These are generally known as plant galls. The Master Gardener Help Desks are seeing quite a few branches with galls on the leaves brought in for identification and recommendations on what to do. Plant galls, while they can be alarming to find growing on the leaves, are typically harmless to the tree.
Very early in the spring of each year at the time of bud break, an overwintering adult insect will very carefully deposit an egg between the upper and lower epidermis of a very immature leave or as the leaves are just expanding. Those insects may lay just one egg to form a rather large gall or lay many eggs creating a leaf covered later with galls. In all cases there can be a just a bit of leaf distortion to the leaves looking all gnarled up.
Master Gardeners are seeing a lot of oak leaves that have had a variety of different looking galls on them. Each insect makes a distinct looking gall. Some of these gall forming insects are host specific, like the Hackberry Nipple Gall or the Maple Bladder Gall on silver maples. Oaks likely win the contest when it comes to hosting insects that create galls with over 700 gall forming insects using oaks. Common names for the galls on oak are; Horned Oak Gall, Wool Sower Gall, hedgehog gall, Midge Galls and Oak Apple Gall to name some of them. There are lots other galls on different kinds of oaks across the country. Literature notes between 1500 and 2000 different kinds of galls in North America, so Illinois certainly has its share.
Gall forming insects themselves are quite different as well. There are midges, many very small wasps (cynipid spp.) and mites. There are additional insects that can also cause galls on twigs and stems. If gall forming insect weren't enough, bacteria, fungi and nematodes can also cause growth deformities. The wasps are not able to sting as the insect is designed to deposit eggs early on in the season. This might be into a bud that has yet to even begin to grow or as the bud is expanding as noted above. These wasps are so small they go completely unnoticed until you see what they left behind – the gall.There really is no need to attempt a treatment. On occasion a heavy infestation of galls on a young tree may need to be addressed, treatments being planned for the next growing season. Gardeners, who have attempted prevention sprays get mixed results as timing is everything, miss the treatment by a day and all is lost as the eggs are already in the bud or tender leaves. Consider those trees with galls decorated by nature and enjoy the unique relationship gall forming insects have with their host plants.