Insects form galls on the leaves of trees and shrubs, including elm (Ulmus spp.).
As an arborist, I work with a lot of people who care deeply about their trees and shrubs. Almost once a week, I will get a call from someone who is alarmed that something new they’ve noticed on their tree might be a major problem. Sometimes it is a problem that needs help, but often it is something that looks bad, but isn’t. Here are some of the common issues that arise.
Leaf and Twig Galls
Some insects, wasps and mites use a chemical to lay eggs inside of leaves or twigs that causes a swollen area, called a gall, to form. This provides a nice place for the egg to grow into an adult. Most of the time these are not significant enough to harm the tree and no treatment is needed. However, treatment may be warranted if the majority of the leaves are heavily damaged, or if a lot of branches and twigs are dying.
Seasonal Evergreen Needle Drop
Evergreens shed their leaves, just like deciduous trees. Although deciduous tree leaves last one season, evergreen leaves, called needles, last for two to seven years, then turn yellow and fall. This can look pretty alarming, especially on white pine (Pinus strobus), but it is normal. It is still worth taking a close look because there are several fungal diseases that will cause early needle loss. Normal needle loss will have needles that are uniformly yellow, whereas diseased needles will have black spots and uneven coloration.
White pine and many other conifers normally shed needles in fall.
Trees naturally shed branches, and some species are more prone to this than others. Honey locust (Gleditsia triacanthos var. inermis), ash (Fraxinus spp.), pin oak (Quercus palustris) and birch (Betula spp.) are examples of trees that shed branches. As long as the leaves have normal color, size and density, there is no issue. Be on the lookout for several branches dying from the tip back, because this is a sign of a problem.
Moss and Lichens
Trees provide habit for many other organisms, including mosses and lichens that grow on the trunk and branches. These are not causing any harm.
These harmless holes in the trunk of an Austrian pine (Pinus nigra) have been made by a sapsucker.
A sapsucker is a type of woodpecker that drills lines of holes on the trunk to feed on the sap and the bugs it attracts. Most of the time a healthy tree can deal with the damage.
There are enough bugs in the world to keep entomologists busy for several lifetimes. Watch for damage to the plants, and become familiar with the common culprits for plant damage in your area, but realize that most bugs you see are harmless.
‘Harmless fungi form a condition called smooth patch on a bur oak.
This is found especially on bur oaks (Quercus macrocarpa). Fungi feed on the rough, dead outer bark of the tree leaving smooth patches. This doesn’t cause any harm to the tree.
Anthracnose and Leaf Spot
Anthracnose and leaf spot are generic terms for fungi that damage the leaves of plants. Oaks and ashes are the most common trees to see anthracnose. Look for distorted and curled leaves with black and brown dead spots. Leaves affected by anthracnose will fall from the tree in late spring to early summer. Unless the tree has lost a majority of its leaves, or is severely impacted for more than one year, treatment is not generally warranted. Treatments are available for severe cases, or where aesthetic impact is important. Most of the time, the best defense is good general care, such as water and mulch. Fertilizer if a soil test shows that it is needed.
Many people would disagree that squirrels, the insatiable chewers that they are, are not a serious problem. At least for the trees, they are not. Squirrels make nests out of plant material. Sometimes they will pick a tree to harvest twigs from and will chew off the tips of dozens of branches, leaving a carpet of branch tips on the ground. It is alarming, but won’t cause serious issues for the tree.
These caterpillars eat foliage from trees and build silky tents around the branches. Except in severe cases, they are not a problem for the tree and treatment is not needed. To help keep the population under control, clip out the tents and throw them in the trash.
This bug feeds on plants and covers itself in a substance that looks like spit. It does not cause severe damage. If the population is becoming large enough to be a nuisance, treat them with the spray of a hose, insecticidal soap or horticulture oil.
The spittlebug covers itself with a harmless, spit-like substance, which gives the insect its name.
I’m listing this with the caveat that they are a serious problem for your home and can be a sign of a serious problem in a tree. However, with trees, ants are not the cause of the problem. These large black ants eat wood that is already dead. Carpenter ants signal there is dead wood in the tree. Dead wood, or decay, can be a structural weakness for the tree that the ants can make worse. If the ants are present, I recommend having an arborist inspect the tree.
Thin-skinned trees, such as a young red maple (A. rubrum), frequently develop a crack in the bark as a result of growth.
Ash Flower Gall
These tiny eriophyid mites attack the flowers of ash trees, making them distorted and black. It is unsightly, but doesn’t cause any harm. I don’t recommend treatment for this pest, unless you absolutely cannot live with the way it looks.
Bark Cracking on Maples
Many smooth-barked trees, especially maples (Acer spp.), have vertical cracks that appear in the trunk. This is because of the way that trees grow, not any kind of weakness or problem. A situation that can look similar that is a problem is when the bark is damaged during the winter, due to rapid heating from the sun. The difference is that there will be dead wood and peeling bark in the area damaged. Wrapping the trunk of thin-barked species with fabric during the winter can prevent this.
Ultimately, I’m always happy to take these calls, because it means that people are paying close attention to their trees. I encourage you to pay attention throughout the year. If you’re in doubt about something you see, do a little research or check with a local expert to be sure. If nothing else, you will gain a greater appreciation for the many nuances and complexities of nature.
A version of this article appeared in a January/February 2014 print edition of State-by-State Gardening.
Photography courtesy of Jonathan Heaton and Bartlett Tree Experts.