By the way, the hole is on the trunk, about six feet from the ground, just before the trunk branches off.

do you have woodpeckers???

borer didnt chip away a larger flake of bark ... which appears to be missing ....

besides. im not sure i would go all nuclear chemical warfare on one hole ....

ken

Do a web search for "chinese pistache borer" or similar phrasing and see if confirmation that these do in fact get bored on a routine basis comes up. And what insect does it, what to do about it.

it appears you are cheating on us ... lol....

https://www.arboristsite.com/community/threads/hole-in-tree.349620/

or on them with us ... lol

ken

LOL There's a funny story behind that. I tried to log-in to Houzz/Garden Web with my old credentials to ask this question, but I kept getting an error about Houzz trying to load the page too many times. So, I went to the Arborist. Finally, I just created a brand new profile on Houzz. I must say Ken, you really make your way around. :-)

And what do you mean about the borer chipping away a larger piece of bark/ Is that a common mark of borer holes?

I did an internet search on woodpecker holes in trees. I will say that the perfectly round hole in my tree does look exactly like the photos of woodpecker drill holes I found online. But the photos online showed dozens of holes all together. My tree only has one single hole. Perhaps the woodpecker that visited my tree was an underachiever. Lol

Hard to say for sure, but the exit hole appears to be somewhat flat on one side which is characteristic of the flat headed borers (family Buprestidae), which includes the emerald ash borer and the flatheaded appletree borer. The latter species has a wide host range.

in your pix.. see the brown stain at 7 oclock below the hole .... a piece of gray bark is missing .... fwiw ...

keep us posted if you get a reply on the other site

ken

So, it is an insect borer hole afterall?

It does seem unlikely on a healthy, established tree, but I think it's possible. Was the tree under some kind of stress during the last year or two? Is there any evidence of larval tunneling under the bark, maybe to the upper right in your photo? Does the tree show any signs of decline? Which side of the trunk are we looking at (flatheaded appletree borers often attack the south side of the trunk)?

It is in good health and the hole is on the North side of the tree. Someone suggested I squirt some insecticide into the hole. I'm not sure if that is safe for the tree though and if that would even be effective.

And like I said before zone 6 is on the very northern edge of a Chinese Pistache tree's hardiness zone. I have seen a few other Chinese Pistache trees die that were planted in my area. It seems like about half of this species of tree planted around here die after about 10 years. I think the environment itself can stress this drought and heat loving species of tree.

Someone suggested I squirt some insecticide into the hole

As arbordave posted this would be an exit hole - the insect has completed its development inside the tree and left.

So, are all holes like this, if they are borer holes, exit holes?

It does have the typical appearance of a Buprestid beetle exit hole. See also this Colorado State University fact sheet that describes the exit holes and feeding damage of some of the common borers of ash trees.

"Adults of the flatheaded appletree borer similarly emerge from trees primarily during May and June, cutting their way through the bark and producing a D-shaped exit hole. However, flatheaded appletree borer is a larger insect than emerald ash borer with a somewhat broader body form. The D-shaped exit hole it produces is slightly larger and wider than what an emerald ash borer will produce."