Tree galls are formed when the chemicals from feeding insects react with the chemicals of the trees. Galls can come in a variety of shapes and forms depending on the type of insect that caused them. Linden trees are quite susceptible to leaf galls, which first become noticeable in spring when the leaves are developing. Most likely, linden leaf galls are caused by the linden gall mite or the linden wart gall midge. Control of these tiny pests can be difficult.
Identify whether mites or midges are causing the galls on your linden tree. The linden gall mite's galls are red, conical in shape and protrude from the top surface of the leaf. The linden wart gall midge causes rounded wart-like galls with indentations in their middle. They start out green or yellow and eventually turn brown and woody over time.
Prune the affected leaves as soon as the galls appear. The galls house the eggs and larvae of the mites or midges who caused them. By pruning the leaves before they hatch, you can significantly reduce the population. Successive pruning year after year will reduce their population.
Destroy the pruned leaves. Either burn them or place them in a black plastic bag and leave them out in the hot sun for a week or two. Do not compost them.
Spray linden trees infested with the linden leaf gall mite with a miticide specifically prescribed for the control of eriophyid mites such as Akari or Judo. Follow the manufacturer's instructions for mixing and application rates. But as a general rule, miticides should be applied when the leaf buds first appear and reapplied at two-to-three-week intervals until the leaves emerge.