Mr. Smarty plants.
I have purple prickly pear cactus that are developing small white flake like spots, mostly where the thorns would be. Why is this happening and how can I cure it?
Your cactus sounds as if it is infested with cochineal bugs (Dactylopius sp.). They are small scale insects that feed on the cactus. They produce fluffy white wax that hides their bodies as they feed on the cactus and protects them from the elements (drying out, in particular) and from predation. The fluffy wax also serves as a sail or balloon to float on the wind and take the bugs to a new patch of cactus. The bugs produce carminic acid that also helps protect them from predation, especially from ants. This carminic acid in the bugs has been used by indigenous peoples of southwestern North America, Central America and sub-tropical South America to make a brilliant red dye for centuries, perhaps millennia, to produce beautifully colored textiles. Originally, the cochineal bugs were limited to the New World. When the European explorers visited and saw the beautiful red cloth of the natives, they took the cochineal bugs back with them and now they occur all over the world. When a synthetic red dye was produced the demand for cochineal bugs decreased, although they have also been used to help control cactus popuations. Recently, however, after it was determined that the synthetic red dyes can have adverse health side effects, there has been a renewed interest in growing cochineal bugs for red dye. The dye made from the bugs is currently used in cosmetics and as food coloring. Because of this, controlling cochineal bugs hasn't really been a priority and, therefore, there isn't a lot of information that I have been able to find for controlling them. However, click here to read what one grower recommends. If your infestation is small, I suggest scraping them off (carefully, to avoid the sharp cactus spines) and disposing of them. You might also be able to wash them off with a water under pressure. Test a small area first to be sure that you don't injure your cactus and gather up and dispose of any of the insects that you wash off the cactus.
Here are a few links to more information about the cochineal bugs:
Lesions on Agave Leaves in Spring Branch, Texas January 14, 2011 - I have a giant Agave americana that was beautiful until recently. Several of the leaves have lesions, some of them quite drastic, on the last third of the leaf. What is causing this? view the full question and answer
Removing grassy weeds from cactus garden in Corpus Christi TX May 07, 2013 - How do you get grassy weeds out of a cactus garden? I don't want to use roundup around the cacti? Can you recommend a herbicide that won't harm the cactus? view the full question and answer
Possibly non-native succulent identification March 27, 2008 - My mother has a plant that grows on a stalk, 2.5' tall, leaves are about 4-5" on the mature plant. The leaves are scalloped on the edges and seeds grown in the scallops & fall off and make new plan... view the full question and answer
Use of Century Plant bloom as indoor decoration December 09, 2008 - Mr. Smarty Pants,
After living in Arizona and now San Antonio, I am in search of a Century Plant Bloom. I cannot have an evergreen because my cats climb them. I would shellac the stalk and blooms a... view the full question and answer
Container plants for cool weather in Cypress TX October 02, 2011 - I am a novice gardener and I am looking for some ideas on potted plants for the fall/winter. They would be covered by a roof, but still susceptible to the elements. What can be planted now that will... view the full question and answer