A: Thank you for bringing samples of your oak tree into the plant clinic. These small structures on the stem are called bullet galls. These growths are caused by a very tiny female wasp, Disholcaspis quercusvirens, placing an egg into the stem. The plant actually forms tissue around the egg, which develops into the gall. The egg pupates inside the gall and when it is fully developed it cuts an exit hole into the gall. The bullet galls have a sugary exudate that attracts stinging insects and ants from late summer to fall. In the home landscape, these galls are of little economic value. However, if these galls are formed on trees grown in the nursery then they can become a huge economic reduction. Chemical pesticides are generally applied during December for moderate control. Light pruning to remove stems may also prove beneficial.