Fence disintegrating.... baffled!

Backyard fence has been literally falling apart before my eyes..... I've been waiting for Spring and money to do something about it, but it is just literally collapsing at breakneck speed.
If you look at the photos, it looks like termites. But I've scanned everywhere, all over the place and have yet to see one single bug, anywhere. Nothing.
Has anyone come across this? I would think you'd see at least one bug if it was infested!
And then if it is, what in the world to do? If I take this fence out and replace it, won't they just start lunching on the new fence?
It was 70'F out yesterday, so you'd think if they were hiding due to winter they'd be out in force. Just bizarre.

 

It looks like rot. I can't tell what wood the species is but it might be pine. I would not expected an untreated pine fenceto last more than a few years.

 

I honestly knew nothing about dry rot fungi, just learned about this today online.
Could that be what it possibly is? A fungus attacking the fence.
Would termites on a fence, or carpenter ants be visible, somewhere? Because again I can't find a single bug, even when prying into the damage.
If its fungus, would it have to be completely vaccummed up, will this fungus infect/spread to a new wood fence put in its place?

 

What region are you located in?
Subterranean termites live in the ground and are seldom seen above ground unless you dig into the wood although you should be able to find their path to the ground.

 

#'s 5 & 6 are the best pictures for detail but even they aren't that good. Those pics appear to be subterranean termite damage, as there are "channels" or "galleries" along with what appears to be dried mud. Termites and other wood destroying insects like chronically damp/wet wood, so it's common to have rot and insect damage in the same structure. The direct wood to ground contact will facilitate both. Termites stay inside the wood to maintain a humidity level that they like. Take a stout screwdriver and probe into the wood near the ground and look for the white, worker termites. I would only replace that wood with pressure treated, especially anything with wood to ground contact.

What region/state/country are you in? Knowing this can help a lot with insect ID.

 

Really the only solution is to replace the fence. The old fence has reached the end of it's life. Replacing it with a pressure treated fence can provide a longer life but nothing lasts forever, especially wood outside in the weather.