I too am not an EHD scientist, but I would first suspect weather/climatic conditions as to why it hits some areas harder in a given year. Even though a good part of southern Iowa was very wet this year(June and July in particular), when the rain stopped falling...it stopped pretty good for several weeks. We never had what I would call drought in my area of SE Iowa this summer, but it also did not rain much, or at all, for about 6-7 weeks in August/Sept.
So...how about this scenario...it isn't as much about drought as what many of us have thought...it is more about the water levels flucuating in the ponds and seasonal creeks...mostly as it relates to
dropping water levels. Clearly, in a drought year, we all know that there are plenty of exposed creek banks and pond banks that the midges emerge from. And the worst EHD outbreaks seem to be highly correlated with drought years.
But maybe even in a wet year when the water levels are up...the EHD problem emerges, but to a lesser degree than in a widespread drought, when the water levels inevitably fall, then there is still previously submerged banks that have midges waiting to come on out, etc.
I think the answer to ring all ponds, rivers and lakes with plywood...to keep the midges at bay!