This cactus used to look entirely different a week ago. A lot chunkier and healthier.

I've been noticing tiny flies around my house for some weeks (upstairs, downstairs). I figured it may be related to a dodgy spider plant elsewhere in the house.

But I noticed today that the tip had shrivelled up. On closer inspection, I saw that the flies were congregating on around the cactus (tapping it revealed a couple more).

Also of concern - prodding the cactus reveals it to be rather flimsy; feels hollowed out inside somewhat.

Is there any way to turn this around?! I'm beside myself, because I inherited this from a late friend.

On my father's advice, I filled a squirty bottle with water and some washing up liquid and sprayed the plant with that. In his experience, that's killed blackfly on leaves.