I keep neem oil for the occasional insect invasion - 5% solution in water works quite well generally. But there will probably be loads of alternative ideas.

How long do you let go in between treatments?

These look like immature scale. So it looks like most are not getting to adult size. You could use rubbing alcohol and a very soft toothbrush. Maybe every 4 days do the treatment. When you notice they are gone the mix needs to be changed. Clean the pot like you've never cleaned a pot before;). While your changing the mix what I do is give it a good blast of rubbing alcohol. Then I sit it on a paper towel for a couple mins upside down. Roots up. After the couple mins are up I will rinse the cactus with water. I rinse with water because I don't want the roots to get rubbing alcohol on them, and I think I may rinse off any residual whatever that may be left behind;). Idk if the rubbing alcohol will affect the roots, but it makes me feel better to not let isopropyl alcohol get on them. Hopefully this will be the last time you will need to do this:).

After repotting you still need to check.

You can try Neem oil. I hate the way it smells, but everybody does it different;). Plus I don't the like the greasiness it leaves behind. But usually this is a one shot thing. If I remember correctly neem affects their hormones and they don't reproduce. Keep out of harsh sun when using this because it is an oil.

Oh ya one last thing;). Clean really well around those areoles. Those little jerks love hiding there.

Are you sure those are bugs? The little white things don't look like scale, but the bigger ones look normal. Looks what they are suppose to look like to me. Those cactus grow cotton on their thorns just like that. I would use 'Safers' insecticidal soap. as a preventative if need be. Neem has burnt mine in the past even in winter

For example:

https://www.google.com/search?q=echinopsis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjlxLq37YDZAhXmkOAKHRYeAmMQ_AUICigB&biw=1118&bih=456#imgrc=uw_iFBV-eIM15M:

https://www.google.com/search?q=echinopsis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjlxLq37YDZAhXmkOAKHRYeAmMQ_AUICigB&biw=1118&bih=456#imgrc=KnG70R5DRTPL0M:

https://www.google.com/search?q=echinopsis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjlxLq37YDZAhXmkOAKHRYeAmMQ_AUICigB&biw=1118&bih=456#imgrc=KnG70R5DRTPL0M:https://www.google.com/search?q=echinopsis&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjlxLq37YDZAhXmkOAKHRYeAmMQ_AUICigB&biw=1118&bih=456#imgrc=fcFdU25GjQnC3M:

myermike,

yes they are bugs. This is Echinopsis “dominos,” but these little white spots move around. Thank you for the sources!

Oh jeez...I know the little white things are buggers but I thought everyone was concerned with the huge white thingies)

Use Safer's Insecticidal Soap easily bought and Lowe;s of Home Depot and you will rid them fast and safe without any harm to your cactus. I have quite a few of these ones)) They will not scar but will with Neem if used to many times or even just once.

Mike

I’m curious if diatomaceous earth is effective against this. Does anyone have any experience on this?

If the pot isn't too big, you can try immersing it in a bucket of water. Wait for the bugs to float to the top, pour them off and drain your cactus. We sometimes do that to eliminate pests before doing a plant exchange.