Hello Amy,

The most common pests of the Tulip tree (Liriodendron tulipifera) are scale and tulip tree aphids. The ants are attracted to the sweet substance, honeydew, which the aphids excrete. You are correct that the ants farm the aphids. Farmed aphids supposedly produce larger drops of honeydew and it is also used to feed the ant larvae. According to the article by the University of Illinois; Honeydew is highly sought by ants, parasitic wasp adults, and other insects as a food source. It is so highly valued by ants that some ant species farm aphids, moving them to uninfested areas of plants, moving them to other plants, and even carrying them underground into their nests for the winter and then bringing them out in the spring to place them on the proper plants.

For help in reducing the ant population you can try banding the tree with a sticky substance known as Tanglefoot . This method can be a bit messy but has been used successfully to control gypsy moth larvae. Also try using large ant bait stations around the base of the tree. This may help as they will take the poisoned bait back to the nest. Caution with this method as to keep the ant traps away from any pets or curious children you may have in the area

More information on use of sticky bands.

Managing ants on trees and shrubs (ucanr.edu)

Making a Sticky Barrier Band – Gypsy Moth in Wisconsin

Once the ant population is reduced this will allow natural aphid predators such as ladybird beetles, lacewings and tiny parasitic wasps, to move in and help reduce the aphid infestation.

More information on aphids.

Aphids (illinois.edu)

Aphids, in-depth – Wisconsin Horticulture

Be Ready for Aphid Attacks | USU

Information on Scale

Scale on Tulip Poplar Trees #470088 - Ask Extension

For aphid control it will depend on how high the infestation is. Sometimes a blast of water a few times a week will help knock them down. But the size of your tree will make this difficult along with any type of control. Horticultural soap sprays or neem oil work well for aphid control but this will also kill the beneficial predators. If you feel the need to treat the aphids it would be best consult with a certified arborist, due to the size of the tree. A certified arborist can be found in your area by zip code at the following website.

Find an Arborist (treesaregood.org)

I hope this helps you out. If you have any further questions don't hesitate to Ask Extension. 

Your quite welcome!