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5 Tree Pests to Lookout for This Month

Tree pests & disease have been sweeping across the country, overtaking millions of trees and destroying what were once dense, tree canopies.

Tree pests and disease have been sweeping across the country, overtaking millions of urban trees and destroying what were once dense, tree canopies. The wave of outbreaks has cost cities millions of dollars in tree maintenance and removal costs, making it critical to identify tree pests early on and prepare a plan of action before trees are infested.

But identifying pests can be a challenge. Trees can share many of the same symptoms if they are stressed or infested. In some instances, the only way to confirm whether your tree is infested is to hire a certified arborist to examine it. However, there are early signs to keep an eye out for, especially with some common tree pests.  

Watch Ask an Arborist: How do I Check for Tree Pests?

The end of the summer is a great time to check trees for pests. Spending 10 minutes of your day to check trees for signs of damage can help save your trees, and your wallet.

Emerald Ash Borer

The emerald ash borer kills millions of ash trees each year. Adult borers are very small and difficult to spot, but trees infested with the emerald ash borer will exhibit serpentine, S-shaped feeding galleries packed with frass and sawdust. Later in the season, you may see D-shaped holes in the bark as the adults emerge. Ash trees with crown dieback from the top down and yellowing foliage are typically infested with the emerald ash borer.

Tent Caterpillar

There are two types of tent caterpillar: eastern tent and western.

Eastern tent caterpillars feed on oaks, aspens, sugar maples, and other hardwood shade trees. The caterpillars emerge in the early spring, around the same time trees begin to bud. Signs of infestation include the presence of large silken tents, stunted tree growth and stripped leaves.

The western tent caterpillar attacks a wide range of trees including poplar, willow, cottonwood, quaking aspen and especially fruit trees. A tell-tale sign is their white silken tents in mid to late spring. The larvae live in a colony, feeding together and expanding their tent as the season progresses. Shriveled or brown foliage and defoliation beginning at the top and outer branches are the first signs of a western tent caterpillar infestation.


Aphids eat away at the sap in tree leaves and stems, leaving behind a sticky syrup called honeydew. Large infestations of aphids can stunt new leaf growth and cause misshapen, curling or yellow leaves. You can spot an aphid infestation by checking the underside of leaves for the pests, or checking for sooty mold, which is often attracted to honeydew.

Read Common Tree Pests and How to Spot Them

Spruce Budworm

Spruce budworm caterpillars feed on the buds and needles of fir and spruce trees. Defoliation of the tree usually starts at the top and works its way to the crown. Signs of an infected tree include the destruction of buds, abnormal spreading of new twigs, defoliation of current-year shoots and, if an affected branch is disturbed, the presence of large numbers of larvae suspended from strands of silk. Every 30-60 years spruce budworm undergo a population outbreak, overtaking thousands of conifers.

Mountain Pine Beetle

The mountain pine beetle is a small insect, native to western North America. It lives most of its life in the inner bark of ponderosa and limber pine trees. The beetles leave blue stain fungus in the sapwood that prevents the tree from repelling. Other signs of infestation include popcorn-shaped masses of resin, called “pitch tubes”, where the beetles have entered. The needles on affected trees generally turn red.

What do I do if my tree is infested?

For many infestations, treatment may only slow down the infestation, but depending on the severity and type of infestation, it may not be a cure. If your tree shows signs an infestation, contact a certified arborist to examine your tree.

  • Make note of what you found, where you saw it and take a photo, if possible.
  • Also, try to capture the insect. Place it in a container and freeze it. Doing so will preserve it for easy identification by the USDA.
  • Finally, report your findings to your state department of agriculture.
  • Contact Princeton Tree Care at (609) 256-6722 for a complimentary tree assessment to ensure the health of your tree. You can also click “contact” at the top of this page to book with us online or email us at


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