While walking with Jello on a wooded trail in our neighborhood, I stepped over this strange looking outgrowth. I leaned in closer to take a picture and was soon threatened by one of the hornets that were working the outgrowth.
I don’t know what it is – or if it’s a hornet – though it likely is. I sent these photos to the local Agricultural Extension Service asking for assistance identifying it and to find out if it’s a nuisance pest.
One thing I’m sure of, I’m unlikely to take that trail again until I know it is safe!
Lindy Low LeCoq 
It’s likely you get an answer, right? Please let us know if you find out.
The answer so far is, “Insect ID can be very difficult, but we are inclined to think these are native bees. At any rate, there is nothing that needs to be done if they aren’t in the direct route of people. Sorry we aren’t able to be more specific.”
I think I’ll put a flag of some sort by the nest to alert others. Thankfully it’s not a well-used trail.
Here’s the latest!
“The photo is of a species of Bumble bee, or Bombus. Since they are normally hidden away, such as in an underground mouse nest, it may have been disturbed hence the exposed cells.
Ground hornet is a folk name for species of Vespula that put their nests underground. In this area that would usually be Vespula vulgaris, Vespula pensylvanica, and sometimes Vespula germanica.
I would need a specimen or better close up photos to determine the species of Bombus.”
Peter Landolt
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The hazards of leaning in to take a picture 😉 I would share your caution.
I didn’t see the bees at first, just thought it was some weird plant!
That would have been a startle!