Here's a warning about table salt and bees that was published in our NWDBA (Northwest District Beekeepers Association) newsletter.
"The latest newsletter mentioned that salt could be added to water placed out for bees to use. Jim Bach pointed out that use of table salt is harmful to bees and any use of salt should be mineral salt such as that found in salt licks. We had mentioned minerals salts being preferable at the meeting but I don't think we knew that table salt could be a problem.
A note on beesource.com has this mention:
Piskovoi et al. (1964) found that common table salt, sodium chloride, in levels as low as 0.125% in sugar syrup, caused dysentery and mortality in caged bees. Bees in overwintering colonies with honey stores containing 0.35 to 1.16% salt were dying prematurely.
http://www.beesource.com/pov/usda/abjfeb1977.htm
2 teaspoons in a gallon is 0.26% (volume)"
Mark