What's the best way to repel flies and bugs when hiking in summer? Where I hike is not really humid (southern California) but it's hot on summer days and a lot of insects and flies gather around me when I hike or sit.
EDIT UPDATE:
My hiking is not heavy, usually ~7miles in the sun, and there are not dangerous mosquitoes but they are extremely annoying and always on my face and bare hands.
I use a repellent that contains 30% Lemon Eucalypus Oil and have found it to be effective against flying insects. According to testing done by consumer reports, it is also effective against ticks (despite this being an "off label" use).
It does not contain DEET, (which can melt certain plastics and rubbers and ruin your $500 hardshell), though it is not quite as effective. As pointed out in the comments, it is poisonous if ingested, so it is important to avoid drinking it. However, it is approved by the EPA as safe and effective when applied to the skin as intended. The main downside of this product is that it can cause some irritation and itching, though it isn't too bad in my case.
You can also treat your clothes with Permethrin (also used as a pesticide) to deter insects from biting through them. It kills insects that linger on treated clothing for too long, works for several washings after being applied to clothing and is effective against both flying insects like mosquitos as well as ticks. (I have not tried it myself.) Ticks apparently do not bite immediately, so if they get on your clothing it should kill them before they have a chance to bite.
Repellants containing DEET are the most effective and the only method I've been happy with. It is a harsh chemical but does NOT need to go on your skin. Spray 95%-100% DEET on the top and bottom of your hat brim, your backpack shoulder straps or the neck and shoulders of your shirt and socks. This works for me most of the time. When it doesn't I put on a head net.
Also most flying bugs can be out hiked in my experience. It seems like >3 miles per hour is just enough to keep the cloud of mosquitoes just behind you.
You don't say what you're wearing. The best solution can be covering yourself with light-colored lightweight breathable clothing. You might be reluctant to cover up on a hot day, but with the right kind of clothing, it shouldn't make you too much hotter. Not only can this be an effective way to avoid bug bites and the annoyance of bugs, but it also can be a good protection against sunburn.
I always wear long pants and often long sleeves and spray my pants, shoes, and socks with permethrin (which lasts a few washes). Here in Kansas, I've found tens of dead ticks in my shoes after a hike, some of which would have been sucking on my ankles without the permethrin. Gaiters also can be effective for keeping the ticks and chiggers away from your ankles.
The US military uses permethrin-impregnated uniforms coupled with skin-applied DEET to repel insects, which together are extremely effective. Both DEET and permethrin may have mild toxic effects in humans, especially if used together over long periods. Research is still ongoing. "Natural oils" and other such remedies may also have such toxic effects, which have not been studied. However, the threat of tick-borne and mosquito-borne diseases must be part of the cost-benefit analysis. Know what tick-borne and mosquito-borne diseases are common in your area and what their impact on your life could be. In the US, the CDC is a good source (tick-borne diseases in the US).
There are proven ways using commercial bug-repellents sprays and lotions.
I'd consider non-commercial, DIY ways using Eucalyptus.
Eucalyptus oil: If you are carrying Eucalyptus oil, you can rub a few drops of oil on your forehead, neck and nose. Such oil on skin-scratch or a pimple may make burning sensation, but there is no harm. In such a case, you can use your buff or a handkerchief with a few drops of Eucalyptus oil on it, and use it cover your face, neck and/or elbows (If you aren't with a full-sleeves attire). I have heard (but haven't tried) that Soyabean oil, Cinnamon oil does the same, but I can't imagine you hiking with me as I am smelling like a Soyabean all the way :)
Eucalyptus leaves: If you aren't carrying Eucalyptus oil, well, then find a Eucalyptus tree. Get some leaves from the tree, squash them and rub them over your backpack, hat/cap, shoes, clothes, etc. Bugs and Mosquitoes stay away! I have tried this, and this works.
There is another way but that doesn't guarantee the effectiveness in all conditions. Camphor is another repellent with a bearable/tolerable smell unless someone is allergic to it, I have seen people who start sneezing for hours when they sniff a camphor ball/tab, while some people (including me) love the smell! If you are of the latter category, keep camphor tabs/balls in a handkerchief and tie it your backpack, and on hat. Looks funny, but works. May not work that well when its raining all the way, but otherwise it works.
You might be aware of some products like DEET, which is anyway an artificial, commercial product. I typically defer from using it because even though its a proven product, it has a strong smell. It's harmful for some fabrics and rubber products, and pets (and animals) suffer badly if they sniff/lick anything thats heavily sprayed with DEET.
Update: Based on some data provided in a comment by Qudit, it's recommended to use Eucalyptus oil for its effectiveness based on a consumer report.
The barrier method is the only method that is 100% effective...and then only when used properly.
Long pants tucked into high socks. Gloves (light weight, you get used to them faster than you would think). Long sleeves tucked into your gloves or with a cuff tight enough that there are no gaps. Shirt should overlap pants enough that there are no gaps. Ideally you would have a net to cover your head, but if you don't want to look too different from a normal hiker you can get away with sunglasses for your eyes and a headband/scarf/handkerchief/"Buff" to cover your ears and a high-ish necked shirt. (Prefer a larger "Buff"/handkerchief/etc. over a small one, since it can be rearranged to cover your face and nose, too, if you go through a particularly bad area, and then back to a headband when you no longer need as much protection.)
Clothing works best if it is somewhat loose (except around the cuffs), even for insects which will bite through clothing, if the outer layer is far enough from your skin they will not be able to reach you.
As a bonus, this will somewhat protect you from other common hazards, such as casual contact with poison ivy and most sunburn.
A few tricks in no particular order:
Of course, no matter what you do, you will be bitten and stung. A few more tricks:
I live and play in Florida, and I feel you.
First, don't be afraid of insect repellant. It's not a total solution but it can really help. Coat your clothes in it, then spray a little on your self. The ones made with DEET are very effective, though they can discolor some clothes and will harm some plastics and rubbers.
Next use oil. A "layer" of olive oil will stop the bug bites. It will also stop the gnats and other annoying insects. Just smear some on, your exposed skin and off you go.
Long, thin cloths work wonders. Most annoying insects can't bite through any clothes.
Finally, just ignore them. At a certain point, it just becomes better to ignore them.
Once bitten
You will get bitten, here are a few things you can do.
Use a hat with an insect mesh covering which covers your head and neck right down to the chest and same around the back so you don't have to spray a repellent on your face. bush hat with netting
Having tried all known expensive remedies to little effect, here is one that was recommended by a friend: equal parts of tap water & apple cider vinegar plus a few drops each of rose geranium and lavender oil, place all ingredients in an atomiser; This is perfectly safe for use on pets against ticks & fleas (it works) and more importantly the ONLY thing I've found works against the Scottish midge a ferocious wee beastie (particularly bad this year). It got so bad the other week, I was stood in the garden with a net curtain wrapped around my head whilst the dogs did their thing...slowly as usual 😉 (not gotten round to mixing my potion for this season)
We get loads of other flying things, horse flies, mossies, hornets and it helps with those too.
Another option if you don't want to go the bug spray or full body cover route could be one of the battery powered thermocell products.
These are designed to be used in a stationary place (and are incredibly effective in normal use), but I can attest that it also works reasonably well when carried around - just with presumably a much smaller sphere of influence.
This would also have the benefit of providing some protection to the others in your group (if any)
In addition to DEET and permethrin, which were mentioned in other answers, there are a couple other insect repellent possibilities:
Picaridin (also known as icaridin)
IR3535
