Stapeliads on the whole are robust plants that aren’t vulnerable to many problems. The two maladies to be on the watch for are root rot and mealybugs. When a plant falls ill, I find it often has both: plants stressed by mealies more easily succumb to root rot, and plants weakened by a run-in with root rot will attract pests.

What are mealybugs?

Boo. Hiss. Plant owners are no fans of mealybugs. Unfortunately mealybugs are a fan of many cacti and succulents including stapeliads!

“[Chorizococcus brevicruris] is considered a rather serious pest of succulent plants, Huernia spp. and Stapelia spp., belonging to the plant family Asclepiadaceae, growing in many nurseries throughout California.” (p. 95)

Color plate from Mealybugs of California with Taxonomy, Biology and Control of North American Species, by Howard L. McKenzie with color illustrations by Mary Foley Benson, 1967. 

"Mealybugs" is the colloquial name for the Pseudococcidae family of insects. They are commonly also called "scale," though scale insects are technically close relatives within the Coccoidea superfamily. There are plenty of varieties of both scale and mealybugs, so you may observe variations in appearance. Scale is typically a smooth bump while mealybugs have a fuzzy appearance.

The insects are considered to be pests because they attach to plants and feed on the sap. Left unchecked, an infestation can grow heavy enough to kill a plant. Mealybugs are a major agricultural crop pest, and have a reputation among gardeners as an especially sneaky and tenacious infestation that can destroy whole collections.

Here's how to protect your stapeliads!

How can I fight mealies?

As illustrated above, fully grown adults appear as very small fuzzy white oval bumps, and form globs of cottony substance on the plant. Advanced infestations will also grow white waxy protrusions. The younger nymphs are even smaller, a peach-fuzzy light red, pink or purplish color. There are more detailed photographs of mealybugs and an infestation in the 'prevent' section.

Inspect

Mealybugs like stapeliads because of their compact growth form which provides hiding places. Check plants before buying, and inspect regularly thereafter.

Don’t harass a plant’s roots just to check unless you have reason to suspect infestation (Repotting is always a good time for a wellness check, otherwise frequent disruptions will delay growth). If the soil permits it, you may be able to slide the pot off to get a look a roots—and the inside of the pot. Established infestations will leave white spots where the mealybugs in the roots made contact with the pot.

Lightly lift the edges of your plant to check for white cottony substance where stems contact the soil. If your eyes are good you may be able to spot the nymphs, but they can be easy to miss.

Next check the nooks and crannies around the base of the plant, especially where stems branch or overlap. Then skip up to the tips of the stems, where the tight folds of new growth can hide pests—sometimes all you notice are white speckles on the ends!

Take misshapen and stunted new growth as a warning sign, like the bent and curling tops on this Stapelia grandiflora x. Natural scarring from other causes like a sunburn can similarly cause stems to curl, and hanging stems often curl a bit naturally, so stunting may not be obvious at first.

Treat

When you discover a plant is infested, plan to isolate, treat, and repot. There are several easy home-treatment options that will kill mealies, in no particular order:

• Rubbing alcohol: 91% isopropyl alcohol, dilute in a spray bottle or dab directly on insects with a q-tip.
• Dawn dish soap: a small amount of dish soap in water can near-instantly kill garden pests including ants, earwigs, and aphids. Less than a quarter teaspoon in a 32oz spray bottle is plenty.
• Horticultural sulfur: say, didn't I mention this for root rot too?! Sulfur can kill and help dissuade mealybug infestations, as well as spider mites. Sprinkle on the soil as a preventative, or mix in water to give your plants a spritz.
• Insecticidal soaps: there are a lot of options available at your local nursery supply, like Safer Brand Insect Killing Soap.
• Neem oil: neem is always a solid choice, and works as a preventative as well as an insecticide.
• Systemic pesticides: options like Bonide are absorbed by the plant and kill sap suckers when they attempt to feed. This helps prevent infestations in the first place, and provides longer-term protection; home-use systemic pesticides typically stay in the plant's system for several weeks or months depending on the variety.
• A witch's blend: people often use combos of these methods rather than relying on one. I use multiple, and alternate between types to keep pests on their toes.

