I just overheard this bit of lore that pinged my BS meter:
In the summer, drink Guinness Extra Stout. It will make your blood bitter and mosquitos will leave you alone.
The person who said admitted to not ever having tried it.
Well?
Well, Guinness has various B vitamins, and my local health food shops suggest taking vitamin B complex to ward off insects, but I have no idea if it works.
Why would Guinness have vitamins not found in other beers? AFAIK, the only real difference is the addition of nitrogen.
Unfiltered beer still contains yeast cells, which are rich in B vitamins. cite (not a very good one, but I’m virtually certain this is true).
Guinness won’t contain more B than other unfiltered beers necessarily, but it’ll contain more than filtered beers.
Daniel
Will the miracles of beer never cease?
Brilliant!
But . . . but . . . even assuming that they care, how do mosquitos know how much Vitamin B you have in your blood without biting? 
And once they’ve bitten, what’s the use?
So why Guinness specifically, then?
Unless the people saying this think that most quality brews are Heineken, Molson, and Colt. Maybe they don’t know of any other unfiltered beers.
I think you’ve got it. Guinness is, and this is a total WAG, the bestselling unfiltered beer in the US (and probably in the English-speaking world).
Daniel
If you have the beer sitting out in front of you, it’s conceivable that Guiness would be better than most. One of the things mosquitoes home in on is carbon dioxide, since many sources of carbon dioxide contain blood. The bubbles in a regular beer will be CO[sub]2[/sub], and so might conceivably attract mosquitoes, but the nitrogen bubbles in Guiness would not (I presume that Guiness still contains some CO[sub]2[/sub], but not as much as other beers).
On the other hand, if this is the mechanism, you’d be better yet with flat beer of any variety, or no beer at all.
In my experience, people who take large doses of B vitamins smell of B vitamins. I guess the smell is supposed to put the mozzies off.
Similar claims have been made for marmite on the same basis. Trouble is, vitamin B doesn’t seem to do anything against mosquitoes in controlled trials. Although Google “vitamin B” with “mosquitoes” and you’ll find no end of people claiming their B supplements stopped them getting bitten.
No, no. You need a Guiness in your hand and a bucket of dry ice way over yonder at the other end of the yard!
Correct me if I’m wrong, but isn’t Guinness Extra Stout only available in CO2 carbonated bottles? All the ones I’ve seen with widgets and at the pub are not extra stout.