Termidor applied in a trench really helps to prevent pest that are traveling through the soil. It sounds like you will need to re trench since over 10 years and then spray around the foundation 1 foot up and out with Termidor. You may also want to use apply dust like Delta Dust into wall voids for the spiders and other pest.
If the footer is right below the dirt level, within 6 inches, and it comes out 12 inches, then you should make your trench out where the footer ends. In other words, instead of digging a 6 inch trench directly against the structure, you will move the trench away from the house where you can dig down past the footer. You should still treat the soil in between the trench and the house (where the soil sits on top of the footer) by just drenching the soil if you can.
You can either drill into the deck to get to the soil underneath, or you may need to remove the plank or two closest to the structure so be able to treat the soil.
Termidor SC is a great option for fire ants inside of a tree. There are two options that you have when applying this product. It can either be done as a drench around the tree or you will inject the product directly into the termite cartons. The rate is 0.8 fl oz per gallon of water.
Please check out the Supplemental Document that explains how to do treatments.
Termidor SC will treat spiders but you would only be able to use the product outside. Termidor SC does not have an immediate knockdown but instead uses a delayed reaction kill which makes it especially effective for social insects like ants, termites and certain species of wasps. Most folks will use a residual product with a broader application area. Please check out our spider control kit for a better option.