While these specimens have been in captivity in a plastic box, the characteristic 'cotton wool structures belonging to cochineal bugs have appeared. These, and the valuable red dye (cochineal) obtained from them are the reason that Opuntias was introduced to Australia. You can see the distinctive red colour from a crushed insect on the tissue (pictured).
The fruits of many cacti are edible, some like the spine/prickle-free Dragon fruit being commercially cultivated. But beware both of these if foraging. O.tomentosa has thousands of minute irritant hairs which embed in skin! As for Harrisia, the spines are strong and sharp.
As both these plants are environmental weeds, they will be heat killed above 100deg. overnight in an oven.
Australia's biological control of Opuntia is possibly one of the most spectacular successes in history (by contrast with the spectacular environmental disaster of cane toad introduction) The story deserves a post of its own.