I've got a large Copiapoa hypogaea which I noticed was infested with mealy bugs today. I've also noticed sciarid flies and ants have been particularly attracted to it of late.

When I had a close look at it, I saw that two small offsets had been killed. On removal, a mealy bug was found inside. On closer inspection, there were many all over the cactus, including several egg sacs camouflaged amongst the wool. I've taken it out of the pot and washed in roots as thouroughly as possible.

My question now is, can I use pyrethrum to eliminate the mealy bugs? It's currently all I have on hand. I'm hesitant to use it as I recall having used it in the past and it causing burning of the plants leaves. That was an anomaly though.

Pyrethrum is a contact killer, if you are confident you can find every one of the little blighters it will work, make sure you keep the plants out of direct sun for a few days.
Personaly my eyes arent what they once were so i use Confidor 200 SC it is a bit pricey but its systemic & long lasting so it kills pests including root mealy's for months without bothering the native bees, unless i'm forced to spray it & then its only the day of application.
Its main limitation is it kills small seedlings & I wouldnt use it on anything being grown for fruit or nopal consumption that season.

I probably would have used Confidor, although I don't like how toxic it is in comparison to 'pyrethrum, but I didn't have any on hand.

Anyway, I used pyrethrum in the end. While it was still bare-rooted, I picked over it then doused it heavily in pyrethrum, roots and all. Gave it an hour to sit, then washed it all off. During that time I checked it periodically. Only saw one living mealy bug which I promptly killed. Hopefully all the eggs were killed too. If they come back, I'll use a systemic pesticide next, probably Confidor.

At least I've learnt why there were so many ants around it. I knew there was something wrong, but thought perhaps they were attacking the root system, like I've seen happen to grafted Lophs. But no, they were milking the mealy bugs for their honey-like secretions. Next time I'll know.

Just be careful with pyrethrum based sprays, it destroys some cacti. Pereskiopsis are the one that comes to mind, any areas contacted will literally melt and destroy the plant. Lophs from memory are hit and miss, some will react badly while others wont. I don't use it for this reason.

Thanks for the info, that's really good to know.

So far, it hasn't melted away, but it's skin looks a little burnt/faded. i think it'll be okay, but I wonder whether it'll regain it's normal colouring?