Help please... think nearly all my orchids have spider mites.
 

The bottom of the leaves on most of my orchids have a dusty looking residue, with spots of reddish dust. Some of the leaves have started to go yellow and few have already dropped off. :(
Never had a problem with my orchids before so didn't realise things had started to go wrong till nearly all the orchids were involved (mainly phals and dens). :shock:
Anyone know what I can use to get rid of the mites, anything I'd have in the house or could buy in a local garden store (in the UK)..?
My orchids all sit on my front windowsill, with radiators underneath (the only place light enough to let them grow well).
Any help/advice would be appreciated.

Wash your plants well. You can use a soap based spray to spray them with. One of my favourite mixtures for insects is garlic and hot peppers put through the blender with some water. Strain it thought a coffee filter and add a few drops of dish soap. Keep an eye on them and spray again when needed.
Are you sure it's spider mites? Are there any tiny webs?

Use a 5x magnifying glass and you can tell if it's mites. I use Insectcide soap, there are different brands. It doesn't harm the plants as long as you don't put them in sunlight after spraying. I get a spray bottle of the stuff then a bottle of concentrate to refill it with later.

If it is spider mites, keeping them warm and dry encourages the mites. That may be why they are showing up in numbers now near a heater and likely dryer air in the house in winter. Misting the plants will help control them until you are able to completely eradicate them.

I agree with silken, the dryness encourages spider mites. Wipe the back of a leaf with a damp wad of cotton and if it shows rusty pink it's probably mites. I use a cotton wad wet with isopropyll alcohol and wipe off all the leaves particularly the undersides. You'll have to keep at it for quite a while as they keep coming back. Is there any way that you can increase the humidity? Good luck!

:)