There are several identifiable signs of injury left by each stage of the beetle’s development in and on trees, some of which can be seen year round.

Trees stressed by Asian longhorned beetle may exhibit early fall color by mid-summer, and weakened branches can break causing liability issues for people and property.

The beetle prefers maple species (Acer spp.), including boxelder, Norway, red, silver, and sugar maples. Other preferred hosts are birches, Ohio buckeye, elms, horse chestnut, and willows. Occasional to rare hosts include ash, European mountain ash, London planetree, mimosa, and poplars.

Eradication attempts and quarantines have been successful regarding this pest, especially when it was found in the beginning stages of infestation.

Adult beetles often reinfest the same host tree from which they emerged. The damage to the tree can be tremendous, and can build up over a number of years, eventually killing the tree or weaken it to a point where limbs begin to break or other structural failure occurs.

Perfectly round exit holes can be found when the adult beetles emerge from the tree beginning in the month of May and June and continuing through mid-summer. Exit holes are 3/8 to 1/2 inch in diameter, and may extend well into the wood of the tree. Both exit holes and egg pits may become callused over with time, as the tree tries to contain the injury.

Link - The Birdwatcher's Field Guide to Holes in Trees (.pdf)

Adults emerge in early to mid-summer and egg laying begins soon after. Adult female beetles will chew an oviposition pit on the outer bark for each egg that she lays. The shape of the pits will vary depending on the thickness of the outer bark, ranging from circular, to oval, to just a slit on thin-barked trees. Mandible or teeth marks are often visible around the edges of the pit.

Freshly chewed egg pits are the easiest to see because the inner bark will contrast in color with the outer bark. Older pits will begin to weather and fade to a similar color as the outer bark. Pits can be identified for several years by careful inspection due to the callus wood that forms around the pit over time.

Adults feed on young bark and leaf veins before seeking a mate. Shoots can be completely stripped of bark and girdled, and there are often holes in the leaves where veins were once present.

Sap may flow from egg pits and early larva galleries, especially on maple trees, as the larvae feed inside the tree. Sap may draw other insects such as flies, wasp, and various beetles; or may be blackened in color due to a common sooty mold that may begin to grow on the surface of the bark where the sap runs.

Link - USDA-Forest Service; Asian Longhorned Beetle and its Host Trees (.pdf)

Early stage larvae tunnel through the outer sapwood. As larvae mature, they turn and make long tunnels deeper into the branches and trunk of the host tree.

The damage to the tree can be quite extensive considering the large size of the larva as they develop and consecutive generations that are present in the host tree.

Long splintered frass, or sometimes referred to as 'sawdust,' will be evident from the larva feeding on wood as it tunnels through the tree. Frass is often noticed falling from cracks in the bark and branches of the tree, and may even collect in branch crotches of the tree, or collect on the ground.

Frass will also be packed in the long tunnels left by the larva. The internal damage from tunneling larvae and pupal chambers are only visible when the tree is completely dissected and split apart.