I've noticed (ouch, and the impression seems generally shared by beekeepers
here) that bees working a Canola honeyflow (Brassica, oilseed rape) in mid June
to mid July  sting more than one would expect from bees on other flows. One can
expect bees to have a better temperament on an apparently similar flow from red
or alsike clover or sweetclover (a week or more later, maybe warmer but
overlapping conditions).
It's hard to get good opportunities for controlled observations, though, because
there are other variables: hive population, average age of the bees, weather
correlated with blooming of a particular crop. A beekeeper's impression of the
difference between "unwarranted" stinging vs rather placid bee behavior may be
very memorable, but subtle.
 
Kerry Clark, Apiculture Specialist
B.C. Ministry of Agriculture
1201 103 Ave
Dawson Creek B.C.
        V1G 4J2  CANADA          Tel (604) 784-2225     fax (604) 784-2299
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(it's heartening to hear of almond pollination starting: although daylength here
is noticeably longer than Christmas time, we have solid winter still: -30 C and
1.5  m of dry snow drifting. It's enough more than normal that anyone who hasn't
shovelled their roof is feeling uneasy, a few roofs have collapsed.)