Question
We have a wonderful rock garden full of century cactus in our yard in Austin Texas.
We have about 20 or so plants, the largest about 6-7 feet tall/ wide or larger. The "mother" bloomed around 4 years ago and has since died and been removed...we haven't had any other stalks since then.  We hadn't really cleaned out the patch much; we were told that the original owner used to clean out the rocks, replace the plastic lining, and prune the plants every year.
A few months ago, we finally found someone (after being in the house for 4 years) who specializes in cactus and "native" landscaping (we are in Austin TX), and we had him clean out the rocks, relay new plastic lining, and cut the dead leaves off the bottoms of the larger cactus. We also moved some babies to other areas.
Since then, we have had three or four of the large cactus tip over and die. While cleaning the garden, the landscaper did show us where some of the others he removed (that were yellowed and already dead), that had had their roots eaten by what he called a "worm" (I am assuming he means the type that bores through the root system).
I am wondering if these recent "deaths" are:
a). related to the worm (maybe searching for a new home after we pulled out where he was previously living)-if so, what do we do other than toxic chemical treatment?
B). related to the trimming of the lower leaves (did we shock the plant by doing this?)
ac). Could the plastic lining be "suffocating" the root system? There has always been plastic there, but I am wondering if it was placed too close to the base of the plants?
d). could just the physical disruption of the patch (the landscaper crawling amongst them, trimming and raking the rocks)have damaged a few of the big cactus/ root systems and caused them to fall over?

It seems as though the newer plants are doing fine, and the medium ones seem to be shooting off babies as usual.
I am stumped as to why we are loosing some of our big beauties. We love these plants and feel such a responsibility to take care of them. Unfortunately we do not know much about them, and are afraid we may have done more harm than good.
Any thoughts? Your help is appreciated.
Thank you.

Answer
Caroline,

I'm not sure if you are talking about cactus, Agave's or both. However, if you are talking about Agave's, there is a bug called an Agave snout nosed borer that can decimate your Agave's (I had a pretty severe case this year myself). Here is a link for treatment: http://www.arizonacactussales.com/tips/snout_beetle.htm

If you are talking about cactus, it could be an over abundance of moisture caused by the plastic. There is a product out now called "weed cloth" that serves the same purpose as plastic, but allows water to pass through it in both directions. If you have had a lot of rain, the plastic could be holding in the moisture too long and causing rot. Typically, a large established cactus would do OK with the change, but depending on its age (very old) or some other factor, the extra moisture could be deadly.

Hope this helps.

Sincerely,

Greg