There are some biting flies and mosquitos at Machu Picchu as it is cloud forest. But, when I did the Inca Trail in May, I didn't encounter any while hiking. I usually bring some insect repellent with me, just in case, though. Haven't tried any bracelet/anklet type devices, though. over 5 years ago
I have hiked the 4 day Inca Trail once and the 1-day Inca Trail at least 3 times. There are very few mosquitos because the altitude does not get below 8000 fasl.
Even when I went to the Amazon, the mosquitos were not as bothersome as the sand fleas!! I think we have bigger mosquitos in Virginia US. over 5 years ago