Answer: It sounds as if you have cochineal scale. The scale insect isn?t noticeable but it secretes a tell-tale sticky, white, cotton-like substance that can spread over an entire cactus with time. I find that a strong blast of water from the hose controls cochineal, although I never seem to completely eliminate it. The more frequently you wash it off, the less trouble you'll have with it. Luckily, the scale won?t hurt the plant as long as it?s kept under control.
I remember years ago when I first hosed off a cholla that had cochineal. I was dumbfounded when the water running off the cactus towards my feet turned bright red! I learned that the insect body (not its white secretion) was traditionally used to make a red dye and many weavers and artisans still collect it from cactus to use in their work.
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