http://blog.apastyle.org/apastyle/2012/05/missing-pieces.html

Most APA Style references are straightforward to write—the guidance and examples in Chapter 7 of the Publication Manual and on this blog make that possible. We’ve written a good deal about the architecture of a generic reference (the four basic pieces of author, date, title, and source). Sometimes, however, one or more of those pieces is missing, and writing the reference can get more difficult. This post will help you adapt the classic APA Style reference template to fit any situation where information might be missing, as well as show you how to create the corresponding in-text citations for those
references.