We were in Beddgelert in June and I am allergic to the little blitters and they were everywhere apart from from right at the top of the Mountain, bliss. Shame we couldn't have camped up there. I always use Tiger Balm but I forgot it grrrr! Even the dogs were covered, my old black dog looked as if he had a grey wig on poor old bu**er! He had a reaction to them too and that's cost us loads to sort out at the vets. If anyone does come up with the best deterrent I'd love to know cus' I'm contemplating not going out again until the winter!!!!!
About marmite: I've eaten it probably every day of my life since I was about 2 (I LOVE IT!) and I get bitten by midges worse than anyone I know.
I also react badly to bites so get big lumps and itchy itchy red marks. As someone else points out I believe it is to do with sweat (I get quite sweaty!) and body temperature.
100% DEET works really well, but it is nasty stuff. Tempted to try Catnip oil as anything is worth a go.
The best answer is to go places where there are no midges and avoid Scotland like the plague in the summer months.
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