Has any consideration been given to the possibility of a foreign state silently attacking the US by infecting honey bees?

Some people call colony collapse disorder (CCD) mysterious, but when I watch films about commercial U.S. beekeeping it doesn't seem that mysterious to me. Scientists so far mostly seem to put the causes of CCD down to a range of interacting factors. Bees are being trucked huge distances nearly all year round from one monocrop to the next. Their colonies rock back and forth and they're unable to leave their hive to go to the toilet whilst on the road. The large concentrations of hives from different beekeepers being kept on plantations is a perfect way to spread disease. Spraying of crops also occurs. In the film 'More than Honey' they show pesticide spraying occurring on almond crops in the daytime whilst bees are flying, because the drivers would get lost spraying at night. Added to that is the varroa mite, a parasite which European honeybees have not yet evolved to cope with. The spore forming parasite Nosema ceranae - http://scientificbeekeeping.com/nosema-ceranae-kiss-of-death-or-much-ado-about-nothing/ has also reached America. And U.S. commercial beekeepers are sometimes using the antibiotic oxytetracycline (OTC) as a preventative measure against American Foul Brood (AFB) - doing this is illegal in the U.K. as it can suppress the symptoms of the disease, allowing it to spread to other hives. Taking all these stresses into account, does a foreign enemy state need to do anything more to attack honey bees? All over the world, people seem to be doing a great job wrecking their own environments for pollinators and wildlife in general. Humans are their own worst enemy. Luckily a few people care and do great things for the rest of us.

Very unlikely.  First of all, other than making it hard for Americans to enjoy honey and putting a few thousand beekeepers out of business, there really is very little economic or other impact to be achieved by killing off US honey bees. Contrary to some of the more alarmist voices regarding the honey bee collapse, most US crops do not require honey bees for pollination, and of those that do, such as blueberries or almonds, there are other insects available that have been filling in much the gaps left by commercial honey bees.  Research has so far failed to find a substantial decline in pollinated crop yields due to declines in honey bee populations, for example, and the reason is likely that other pollinators -- wild insects, bats, etc. -- may very well be doing the job almost as well as honey bees, which were never native the Americas in the first place.