Insect Pests
There are many, many insects that live in and around trees. Some are serious pests that will likely kill the tree (e.g. emerald ash borer, damage pictured above). Other insects are low-level pests that may kill a tree if the tree is already stressed by other issues such as drought, and many insects are beneficial to trees. Below are a few common examples of insects that you may have seen in and around your trees.
Emerald ash borer
This beetle is native to northeastern China, the Korean peninsula, and eastern Russia. It was first detected in North America in 2002 but historical evidence shows the insect was already established in southeast Michigan in the early 1990’s. Emerald ash borer was likely introduced to North America in wood used for shipping.
Emerald ash borer larvae kill trees by eating the living tissue between the bark and woody center of the tree. There are many other bark beetles (~6000 species worldwide) that follow the same life strategy, but because this beetle is not native to our area, our ash trees do not have any defense against them.
Although most of the mature ash trees are likely to die in our area of upstate New York, there may be hope for the future. The Department of Agriculture has been releasing parasites that feed on the ash borer since 2007; these natural enemies of the ash borer could reduce their population to the point where ash trees can make a comeback. Private landowners can also participate in the effort; click here for more info.
Carpenter ants
Despite what you may have heard, carpenter ants do not eat wood and they are not causing your tree to decline. However, they can be a sign that a tree is unstable and potentially hazardous. A large colony can spread from trees, stumps, and wood piles into decks and homes.
Carpenter ants mostly eat other insects and prefer to dig their nests into rotten wood. If you see carpenter ants in one of your trees it’s a very good sign that the inside of the tree has been decaying for some time. Their burrows don’t usually make the tree less stable unless the colony has grown very large and has begun excavating solid wood, which is rare.
The tree in the picture was able to (mostly) contain the decay and the ants. Yes, it looks bad, but the section of the tree in the picture had only lost about 20-30% of its strength, and pruning might have been a reasonable option to preserve the tree.
Any tree with carpenter ants that is near a house or other structure should be inspected and may need to be removed. In some cases a tree can be pruned to reduce any hazards, but the lifespan of a tree with carpenter ants is limited and it will likely be safer and more cost-effective to remove the tree sooner rather than later.
Gall insects
Galls are abnormal growths that can be caused by many things: insects, nematodes, fungi, bacteria and viruses. These pests cause galls by stimulating the tree to produce more growth hormone in a small area, which causes the tree to send extra nutrients there. These hormones cause the enlarged gall to form while the pests feed off of the excess nutrients. Insects are the usual suspects.
If the tree is young and the infestation is heavy the tree may suffer, but gall-causing insects are generally not a threat to the health of a tree. They’re more of an eyesore than anything else.
After the galls form they protect the insect and pesticide treatment is generally ineffective. Also, after a gall begins to form it will continue growing even if the insect inside dies! The severity of infestation varies from year-to-year and depends mostly on weather conditions.