Loading/Unloading Web User Controls at Runtime

I have an ASP.NET page on which I have a dropdown list and a bunch of Web User Controls that I have created. The user controls are all added to the page at design time with their visible property set to false. When the user selects an item from the dropdown list, the corresponding user control becomes visible. This is all working fine.

However, right now, all of the controls on each of the user controls (e.g. dropdown lists, text boxes, etc.) are populated when the main page is loaded. I would like to only populate the controls on a specific user control when the user selects that user control from the dropdown list on the main page.

What is the best way to accomplish this?