When I watered my cactus earlier this week, I noticed that it had what looked like blisters, some of them black, and a wound with dried mucilage attached. I’ve never seen this before, so I went down the hall and asked some biologists. Their guess was that it’s caused by either an insect or a virus carried by an insect.
I didn’t know what insects might attack cactus, but an internet search turned up the cactus moth, which targets prickly pear cactus like mine. This moth is native to South America but it has invaded the southeast U.S. It probably hasn’t come as far north as Pennsylvania.
Yesterday, I removed the injured pad and the pad it was growing on (and the pads growing from it, of course) to get whatever this is as far from the (apparently) healthy remaining cactus as I could. I also dissected the injured pad.
The big needles appear frightening, but the real danger is from the tiny needles that hurt as much as the big ones and are much more difficult to find and remove.
The affected areas appear not to have penetrated very far into the cactus, only into the outer, green part.
I didn’t find any caterpillars, or even any evidence for them like tunnels through the soft, inner part of the cactus pads. The source of the problem is probably a pathogen, but whether it’s fungal, bacterial, or viral, I cannot guess. If anyone knows what caused these blisters, please leave a comment.
I wanted to pull out the entire infected stem, but I couldn’t get a good enough grip without getting poked by a lot of needles. I also couldn’t get a clean break where the pads are joined, so my cactus was left with this large injury. I hope it heals.