Earlier today I used "ad hoc" in a posting, and it reminded me that someone posted some misleading advice about it here not long ago.
"Ad hoc" is never italicized and it's never hyphenated, even when it's used as an attributive adjective.
That "ad hoc" is never hyphenated when used attributively follows from both commonsense hyphenation principles and from what reliable usage guides say about it.
Commonsense hyphenation says to hyphenate an attributive open compound only if hyphenation is needed to avoid false scent or ambiguity. "Ad hoc" should seldom produce false scent or ambiguity, although examples could be contrived based on the noun "ad" that is short for "advertisement"..

In contrast with this is the word "in absentia". Because "in" is a common English word, a striking example of false scent is shown in
He doesn't believe in absentia voting is legitimate.

First time through, a reader may assume there's something called "absentia voting" and "he" doesn't believe in it. When the reader gets to the "is" he or she has to backtrack and realize the "in" is part of the import from Latin "in absentia". I found that example in Edward Johnson's Handbook of Good English . He recommends fixing it by inserting "that" before "in". He can't recommend
hyphenating "in absentia", because he has previously made it clear that Latin imports of that sort are not hyphenated, even when used attributively.
As for italics, the principle is that if a Latin import has been in the language long enough to be felt as an English word, it's not italicized. If it still feels like a foreign import, it's italicized. Johnson and others tell us to rely on a dictionary to tell us which ones are italicized and which ones are not.
In following that advice, we need to pick one dictionary and stick with it, because dictionaries differ in their recommendations about italicizing various Latin imports.

But every dictionary I've looked at has "ad hoc" in roman type.
Looking at a large number of Google hits on "ad hoc", I find a couple that are italicized. These are to be regarded simply as errors.
Re: ad hoc and in absentia
As for italics, the principle is that if a Latin import has been in the language long enough to be ... in their recommendations about italicizing various Latin imports. But every dictionary I've looked at has "ad hoc" in roman type.

How much variation did you find for "in absentia"?

(Collins doesn't italicise "ad hoc" but does italicise "in absentia".)

Cheers, Harvey
Ottawa/Toronto/Edmonton for 30 years;
Southern England for the past 22 years.
(for e-mail, change harvey.news to harvey.van)
On 08 Feb 2005, Bob Cunningham wrote Re: ad hoc and in absentia

As for italics, the principle is that if a Latin ... dictionary I've looked at has "ad hoc" in roman type.

How much variation did you find for "in absentia"?

I don't remember, but I'll look again:
"in absentia", "ad libitum", "ad hoc" Italicized?:

Random House Webster's Unabridged , yes, no, no
New Shorter Oxford English Dictionary , yes, yes, no Webster's New World College Dictionary , no, yes, no

So far I haven't found any entries in Merriam-Webster's 11th Collegiate to show that they ever* italicize any main entry. Same with the online Cambridge Advanced Learner's Dictionary and the online American Heritage Fourth Edition . That's not to say there *are no italicized entries in any of them.
Merriam-Webster's Unabridged Dictionary , which is what they call Webster's Third New International when it's on CD ROM, has the very strange entry
in ab sentia
That is, the "in ab" is roman and the "sentia" is italic. I don't know what to make of that.
Now I see they do the same thing with "ad libitum" ("ad lib itum ").
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Merriam-Webster's Unabridged Dictionary , which is what they call Webster's Third New International when it's on CD ROM, ... what to make of that. Now I see they do the same thing with "ad libitum" ("ad lib itum ").

Very odd; perhaps a direction to "italicise when spelled out but not when abbreviated"?
(Then again, although that might make sense for "ad lib" and other commonly-used abbreviations etc.; ibid.; op.cit I wouldn't place "in ab" in that class of common abbreviation.)

Cheers, Harvey
Ottawa/Toronto/Edmonton for 30 years;
Southern England for the past 22 years.
(for e-mail, change harvey.news to harvey.van)
Earlier today I used "ad hoc" in a posting, and it reminded me that someone posted some misleading advice about ... that example in Edward Johnson's Handbook of Good English . He recommends fixing it by inserting "that" before "in".

Which should be done in any case.
As for italics, the principle is that if a Latin import has been in the language long enough to be ... others tell us to rely on a dictionary to tell us which ones are italicized and which ones are not.

I always italicise "ad hoc", as well as "viz.", "qua" and other Latin words.
In following that advice, we need to pick one dictionary and stick with it, because dictionaries differ in their recommendations ... Google hits on "ad hoc", I find a couple that are italicized. These are to be regarded simply as errors.

Prescriptivist, eh? I met your type in the war...

Will.