2010 may go down as the year of the oak gall, said a University of Missouri Extension horticulturist.
 
     MU Extension centers have been deluged with questions about two different oak gall pests, said Chris Starbuck.
 
     “Many people are curious about what is causing golf-ball-sized ‘tumors’ on branches of their pin oak trees. Others wonder why the leaves of their white oaks are turning brown. In either case, it is a tiny wasp,” he said.
 
     The good news is that oak galls are seldom fatal to trees.
 
Pin oak galls
 
     Pin oak galls are caused by certain species of wasp that lay eggs on twigs. These young branches swell as the larvae hatch and mature, which can take a couple years. Galls can grow as large as 3 inches in diameter before adults emerge to make more galls on the leaves. Adults emerging from the leaf galls then go on to make more twig galls, and the cycle continues.
 
     Two types of gall commonly affect pin oaks, caused by two closely related wasp species. In “horned oak gall,” spiky projections protrude from the gall. “Gouty oak galls” lack these projections.
 
     “Generally, even a heavy infestation of gouty or horned oak gall does not kill trees,” Starbuck said. “Over time, however, with increasing branch dieback, trees become unsightly. Unfortunately, there are no control methods that have proven very effective on controlling this pest on large trees.”
 
     Certain insecticide treatments can reduce the leaf gall stage, but they also tend to kill off most of the predators and parasites that help control the wasp population. Long-term studies indicate that even treatment with systemic insecticides for several years can’t eliminate the problem.
 
Jumping oak galls
 
     The galls leading to browning of leaves on white oaks are the work of the tiny “jumping oak gall wasp.” In this case, the galls are only pinhead-sized, but there are hundreds of them on the underside of a single leaf. The button-like galls eventually drop to the ground, leaving a pockmark on the leaves. Badly infested leaves unusually turn brown and may even fall off.
 
     The galls are called jumping galls because, similar to Mexican jumping beans, each one contains an acrobatic larva that causes the fallen gall to appear to jump. This allows the galls to work farther into the litter under the trees, where larvae are sheltered from adverse conditions.
 
     Prolonged snow cover, such as seen last winter, may provide additional protection to the larvae, leading to the current population explosion people are seeing now, Starbuck said.
 
     As with pin oak galls, jumping oak galls almost never cause tree death. Natural enemies drastically reduce the population of this pest after a year or two, so control measures are unnecessary.
 
     Raking and burning fallen leaves may help reduce infestation the following year.
 
     “Although horned, gouty and jumping galls cause dramatic symptoms, they are not fatal,” he said. “Keep in mind that such pests usually go through population fluctuations due to natural enemy buildup and climatic conditions.”