Gardeners have been seeing lots of lumps, bumps, and blobs on different kinds of leaves throughout the home landscape, or in parks and the forest preserves. It is not uncommon, as this occurs annually. What is uncommon is the generous number we are seeing this year.
These growths are generally known as plant galls. Plant galls, while they can be alarming to find growing on the leaves, are typically harmless to the tree. Literature notes between 1,500 and 2,000 different kinds of galls in North America, and Illinois certainly has its share.
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 leaf or as the leaf is beginning to expand. Those insects may lay just one egg to form a rather large gall, or they may lay many eggs creating a leaf covered later with galls. Following suit, leaves afflicted may only have a bit of leaf distortion or they may appear very gnarled.
Many photos of oak leaves have been submitted to the Master Gardener Help Desk, and they show a variety of different galls. 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 more than 700 gall forming insects using oaks. Locally, common galls on oak trees include Horned Oak Gall, Wool Sower Gall, Hedgehog Gall, Midge Gall and Oak Apple Gall, and new to me this year, Jumping Oak Gall. These vary across the country.
Gall forming insects themselves are quite different as well. There are midges, very small wasps (cynipid spp.), and mites. There are additional insects that also can cause galls on twigs and stems. 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.
As if gall forming insects weren’t enough, bacteria, fungi and nematodes also can cause growth deformities. However, 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, with treatments being planned for the next growing season. Gardeners who have attempted prevention sprays get mixed results, as timing is everything.
Consider those trees with galls decorated by nature, and enjoy the unique relationship gall-forming insects have with their host plants.