Procedure

Clean as much dirt away from the roots as possible, then spray down (or dip in solution) the entire plant and its roots. You can use Q-tips or a paintbrush to get your mealybug killer into all areas of the plant, since the pests will wedge themselves into tight folds that manage to stay dry when doused. Be careful not to leave the plant out in direct sun after treatments that can cause sunburns.

Sterilize the pot and any accessories like plant tags. Mealybugs are capable of wedging into cracks in rocks so sanitize or discard any decorative rocks from infected pots. Alcohol is a convenient option; I mix a dilute bleach solution to sterilize batches of pots and garden tools, but the bleach mix becomes inert after about four hours so it isn't practical to keep pre-mixed in a spray bottle like alcohol for spot-cleaning.

Treat the soil as infested: some mealybug varieties leave their cottony eggs (ovisacs) in the dirt and may burrow into the soil to molt. Some people recommend baking or otherwise sanitizing soil for reuse, but I douse it and throw it away. I also dispose of heavily infested sections of the plant; salvage the healthiest looking portions and yeet the rest. It may seem wasteful but I prefer to minimize exposure because it minimizes work in the long run.

Re-treat

One treatment usually is not enough to catch all mealybug eggs and adults. Plan to re-treat for several days.* You can leave cacti and succulents bare-root during this period, though you can expect tender roots to die back and larger roots to callous. Once repotted, continue to check plants regularly for new signs of mealies you may have missed.

* Alternatively some plants will tolerate a one-and-done 'waterboarding' treatment where they are left submerged for hours or even days to catch offenders in all stages of the mealybug life cycle. I have not attempted this with stapeliads due to their sensitivity to root rot.

Deep treat

An infected plant can be so large and mature that it is not practical to repot without fully dismantling the plant... which is unfortunately what you will need to do sometimes. To treat without repotting, make a large batch of your preferred insecticide and SOAK the soil in the pot, and any rock top-dressing. (This is not recommended with sulfur though, which can build up in the soil to a point of toxicity). At the same time thoroughly spray down all above-ground growth, moving branches to get under and between them.

I have been concerned about burning the roots so I run a rinse with plain water after treatments like alcohol. This poses some risk of overwatering the plant and causing root-rot if it’s unable to dry out quickly. Harsher treatments like alcohol also seem to cause some root die-back, so using something like a systemic pesticide or soil granules will be a lot easier on the plant.

Prevent

Where do mealybugs come from? The depths of hell. I’m convinced they can materialize from the aether; they are sneaky little demons and it can take some attention to keep your plant collection clean. A few quotes from Mealybugs of California (1967), p. 19–20, illustrate how tenacious this pest is:

“Specimens [of Scaptococcus milleri] have been discovered at least ten feet away from any vegetation”

“Many mealybug species are found under rocks, deeply embedded in cracks.”

“...some mealybugs are capable of finding their annual hosts before the first shoots break the soil surface.”

Even so, there are a lot of small things you can do to give your plants the best shot at staying healthy.

Quarantine

To avoid mealybugs, it’s always recommended to inspect, repot, and quarantine new plant purchases. Plants from big box stores commonly have pests! A lot of locally-owned nurseries do too.* Plant lovers recommend quarantining new purchases away from your main collection for several months. I always repot in my own soil as soon as possible and throw away the original soil. Repotting provides a good opportunity to closely check the base of the plant and roots for signs of mealybugs (you will occasionally get tricked by rooting hormone or a piece of perlite wedged between stems). Treat and quarantine as necessary.

