Which natural remedy exist against aphids?
 

The nettle macerate.
A preparation based on propolis: 15 ml of water-alcohol solution of propolis on 5 liter of water.
marseille liquid soap: 20 cc over each liter of water (and mix well).
Strong bitter tea: boil for several minutes, a spoonful of tea in 1 liter of water. Then strain, let cool and use undiluted on the plant and also on the paths of the ants.

Water under pressure over the plant.
Generally is a system used over tree but with caution i think could be used even for small plant.

Good entomophagous insects:
Coccinellidi beetles
Hoverflies the Diptera
Different Rincoti Heteroptera
Neuroptera Crisopidi

adalia bipunctata
aphidius colemani

About chrysoperla carnea they are active against differents things but i don't known if the adult are sufficient good or not, given that they like feed on pollen, nectar and other sugary substances.
In summary i don't known if they can ruin the flowers.
Probably no, but i don't known.
Lotta biologica agli afidi con l'ausilio degli insetti utili
While other type of insects require a lot of prey for work good.
Warning:
As usual when are used good insects and larvae is better don't use other system because even biological system could be dangerous even for good insects.

Later i think i add others solutions.
If someone known other system, suggest it.

I was able to completely eliminate spider mite from my collection using this simple solution : put a couple of drops of Eucalyptic oil into the potting medium.
I repeat this application anout once a month and have NOT seen even 1 mites on any of my 100+ orchids

I haven't seen any aphids around here.

But one day i went out to a nursery to check out some 'other' plants. ;) While I was there we watched one of the ladies put ladybugs all over some rose bushes. They are considered a good natural enemy.

Don't know how this translates to orchids, but thought I'd share. :)