Mammillaria pest help

[nwinstead]

So I've been noticing some webbing on my Mammillaria, and just the other day I noticed this gaping hole in one of my stems. It had some of the webbing around it with black specks. I hope the pic is large enough to see. This is probably something common, but I don't know what it is or how to stop it. Anyone have any helpful suggestions? Thanks!

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[BarryRice]

Looks to me just mechanical damage to the growth tip. Did you bang it against another pot? Maybe a caterpillar, slug, or hungry mouse came along?

[nwinstead]

Well, nothing like that has happened since I got it. I've had it about 3 months - and the hole wasn't immediately obvious b/c spines were covering the hole. Maybe it was there before I got it. But should I be concerned about the random webbing I see on the stems now and then? Here's a close-up of the hole and webbing/specks (insects?).

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[Gail]

Those specks almost look like caterpillar poo. Not sure if they would leave the webbing though.

[nwinstead]

I guess it does look like poo. That made me go back and take a closer look for caterpillars. Didn't see any, but beside the stem with the hole, there was a dried up dead stem that had a much deeper hole (in my fingers in the pic). I went ahead and removed the live stem with the hole and sliced it open just to see. The hole didn't go very far, but you can see in the cross-section the webbing/poo in between the spines. This is all over several of the stems. Should I apply some kind of insecticide?

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[Steve Johnson]

I guess it does look like poo. That made me go back and take a closer look for caterpillars. Didn't see any, but beside the stem with the hole, there was a dried up dead stem that had a much deeper hole (in my fingers in the pic). I went ahead and removed the live stem with the hole and sliced it open just to see. The hole didn't go very far, but you can see in the cross-section the webbing/poo in between the spines. This is all over several of the stems. Should I apply some kind of insecticide?

Ooh, it does look like caterpillar damage. If that's the case, unfortunately there's not much you can do -- the only guidance being offered on CactiGuide is here:

http://www.cactiguide.com/cactipests/#worms" onclick="window.open(this.href);return false;

[tumamoc]

I was not aware that borer moths attacked Mammillaria. I have seen them on Peniocereus and Opuntia. You should inspect all of the stems carefully for webs and caterpillar turds. Quarantine the cactus. Remove any affected stems. Un-pot the plant if you need to. Worst case scenario, you may have to throw out the plant. You do not want these nasty critters working their way through the rest of your collection.

[iann]

I've had these. I seem to get one or two each winter, but they prefer Lithops to cacti. One caterpillar can do a lot of damage, but if you catch that caterpillar then the problem is solved.

[nwinstead]

I've looked over the whole plant and the soil surface. I don't see any caterpillars. Hopefully the caterpillar(s) that caused that damage is gone. But a lot of the stems have webbing and poo under the spines. I've pulled most of it out and will keep checking for new webbing/poo. I did apply some systemic imidacloprid which apparently doesn't work well on caterpillars, but I read works well on the neonates (just in case). Thanks all for your help!

[hegar]

Well, tumamoc, the picture of the "purple borer" pictured under the tab "Pests & Diseases" was submitted by me. The insect larva was found feeding inside a Mammillaria sp. cactus. The plant looked perfectly healthy and if I had not seen the green frass (poop) that had fallen off the cactus near the Mammillaria body, I would never have guessed that it was infested. That specific caterpillar was an internal feeder. Those are the worst and will usually kill the plant, because after the fully grown larva leaves the host, there is a big opening for any kind of microorganism (mostly bacteria and fungi) to enter and the plant will rot.
So my advice to you is: Keep a close eye on the cactus. If you do see a hole in it through which frass or sap ooze, there is usually not much that you can do except to discard (destroy) the cactus. Fortunately, caterpillars cannot multiply and internal feeders will remain in the infested plant until they leave it in order to pupate in the soil.

Harald