What are these white spots on my cactus? Note: they are not cochineal bugs (as I know them)

Question

I've had this cactus for about two years now, and this winter in began showing these white, flat dots that look like splattered paint. They can be scrapped off, but there's no sign of an insect or anything moving. They also don't have the typical red dye of cochineal bugs, nor the fuzzy texture. The cactus receives plenty of light, though not direct sun, and has for sure not been over-watered. The dots seem to be spreading fast through-out the leaves, but thankfully other cacti that I have, have not been affected.

I have tried scraping them off with a toothbrush, but I don't think that will get rid of them for good.

Any ideas on what this is and how to get rid of it would be greatly appreciated.

Thank you!

Answer 1

There is a natural flow of nutrients in solution from the root hairs up to the top of all plants where growth is happening. One result of this process is that as the sap reaches the top and the nutrients are used up what ends up on the leaf surface evaporates. Only the water can leave, so this process can leave behind salts and other minerals on the surface of the leaf. In temperate zones the residue commonly is washed off by falling rain or, since most of the loss is from the underside of the leaf where it falls off naturally, is not noticed by plant owners.

Note that there is a tendency for more of the deposit to accumulate where growth is happening. Also, Opuntia does not really have an identifiable underside of the leaf and is rarely rained upon or interfered with. The older the crystals become the drier they are and more likely to fracture off due to minor tremors, wind gusts and so on.

An interesting experiment might be to scrape up a quantity of the deposit say on an old clean toothbrush and with an amount collected, swill it around in a small quantity of pure water, see if the powder dissolves, and set the liquid to evaporate again on a sunny windowsill to see what you get back. If any insects are involved then when the residue is dissolving there should be some signs of old dead body parts visible once the minerals have gone into solution.

Answer 2

I think it is armored scale with gall (the white stuff). The bugs are so small and flat that you don't recognize any insect parts, the white stuff is gall resulting from the infestation. You can use a magnifying glass to see if you can recognize bugs. Simple method to get rid of it is to brush them off with a toothbrush, like described here, the second picture looks very much like your situation I think.