As Allen said, HMF in syrup may be responsible for bee poisoning:

Bees constitute a very important group of livestock that come into a close contact with both natural toxins and contaminants of the environment. From time immemorial, bees come into contact with natural toxicants from nectar, honeydew and plant pollen. In the central European setting, natural toxins represent no major danger for bees. Thus all bee poisoning cases on record are therefore traceable to human activity. 

Nowadays, mass intoxications due to emissions caused by industrial processing of low-grade coal and minerals are a matter of the past. Bee poisoning cases reported included poisoning with hydroxymethylfurfural (HMF) produced when sugar is heated under acidic conditions [or] due to an overdose or an improper administration of drugs. 

Interdisc Toxicol. 2009; Vol. 2(2): 48–51.
 

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