galls are “ripe.” (Photo: Lee Townsend, UK)

Figure 3. Galls on underside of heavily infested leaf. Light galls are a few days away from maturity and exit of the insect, while dark galls are “ripe.” (Photo: Lee Townsend, UK)

Gall development begins when female wasps place eggs in expanding leaves in spring. Chemicals from the insect cause leaf tissue to develop around the grub-like wasp larva, providing it with protection and food.  The more galls that are present on a leaf, the more browning that will occur. Leaf loss is unlikely to affect healthy, established trees. However, it can harm already stressed or damaged trees.

Management

Insecticide applications are not recommend for gall wasp control. It is difficult to time insecticide treatments correctly and often sprays kill many of the natural enemies that reduce gall formation in most years. White oaks in the landscape may benefit from fertilizer next spring and watering if infested trees are under drought stress. Outbreaks of this insect occur at intervals and may last for 2 years; the last significant infestations were reported in 2010. The best strategy is to promote tree health and reduce stress.

 

By Lee Townsend, Extension Entomologist

 

 

Posted in Landscape Trees & Shrubs