I doubt the problem is termites. Depending on your location, only a couple of species can or will infest living trees - most prefer dead wood or wooden structures - so unless you live along the Gulf Coast, California or Hawaii, chances are pretty good there is some other reason for the tree's decline. Possibly bronze birch borers, which are a concern with virtually all birch species other than river birch.

I don’t know if this link will work. The above photos that are close up are screen shots from a video, and are very blurry. I’m trying to add a link to the video here. you can clearly see the insects running around carrying eggs. I’m in CT zone 6.

https://share.icloud.com/photos/0a6Hr9fjQY0oaTNfsIVePxfOQ

So maybe the wood was already dead and decaying when they moved in? In any case, the trees are kaput.

Joanne,

IME: Birch trees can be a bit finicky if not growing under ideal conditions to their likings.

Your pine trees tell me that your soil is on the sandy side and during dry spells or drought, birch can begin having health issues.

Birch will get cankers and root problems, and bronze birch borers can move in, leading to the decline of the tree, eventually killing it.

So, the insects aren't necessarily what kills the tree, the insects just take advantage of a weakened tree that hasn't the resistance to fight them off anymore.

This can all happen over a period of years.

The link didn't work for me. And your previous photos don't show any insects clearly. Also, the tree(s) are clearly not yet dead so still not very likely a chance that the problem is termites as they so rarely infect living wood, especially in CT.

As to the trees, the one trunk with the decay should be removed. Birches are often planted in clump form so if that's the case here, the others look okay....... for the time being. However if it is BBB, they will inevitably succumb so removal of the entire clump maybe advised.

If there are insects, I'd guess they're carpenter ants. They tunnel thru dead wood, and they do promote more rot as their nests contain and conserve moisture.

I saw the video. I couldn’t quite see if they were ants or termites. Not carpenter ants, which are quite large. The video showed winged and wingless insects, mostly transporting eggs and maybe larvae from one location to another. They are a light brown color. When I enlarged the video, it just became blurry. I couldn’t pause it, which would allowed me to capture a good image.

Looking back at the third picture in the original post,

the other two trunks don't look all that great either, i.e. the base of the largest one and the split in the one further back.

You can enjoy them for a time and as long as there isn't anything in the way that will get damaged if they fall in a high wind.

Love birches!