First of all don't scratch.
blood sucking insects inject anti-coagulant under your skin to prevent your blood from clotting and forming a scab so they don't get their mouthes stuck inside you while sucking.
If you scratch, you only manage to spread the anti-coagulant around under your skin, which intensifies the itch and makes things worse.
Train your brain not to unconsciously scratch every itch, and they won't persist as long.
The go-to medicinal plant for pretty much everything is yarrow ( Achillea millefolium ).
It's as common as plants come in North America, but I checked and it's apparently also native to the UK.
We were taught to use it as kids to remedy stinging nettles, but it also works for other types of stings and bites by drawing the toxins out of your skin.
Simply pick the fern-like leaves, chew on them a bit to partially mash them into a paste, then apply to the affected area.
Relief is almost instant.
It also works well as a tea for relief of cold symptoms and fever, which doesn't surprise you after you've tasted it, since it tastes like a kinda minty but mostly sappy-bitter cough syrup.
Whenever I use it on people suffering from a rash out the woods, I always get them to chew it up for me.
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Over the counter antihistamine products - especially tablets (due to light weight and effectiveness), but also topical creams such as stop itch and antihistamines are the most effective solution.
These should be carried in your first aid kit if you have a history of allergy problems.
Even for those that normally don't have problems, the size and weight of antihistamine tablets is so small, and its importance a potential life saving treatment make it an item I always have in my first aid kit
(Wasps are a real problem for us in summer).
As far as plants, the best I can think of is Aloe vera - although you are unlikely to find it when you need it most.
Others I have read about, but have not tried are Peppermint, Basil and apparently banana peel.
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The only thing I can recommend from experience is mud: Cover the itching area with plenty of it and the itching will go away.
After the mud dried out and has fallen off, sometimes the bites start to itch again, just reapply.
But in most cases I never had to do that again.
Generally cold helps by dulling the itching.
The opposite, heat, will temporarily help as all histamines (proteins by the immune system which cause the itching) are released by heat.
But the itching will come back.
Just don't burn yourself.
There are also a variety of plants that have antihistamine properties.
What will be available obviously depends on where you are.
In Europe the ones you might find most likely are jewelweed, chamomile and echinacea.
I
don;t think (though I am not sure) that simply crushing and applying will do the job, I guess extracting by cooking to make a poultice will be needed.
So the difficulty to find such plants and the preparation needed will make it quite unpractical as a solution on the go.
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Plantago works excellent againt nettle because of its anti-histamin properties and, in my experience, also against musquito bites.
It grows usually in the neighbourhoud of nettle and might be the only plants that survices on a pathway.
You need to crush the leaves and apply it on the 'wound'.
(it's like pressing water out of a stone, but it can be done).
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I remember buying in pharmacies a liquid prepared on the spot, from mint powder and saline solution (unfortunately this mix is sort of local-specific to our Romanian pharmacies, so I cannot provide a link).
This was applied on mosquito bites and allergy rash - it stopped the itching for an hour or two.
Based on this experience: try some sort of minty cream (or even toothpaste) to cool down the itchy feeling.
Might sound a bit extreme, but worth a try.
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If you expect itchy insect bites, I can recommend a dedicated heat stick for treating the bites.
Something like this: Therapik Mosquito Bite Reliever
It does work, at least temporarily.
The mechanism is often said to be the denaturing of the proteins due to heat.
However the heat is not sufficient for that.
What apparently really happens is that the local pain receptors are overloaded by the heat and shut down.
For a study to show this actually works: Locally administrated concentrated heat leads to fast amelioration of symptoms.
- PMC
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On a recent walk I got bitten to death by midgies. I was itching all day until I got home and put some antihistamine cream on them. Is there anything available (plants, etc.) that I could of applied to stop (or help with) the itching?
Back country solution to itchy insect bites
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Train your brain not to unconsciously scratch every itch. A variety of plants have antihistamine properties. The go-to medicinal plant for pretty much everything is  yarrow.  Plantago  works excellent against nettle because of its anti-histamine properties and also against mosquito bites. Cover the itching area with plenty of mud and the itching will go away. Avoid scratching, plants and mud are effective. You can also use antihistamines, heat sticks and solutions from pharmacies.
Avoid scratching. Antihistamines, mint powder and saline solution, and heat sticks can help with bites. Plants such as yarrow, plantago work well, as does mud.