On the first page, use
SelectedIndexChanged [
^]event on your dropdown and save its value into Session variable.
Alternatively (since you're using queryString in your code) - redirect directly from SelectedIndexChanged:
Protected Sub Index_Changed(sender As Object, e As EventArgs) Handles your_drop_down_list.SelectedIndexChanged
Response.Redirect(string.Format("your_second_page.aspx?reporttype_value={0}", your_drop_down_list.SelectedValue), true)
End Sub