yup they are right next to/under the shed, thats what i mean though when i say the shed looks similar to the mealies i have, i know mine are not aphids but i will take a pic tomorrow if i can rind them, it has been raining for 2 days straight now so some might have been washed off
also with as little as is known about the imidicloprid and it's long term effects on the environment, that is in the bayer systemic, I have had very good results with it and if carefully applied.....don't be scared of the "hard stuff", it works.
Do you have a linden or other flowering tree over or next to your bog at all? Lindens drop a tragic amount of sticky pollen specs. that can easily blow around for a short distance.
I had this grainy white stuff getting on my cephalotus that for a while I thought was mold. Turns out it was sodium from bricks I use to hold the water tray up inside the tank. Once I moved the plants to another tank the problem went away and I confirmed a sodium buildup.
Not saying that's it, but it could be something similar. Sodium hurts plants, so I hope you don't have that issue like I did.