Tree galls are small, bump-like growths that form on a variety of trees due to insects such as aphids attaching the plant’s tissues. Galls can form on all portions of the plant including leaves, branches, the trunk and root systems. Controlling gall formations means controlling the aphids. The aphid feeds or lays eggs in the live tissue creating the gall. Galls will not injure the tree long-term, though in younger trees, leaf drop may occur.
Horticultural oils, such as neem oil, work well in controlling aphids creating galls. One benefit in using horticultural oils is it does not kill or affect the aphid’s natural predators such as parasite wasps, lady beetles and the syrphid fly. Apply the oil only after the tree is thoroughly water, early in the day and when temperatures are below 90 degrees Fahrenheit, according to the University of California. Gardeners should spray the entire plant’s foliage, especially the underside of leaves where aphids are more likely to occur. Horticultural oils will kill any aphids present on the plant only on the day the product is applied. Continued applications are required to kill future infestations. Spraying trees while they are dormant in early spring kills any aphid eggs before they hatch.
When aphid infestations are heavy and horticultural oils do not seem to have any effect, gardeners can use insecticides to control the insects creating the tree galls. Products such as malathion, permethrin, dicofol and acephate work well in controlling and killing aphids and their eggs. Disulfoton is a pesticide applied to the soil surrounding the tree to control aphids. This product is highly toxic to humans so protective gloves are required when using it. The downfall of using insecticides to control aphids is that it kills the beneficial insects as well as the pests. Spray the entire tree’s foliage with the insecticide, covering the outside and underside of leaves, with repeated weekly applications possibly required. Gardeners should apply insecticides early in the day and before temperatures rise above the 90 degrees Fahrenheit mark. Trees require thorough watering before applying the insecticide.
When aphid infestation creating the tree galls is not too problematic, gardeners can simply prune away the affected areas of the tree. This is the least toxic method of treating the galls and aphids creating them. Gardeners should look for galls on the leaves and branches and see if aphids are present. Aphids make holed galls, allowing them an escape hatch to exit the infected area, according to the University of California. By removing the affected areas of the branches and leaves, gardeners are removing the aphids creating the problem. Any leaves or branches infected with galls and aphids require disposal in a plastic bag or garbage can where they cannot spread to other plants.