Be mindful that Perthshire has biting insects other than the famous Highland midge - but the first Highland midge of a summer crawls out of her bed (Yes, it is the female of the species that bites! She does it for the kids, you know.) at the end of May or early June, hungry for a slurp of human blood.
In past years there has been a Midge Forecast in the local paper (Press & Journal) but it's just a one-day forecast. To me, it's never looked very useful or accurate (then my main Highland experience is East Sutherland which is pretty much midge-free). The web site
http://www.midgeforecast.co.uk/2008/ looks like the same forecast but it remains blank at the moment as the midge season has yet to begin (& the .../2008/ is a bit ominous.) The vox pop on their twitter link is active but, as ever, of unknown provenance.
On balance I'd suggest the following warning ? REMEMBER to pack insect repellent ? although, given the cold winter/spring & not being West coast, participants in the Heart of Scotland 100 will likely be deprived of the world famous Highland midge experience. What a terrible shame!