>   Seems the whistle of the head is what does it.....  

Yes, vibrations from the line.  If you fit a cutting disc to the same machine they will ignore it and you (or at least, I) can trim close to the hives with no problems at all.  I also use a long reach hedge trimmer around the apiary - good for grass, brambles and nettles - and they ignore that too.

So it is not the engine - although, oddly, if I am clearing honey with the blower (Dadant) and I put the machine too near the hive - say five feet away - then they will sting the rubber cap on the sparking plug.  2-stroke vs 4-stroke engines?

Best wishes

Peter 
52°14'44.44"N, 1°50'35"W

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