Can you exclude issue numbers in references based on APA 6th edition? I'm currently working on my thesis (communication science), and I'm wondering about the issue number in the reference list.  

A teacher told me that I don't need to include the issue number. But I can't find anything about it, so I wondered if somebody here knows about it. 

The current mark-up that I'm using now is as follows (I mainly cite scientific journals):
Lastname1, Initials.,  &amp; Lastname2, Initials. (Year). Title. Journal name, volume number (issue number), page-page. DOI / Link  

Can I exclude the issue number or is imperative to include it?
The general rule in APA 6 is that if the journal is paginated by year, then you exclude issue number and just report volume number. From my experience, just about all journals in psychology are paginated by volume/year (and not issue number), so you can almost always exclude issue number. 

There is a general discussion here on how to determine whether a periodical is paginated by issue.

However, I honestly can't see myself going through every reference and checking whether it is from one of the rare journals that is paginated by issue. And presumably if there was occasionally a journal that happened to paginate by issue, it would not be the end of the world if the issue information was excluded from the reference. You still have authors, year, title, and so on. So, that's enough to identify the reference.