Not a tree pest but what is it?

[greg carr]

I live outside of Boston,found this mass of what looked like wasp nest but it was very hard on a chair cushion in a HOT attic. It was 6" long by 2" wide by 1" tall. inside were 8 larvae lined up like cigars in a box. 3/4" to 1" long yellowish. Any ideas?View attachment 37262

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[Elmore]

Mud Daubers

http://ohioline.osu.edu/hyg-fact/2000/2078.html

 

[greg carr]

The nest was not tubular, more of a rectangular mass, like a small grey brick.Very hard.