You should perform a liquid termite treatment in the ground to get the termites where they are entering your house. To perform a treatment just as a professional company would do, please read our How To Do A Termite Treatment Article which explains exactly how you should treat your home properly. Doing a treatment like what is described in the article will not only take care of existing termites that you have, but it will also protect and prevent future termites from entering for about 7 years.
Here is a link to the article: http://www.domyownpestcontrol.com/termite-treatment-c-3_352.html
Please let me know if you have any further questions.
Both would need to be done. You should perform a trench and treat around the perimeter of the screened porch, but you also need to be concerned about the seam where the porch is attached to the house….where the beam is that you described. Any time you have an “add on” to the house, in this case the screen porch, it is possible for termites to enter between the porch and the house. Therefore, you should drill along that seam and treat with a termiticide along the house foundation as well as around the perimeter.
Yes, you could drill through the sides of the porch and treat underneath the top cover from the sides. You would also want to trench and treat around the exterior of the porch or sunroom as well in addition to drilling it from the sides. Please let us know if you have any other questions.