* No shade to local nurseries, in all fairness you gotta be vigilant having that many plants in close proximity, and having just one supplier with pests can introduce an infestation. Most nurseries operate with good intentions surrounding pest control but mealybugs are notoriously sneaky. As the houseplant and exotic plant industry has been expanding rapidly in recent years, unfortunately a commercial industry has ballooned where less and less care seems to be put into plant health, as long as it looks good for long enough to sell—if even that, we’ve all seen the rows of dead plants at Walmart. Box stores have grown notorious in the hobby for introducing pests to peoples' collections, so exercise caution.

Will quarantine keep your collection clean?

No. Absolutely not. I had an existential crisis the day I came across a mealybug while weeding my front yard and it suddenly dawned on me that they can come from nature. Look at these plump assholes:

The good news is not every type of mealybug is attracted to the same kinds of plants, so this may not be a challenge for your scenario! Cactus-infesting mealybugs are common in my region; I see infestations in yards around my neighborhood. There is an opuntia in the center divider on the way to my post office that is particularly egregious. Advanced infestations like below look as though someone spent their day shooting spitballs onto the plant, and eventually cause the cactus to collapse and die.

In the image above you can observe small pinkish nymphs between the white mounds the adult females have produced. There are also a bunch of gnat-like insects hanging out with them. Why? SURPRISE! That's the adult form of the males—they are more capable of spreading than I had originally realized.

This example is actually technically cochineal, a variety of scale native to the southwestern states and Mexico, which is known for producing the vivid color in carmine dye. If you look closely the white puff-balls appear as if composed of rigatoni noodles. These are the white waxy secretions that protrude from the female as it matures, and can vary in shape/appearance in different species. Many types of mealybug form cottony sacs so you would find something more like tiny faux Halloween spider webbing on your plant.

The ground is lava / watch out for ants

Mealybugs live in soil and have a symbiotic relationship with ants: ants help transport the young mealies around to new host plants, and in turn the ants feed off of mealybug "nectar." 

I learned the hard way that leaving pots on the ground gives these partners-in-crime direct access to roots. Plant stands and tables won't stop pests; keep an eye out for ant trails running up the legs. You may find it necessary to treat for ants or, as a cheap fix, create "ant moats" by putting table legs in pie-tins filled with water.

...and they can spread via wind

This is true of practically any insect; wind is powerful, and can transport small and lightweight objects long distances.

Nymphs are known to climb to the edge of a cactus pad and wait to catch a breeze. The gnat-like males are capable of flying, while the female's waxy protrusions can serve as a sail to help whisk her away to new and exotic lands (like your back yard).

What if my plants are indoors?

I have observed an alarming number of cases where people found mealybugs on flowers or produce they brought home from the supermarket. I recently saw bell peppers heavily infested with spider mites at my local grocery store. No matter how careful you are, pests might find their way in through unconventional channels.

Don't let humidity get too high

If you use a greenhouse over winter, ensure proper ventilation or it is easy for conditions to get too humid inside—I have to be careful even in Tucson where daytime humidity is typically below 30%. A warm humid environment means your greenhouse is a tropical paradise for pests like mealies and spider mites, as well as mildew in succulents.

Avoid wood planters

Avoid putting your stapeliads in wooden planters, since wood planks are another of mealies’ favorite habitats. Presumably wooden planters available commercially have been treated to protect against insects but I'm not sure I trust this would be effective against mealybugs. To be honest I'm not sure I even trust wooden shipping pallets, which are notoriously heavily treated against pests.

Take it easy on water and fertilizer, or try an inorganic soil

Your stapeliad may respond well to frequent watering and fertilizer, but it will also be more likely for it to attract mealybugs. For your plant to be in ideal health, it actually needs some stress! I know I said stress causes root rot and mealybugs, but it’s about good stress: some stress, but not too much.

Think about it like trying to mimic what the plant would experience in nature versus pampering it. Pampering will invite mealybugs. In nature the plants are accustomed to poor nutrients and will get thirsty and shrivel sometimes. In fact, one of the more effective ways to deter mealybugs is to use an inorganic soil.