Question
Hello Jim,  I live in Maryland and planted two large river birch in our yard late last year that were doing fine until recently.  It's like something from a horror movie -- the tree is covered in two distinct areas with flies (house flies and deer flies) and wasp and hornets!  We noticed this yesterday and thought to see if they disperse over night; today it was the same situation but we noticed a blister had formed under the bark in the lower area were the critters were gathering.  My husband examined the blister and found a milky, watery fluid with a pungent odor filled the blister, come to think of it the flying creatures were acting a bit lethargic (maybe they're drunk?).
Anyways Jim -- can you help us?  These are two of our favorite trees and are about 20' tall with multiple trunks 2-3" diameter and the one with the problem seems to have fewer leaves than the other (unaffected) tree.

Sincerely,

Mirella

Answer
Sounds like slime flux or wet wood.
Slime flux or Wet Wood is a foul-smelling and unsightly seepage of sap from the trunk of shade trees. It occurs in apple, birch, elm, hemlock, maple, mulberry, oak, poplar and willow. In North Carolina slime flux is very common in large, mature, landscape oaks, tulip poplar and elms. This disease is not normally a serious problem if the tree is otherwise healthy.

Slime flux is a bacterial disease. The infected wood is frequently discolored or appears water soaked (wet wood). Gas (carbon dioxide) is produced by fermentation by bacteria. The gas produces pressure in the wood. This pressure forces sap from the trunk through cracks in branch crotch unions, pruning wounds, lawn mower wounds, other injuries and occasionally unwounded bark. This oozing of sap is termed fluxing. The flux is colorless to tan at first but darkens upon exposure to the air. As fluxing continues, large areas of the bark become soaked. Many different microorganisms grow in the flux producing a foul or alcoholic smell. Various types of insects are attracted to the slime flux. If the fluxing continues for months, leaves on affected branches may be stunted and chlorotic. Grass may be killed where the flux runs down the trunk onto the grass. Sap may continue to ooze for several weeks or months, but usually it eventually stops with no treatment and no apparent damage to the tree. This slime flux may be triggered by heat, drought and other stress.

There is no curative or preventive measures for slime flux except to maintain trees in a general good state of vigor and minimize wounds and injuries. More damage can be done to the tree in attempting to cure slime flux than the flux will do alone. If there is loose or dead bark in the slime flux area, remove all of the loose bark and allow the area to dry.