org-agenda-insert-diary-strategy

org-agenda-insert-diary-strategy is a variable defined in `org-agenda.el'.
Its value is

date-tree

Documentation:
Where in `org-agenda-diary-file' should new entries be added?
Valid values:

date-tree in the date tree, as child of the date
top-level as top-level entries at the end of the file.

You can customize this variable.