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These are large members of the Siricidae family, in the Order Hymenoptera (wasps, bees, etc), often called Pigeon Horntails. This female is laying eggs into a decaying sugar maple. It defies imagination to think of an insect being able to insert her long ovipositor into a trunk as hard as that of maple! Bug Guide says of the larvae that will hatch inside the tree:

"larvae require a symbiotic fungus to digest wood(2). It is a complex interaction between three organisms: the woodwasp, a symbiotic wood-decaying fungus, and the host tree."

The Pigeon tremex larvae are parasitized by the large and spectacular Megarhyssa wasps, members of the giant Ichneumonid group: http://www.pbase.com/laroseforest/image/114209788

These wasps do NOT sting! What looks like a stinger in the above photo is the "horn" which gives them their other common name of horntail.