There are a variety of gall-forming species of small wasps that commonly infest oak trees in our area. Most leaf galls on oak cause little or no harm to the health of a tree. However, twig or branch galls may cause injury or even death to a heavily infested tree. Two common species of twig gall-producing insects are the horned oak gall wasp and the gouty oak gall wasp. The horned oak gall wasp attacks the twigs of pin, scrub, black, blackjack, and water oaks. The gouty oak gall wasp develops in the twigs of pin, scarlet, red, and black oaks. Both of these woody twigs galls on oak look similar, but the horned oak gall has small horns that protrude from around the circumference of the gall. One adult gall wasp emerges from each of these horns. Black oaks are the most commonly attacked trees on long island and the population of gall wasps can become large in one tree. Pesticide applications and even trunk injections are useless for these pests. The best strategy is to prune out and remove infested branches if possible. If an entire tree is infested, removal may become necessary as the tree flags.