Fly reduction in the coop

The most important word is reduction. I have a chicken coop, I have flies.  No way around it. I don't have tons of flies. To be honest I have really thought much about it. Until today.  
I try to keep my coop relatively clean, for a chicken coop. I don't scrub it down with bleach every week, or anything like that.  I just don't want to smell the coop from the back yard, and I want to be able to get eggs without gagging.  The way I accomplish this is I keep a mix of pine and cedar shavings on the ground ( I learned years after I had been using cedar that it's bad for chickens because it can cause respiratory problems.  I think I get away with it because my coop is very open on all sides, only the end with the nesting boxes is enclosed on 3 sides, but the 4th side is wide open.  That's my theory anyway).   I don't have a schedule, when it starts getting stinky I add more chips. On a side note I tried using my free wood chips and my hens had the worst case of mites ever, so I buy the pine and cedar, it's worth the cost for all of us.  
Normally I spray all the poop out of the nesting boxes, once or twice a week as needed. I scrape or spray the roosting bars when needed.  
I didn't realize how much this simple practice helped.  I usually clean the coop on Friday or Saturday. Friday I was busy, and Saturday I started feeling ill, so I didn't clean the coop this week.  I'm still not feeling very well, but the chickens have to be fed. I went out to feed them and there were a ton of flys. Bugging me, and the chickens.  It was horrible.  I think having piles of poop just sitting in the nesting boxes, and on the roosting bars breeds more flies.  I don't know why it makes such a difference, the poop is still there?  Maybe the cedar discourages the flies, maybe because chickens are always moving everything around, I don't know, but a difference it makes.  
I toughened up a bit, and cleaned it out. I'll probably go back to work tomorrow and and I will pick you some pine and cedar chips and spread them in the coop soon. Hopefully things will be back to normal soon.
I have been thinking about making a bar that goes in front of the nesting boxes, so the chickens can't sleep in them.  I would have a lot less to clean if I did this. It means putting it on in the evening, and removing it in the morning.  I'm not sure if it would save me time, or cost me time. It's something to think about. Might be worth it to have cleaner eggs.???  

It would be worth having an extra chore if you think it's worth having cleaner eggs and not having to clean poop out of boxes.

The hens are sleeping and pooping in/on the boxes and that's why there's poop there. It's a bad habit and one you should try to discourage. Whether it's risen to the level of importance enough for you to do something about is your decision. It's definitely a contributing factor to the fly problem.

With the warmer weather, flies are everywhere and their life cycle runs fast! Depending on the type of fly you have, a life cycle usually measured in days, allowing the chickens to scratch around and eat the larvae before they start causing problems, is now measured in hours (or is much, much faster). There are more flies being drawn to your birds than before, and they are reproducing at a faster level than before.

I make sure I have "mucking about" clothes that I use for poultry and gardening, wear gloves, and do very lackadaisical care of my birds. I do a twice a year cleaning of the main congregation areas, do "spot cleaning" as and where needed, but my flock has a very large space for ranging and I tend to lose more birds than people with confined runs. It's a trade-off. We all make the decisions that suit us and our circumstances.

Flies are a problem. Black flies can kill chickens if there are enough of them.
If you were able to stand the smell of it, I would suggest putting a baited fly trap at a different spot in your yard, to draw the flies away from your chickens. I know of people who have good results with fly paper in really bad areas, but you have to deal with fly covered fly paper as clean-up for that. It might be a bit much.

Cedar is bad because it can damage lung tissue in confined spaces. Yes, it's good as a deterrent for a lot of insects, because of the polyphenols which is what makes it so good at it's job.
Whether or not you use cedar shavings is a personal decision based on what you perceive as a benefit to your birds and the price you are willing to pay for what you get. I put that in the same category as feeding a "layer bird" bagged feed to cockerels and roosters. They can eat it and be fine, until the calcium solidifies their kidneys and they die. Some people make the decision to feed a layer feed anyway, with the knowledge that a rooster will not likely live past 5 years. Again, it's a personal decision.

If you have a routine that works and that you use year-round, I wouldn't change it. It works for you. Your birds are used to that level of care and attention. The only reason it stopped working is because your routine was out of whack and you now have to catch-up to what it was. There's going to be some yucky times while you bring everything back into balance, but the summer will end, thankfully.
Once you get the cleaning routine back, it should be fine. There will be a slight delay as you get the current fly population under control, but it will happen.

Old fashioned and less invasive ways to control insects - whitewashing your coop, roosts, nest boxes and all twice a year; doing a deep cleaning twice a year and scrubbing everything (prior to the white wash and then letting things dry in the sun); keeping the ground around your coop  and run as dry as possible to prevent stagnant water areas; keeping the poop buildup down through a regular level of cleaning maintainence.
Any or all of that could help.

At least fly season will pass. It'll be colder and frosty before we're ready for it!

I am starting to notice the increase in fly population around my chicken yard as the weather increases.

I have heard nearby planting of smelly plants such as lavender or mint may provide useful but I haven't found good reliable information to support it. I'm thinking of incorporating some DE and wood ash into their dust bath area just to try and deter any chances of a surprise fly-strike heaven forbid but I am hoping to hear if anyone else has experience regulating fly populations?

Unfortunately at least my experience is with planting aromatic plants is they don't work just growing.  If you brush up against it or crush it then it helps.  
The only thing that really helps me is to keep the coop as clean as I can.  Adding fresh wood chips.
I feel adding D.E. wouldn't hurt, but I'm not sure how much good it will do . It's great for fleas, and mites, but the fly would have to land on it, and even then I don't know if it would kill the fly???
Good luck, wish I could be of more help. There's a bunch of fly products, but I don't use that kind of stuff.

I dust the laying areas with DE, and will "powder" the roosts with it, but other than trying to make sure that mucky places get a bit, I don't worry about it too much. Too much DE can cause damage to lung tissue which leads to weakened respiratory strength and the potential for upper respiratory infections in chickens.

I have the same experience with scented plants. If the chickens are eating them or brushing against them, they help. I have pulled up Giant Ragweed (locally called spicebush) and put a clipping of rosemary and mint in with the chickens, but that only worked a little.
My goslings enjoyed sleeping on the spicebush bedding when they were being moved outside (and in an introduction pen), probably because of the scent driving off insects. But I replaced that bedding daily and crunched it up for them.

Anything you can do to keep things dryer, drain mucky areas, and try to keep the manure away from the animals will help. Shade seems to be attracting mosquitos right now, but my flock is eating all the fly larvae, and flying insects they find. If you can let your chickens search and destroy the fly-generating areas, you'll all be much happier!