Q. I appear to have a massive infestation of Cottony Cochineal Scale on my prickly pear cactus. I can usually get most of it off by hosing, but much is hard to reach. Beyond hosing, what insecticide or other kind of solutions do you recommend? Permanent, if possible!
A. Yes, scale is quite common on cacti in general and prickly pear in particular. This one in particular is most likely cochineal scale rather than cottony cushion scale. This cochineal scale is a close relative of the cochineal scale that was used by the Spaniards in Mexico centuries ago and die making. This scale leaves a purple color on your finger when you rub it.They can be hosed off the pads with a strong stream of water as you are doing but they reinfest the pads very quickly and in full force again in a week or two.
When I am growing prickly pear as a food I am more concerned with insecticides and getting them into the food we eat but if they are grown just as an ornamental plant and not used for food then I feel better about using something stronger than water, soap and water sprays.
The problem with a strong stream of water is that the force from this water is not enough to knock the insect off of the pad, only the white, fluffy coating surrounding the insect. So it is a short time before it regrows this protective cover and the unsightly mass of cotton is back again.
Knocking the fluffy covering off first with a strong stream of water will make these insects more susceptible to an insecticide spray. Most of the common insecticides will work after that; malathion, pyrethrins, Sevin, just about any common insecticide will work after the fluffy covering has been removed. This does not mean they won’t be back. They will. But it will take longer for them to get established on the plant again.