Question
I have a mature maple near the driveway that is oozing an excessive amount of sap.  The sap is visible in a line up the tree about 12 feet and if I park my car under the branches, it is covered with sap in about a day.  This has been going on since Spring which is unusual.  There is now quite a swarm of yellow jackets and flies which is what prompted me to write.  There is no nest visible and they are particularly attracted to the sap near the trunk and what appears to be "bark nodules" on some of the lower branches.  I figure if I treat the sap issue, the insect situation will be treated as well.  Any suggestions?

Answer
Sounds like two problems.
If the sap getting on your car is sticky this is called honeydew a substance excreted by an insect called an aphid. Honeydew is high in sugar which makes it sticky and usually a black sooty mold will grow on the honeydew. Aphids suck the plant juices from the leaves and twigs and this sticky mist falls on the objects below. Control the aphids and you will control the sticky mist.

Chemical Control - Contact Insecticides Numerous contact insecticides are registered for aphid control. Since aphids are often placed under considerable pesticide pressure in field crops and greenhouses, they may be resistant to certain categories of insecticides. Therefore, if you do not obtain reasonable control, consider rotation to another insecticide. Contact insecticides currently registered for aphid control include: acephate (Orthene), , bifenthrin (Talstar), chlorpyrifos (Dursban), ), diazinon, , malathion, nicotine sulfate, pyrethrum,  rotenone, resmethrin, and tetramethrin + sumithrin.

Chemical Control - Systemic Insecticides Several systemic insecticides are useful in aphid control. Aphids have sucking mouthparts and are thus very susceptible to pesticides located in the plant vascular system. Some of the systemic insecticides also have contact activity. Systemics injected or applied to the ground are less harmful to beneficial insects. Systemic insecticides include: acephate (Orthene).

The second problem is the oozing of sap from the trunk. This is called wetwood or slime flux.
The bacterial wetwood is found in the roots, branches, and trunks. The cause of the condition is an infection of the heartwood caused by several common types of anaerobic soil bacteria (bacteria to which oxygen is toxic). These bacteria feed on substances in the wood, releasing fatty acids, methane and carbon dioxide gases. The fatty acids go rancid leaving the wood of the tree water soaked and foul smelling.

The gaseous by-products create a hydraulic pressure which forces liquids out of cracks in the bark, branch crotches and pruning wounds. These fluids will turn brown once they come in contact with air, and leave a slimy, fetid ooze dripping down the bark. The liquid raises the pH of the tree making the interior more alkaline. The bacteria do not cause any wood decay, and infected wood is also very resistant to decay.
There is no satisfactory control for bacterial wet wood. It does cause die back of branches, but the raised pH prevents decay fungi from rotting out the centre of the tree.

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. Do not apply a wound dressing.  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.