White Fungus or Bugs on a Red Cactus

Small plantlets that develop on moon cactus ribs can hide mealybugs.

Small plantlets that develop on moon cactus ribs can hide mealybugs.

If you have a red cactus, you have a cactus with history: The moon cactus, your red cactus, is a product of a distinct variety of rose plaid cactus (Gymnocalycium mihanovichii var. friedrichii) grafted onto a green rootstock cactus that is most likely the night blooming cereus (Hylocereus undatus). Moon cacti arose from mutant seedlings that lacked chlorophyll and couldn't make their own food. To survive, they were grafted onto a green cactus that could make food for itself and the moon cactus. A common pest of moon cactus and cacti in general, mealybugs are small white insects with a fluffy wax covering.


Mealybugs, sometimes known as wooly aphis, are sucking insects related to scale insects and aphids. Like aphids, they secrete sticky honeydew. They tend to locate in crevices, folds and branching areas of cactus stems where they are partially hidden. Once they settle in place, they produce a cottony-looking waxy covering that further hides them and repels moisture. This might lead you to think they were a fungus rather than an insect, since the thread-like waxy filaments resemble the mycelia of some molds.

Life Cycle

Adult female mealybugs lay eggs and give birth to living young. The eggs are orange-colored and can stay alive for a long time, even several years. The young mealybugs are called crawlers, and they move around for a short time. Female crawlers choose a suitable place to live and insert their slender mouthparts into the plant's stem, feeding on the sap. They become fixed in place and don't move for the rest of their lives. Male crawlers go on to become small, short-lived winged insects that live only long enough to mate with and fertilize a female mealybug.


Once an infestation begins, it is difficult to control. Make a habit of closely observing your moon cactus every week or two, looking for evidence of mealybugs. Especially look for the small, newly-settled female insects around the bases of tubercles and offsets and in the valleys between the cactus ribs. Also look at the folds where the top red cactus, or scion, joins the green rootstock. Immediately remove any mealybugs you discover, using a cotton swab dipped in rubbing alcohol. You may wish to remove offsets from the moon cactus so you can expose all the crawlers and mealybugs they might conceal. Mealybugs weaken a plant and can kill it. Moon cacti are produced in greenhouses and are meant to be houseplants because the rootstock can only do well outdoors in U.S. Department of Agriculture plant hardiness zones 10 and 11. Indoor conditions provide more favorable conditions for plant pests, and it is highly inadvisable to use harsh insecticides indoors.


Usually mealybugs come into a home on newly purchased plants. A good practice is to put new plants into quarantine for several weeks, inspecting them frequently for evidence of mealybugs. If they are clean, add them to your collection. If mealybugs show up, keep the plant in isolation and try eliminating the mealybugs. In addition, practice good sanitation. Before reusing pots, wash them in hot soapy water to get rid of any lingering mealybug eggs and discard plant trimmings or debris that might harbor crawlers or eggs. Immediately discard any offsets you prune away from an infected moon cactus. Don't recycle potting medium if mealybug infestations are suspected.

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