Question
We have a huge maple in our back yard that shades our deck.  I've noticed localized areas of drippings about 1-2ft in diameter puddles (brown, sticky, sometimes frothy).  I read about aphids, but the drippings are the tree's only symptoms - it's not losing any leaves and I haven't seen any actual aphids.  It's sweet to the smell and attracts flies.  Could it be the tree dripping sap?  If it is aphids, there is no way we could spray a tree this big.  Will they go away on their own?

Answer
Sounds like slime flux or wetwood.
The foul smelling liquid that oozes from wounds in the bark or wood of various shade trees is known as slime flux or 憌etwood.'  Fluxing occurs commonly on a large number of species including but not limited to maple, oak, beech, birch, sycamore, and willow but elms are most susceptible.  There are two types of slime flux, brown and alcoholic, distinguished on the basis of margin, color and mode of development.  Brown slime flux originates from heart wood sap.  Before reaching wounded surface, the sap is clear, watery fluid containing several nutrients.  On the surface, it soon changes to a brown, slimy ooze as a result of the feeding by fungi, bacteria and insects.  Alcoholic slime flux, on the other hand, develops from the sap of bark and sapwood.  It is white and frothy and usually forms near the base of the trunk.  The sap is rich in starches, sugars and proteins and, thus, makes an excellent growing medium for various bacteria.

As these bacteria develop in the nutrient rich tree sap, extreme gas pressures are commonly reached up to 60 pounds per square inch.  The high gas pressures frequently force the liquid to seep from the wounds and flow down the bark.  This sap composed of nutrients, fatty acids, bacteria and gasses is a readily available source of nutrients for insects of various types.  Bees and wasp can usually be observed feeding on the ooze.

There is very little that can be done to eliminate the bacteria from the cambium.  There have been various injection techniques researched but not have been reliable enough to recommend routinely.  In fact, injection could cause healthy tissue to be inoculated with the bacteria by disrupting the developmental tissue barrier the tree has produced.  A solution of 10% sodium hypochlorite #1 part household bleach to 9 parts water# will help surface disinfect the area around the damaged area.  This will help discourage insect attraction and help restrict damage to the bark below the damaged oozing area.  There is no control.  The good news is that, just because this problem exists does not mean that the tree is in jeopardy of dying, it just means that the tree has been injured.  Water the tree during droughts and do not let the tree go through any undo stress.  Do not fertilize unless called for by a soil test and the tree is capable of utilizing the nutrients.  Fertilizing a stressed tree improperly may cause the tree to decline further.