Chicken has Bald Patch on her Bottom

[aart]

I would guess that that is from feather picking.
If you do find bugs, permethrin is the best route to go, IMO.

I've looked at some organic mite-killing sprays

Depends on what it is, link to actual product would help.
First find out what exactly you're dealing with.
My Bug Check notes:
Have you checked them over real well for mites and/or lice?

Google images of lice/mites and their eggs before the inspection so you'll know what you're looking for.

Part the feathers right down to the skin around vent, head/neck and under wings.

Best done well after dark with a strong flashlight/headlight, easier to 'catch' bird and also to check for the mites that live in structure and only come out at night to feed off roosting birds.

Wipe a white paper towel along the underside of roost to look for red smears(smashed well fed mites).

Good post about mite ID by Lady McCamley:
https://www.backyardchickens.com/threads/my-chicken-has-mites-now-what.1273674/page-2#post-20483008

 

[lipazron]

We had the same issue (see my recent post). I emptied the coop and run, hosed everything down, brushed all surfaces with white vinegar/water and dusted each hen with premethrin. Will repeat in a week's time. Try to catch it before it gets to all hens and all over them (like mine).

 

[aart]

We had the same issue (see my recent post). I emptied the coop and run, hosed everything down, brushed all surfaces with white vinegar/water and dusted each hen with premethrin. Will repeat in a week's time. Try to catch it before it gets to all hens and all over them (like mine).

Did you inspect the birds and find mites/lice or their egg clusters?