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Leaf Galls in Your Trees: Damaging or Not?
We often get calls notifying us of strange, pod-like growths on the foliage of local trees. Due to their ominous appearance, the deformities can cause serious concern for homeowners. When you see such growths in your trees, what you are most likely see are known as leaf galls. And, while they might seem alarming due to their alien appearance and the sheer abundance of them, by the time you notice them, the insect that caused them is long gone. So, are leaf galls harmful to your trees?
What are leaf galls?
Galls are abnormal growths caused by damage from insects and other pathogens. Galls can form on any part of the tree, leaves, twigs, branches and even roots. They can be caused by thousands of different types of organisms. Galls on trees form when insects deposit eggs into leaves or damage the epidermis of your tree with their mouth-parts. While the gall may seem like something created by the insect, it’s actually the tree itself that grows the deformed tissue. The gall results from a hormonal response that the tree produces at the site of the injury that causes scar tissue to grow.
Often, if an insect has laid eggs in the tree tissue, the gall becomes home to the insect’s larvae; they then mature inside the gall and feed on it until they emerge. By the times galls are visible, the larvae has often already matured and is on its way to reproducing in your trees and those nearby.
What’s that weird growth on my pecan tree?
The galls in the image above are are caused by the insect we call Phylloxera, which is similar to an aphid. Some insects will only deposit their eggs in specific tree species. Phylloxera insects overwinter on the stems of pecan trees and become active as soon as the new leaves emerge in the spring. During this time they begin feeding quickly and start depositing eggs again in April. After a few weeks to a month, the mature phylloxera emerge and begin a second generation. Most homeowners do not realize they are active, due to their very small size.
What should I do?
Typically by the time you see galls, the insect has come and gone. Although galls are unsightly, they are typically don’t cause irreparable damage to your tree. Because many galls only cause cosmetic damage, we may not recommend any specific treatment for tree galls.
However, if your tree is in a weak condition or has defoliated then action will need to be taken. In extreme cases certain pesticides can be applied, but this must be done when the insect or pathogen is present in the tree, before it lays its eggs. Insects are opportunistic, and will chose weak plants. Keeping your trees healthy is the best prevention for problems such as galls.
If you are not sure what to do, call an experienced professional arborist. They can help you decide what the next steps should be in eradicating the pest and can offer advice on improving the health of your tree to reduce future outbreaks.