when the need arises

:  necessary duty :  obligation

:  a lack of something requisite, desirable, or useful

:  a physiological or psychological requirement for the well-being of an organism

:  to be needful or necessary

:  to be in want

:  to be in need of :  require

:  to begin to occur or to exist :  to come into being or to attention

:  to originate from a source

:  to get up or stand up :  rise