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," which 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?

Inspect

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.

One of the reasons mealybugs like stapeliads is because of their compact growth form, which provides hiding places. Check plants before buying, and inspect regularly thereafter.

Mealybugs tend to infest the roots and crown of a plant first, so 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 compared to a high-contrast white substance.

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 growth as a warning sign. An infested plant will suffer so keep an eye out for tops that are bent or curling over like this Stapelia grandiflora x. Stem tops often curl naturally, especially on hanging stems, so this may not be obvious at first.

If everything looks clean it’s still possible mealybugs are in the roots or soil. Don’t harass your plant’s roots just to check unless you have reason to suspect infestation. Repotting is always a good time for a wellness check though! Otherwise simply take preventive measures and take a close look at your plants regularly.

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, use in a spray bottle or dab directly on insects with a q-tip.
• Dawn dish soap: a small amount in water in a spray bottle works wonders on a lot of garden pests including ants and aphids. Less than half a teaspoon in a bottle of water 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. Follow direction to make a spray of neem oil in water.
• Systemic pesticides: options like bonide are absorbed by the plant and can help prevent infestations in the first place.
• A witch's blend of these: people often use combos of these, like dish soap and alcohol. I often use multiple, and alternate between techniques to keep pests on their toes.

Clean as much dirt away from the roots as possible, then spray down (or soak) 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 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, since mealybugs will live along the edges of the pot and leave dusty white residue. Alcohol is a convenient option; I mix a dilute bleach solution to sterilize my pots and garden tools, but the bleach becomes inert after about four hours so this isn't practical to keep pre-mixed in a spray bottle like alcohol.

Treat the soil as infested: mealybugs may leave their cottony eggs (ovisacs) in the dirt and may burrow into the soil to molt. Some people recommend baking or otherwise sanitizing the dirt for reuse, but I douse it and throw it away. It may seem wasteful but mealybugs can really wreak havoc! Mealybugs are capable of wedging into cracks in rocks so sanitize or discard any rocks from infected pots. I also dispose of heavily infested sections of the plant—salvage the healthiest looking portions and yeet the rest.

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 new plants regularly for 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. This poses some risk of overwatering the plant and causing root-rot if it’s unable to dry out quickly. Harsher treatments like alcohol do seem to cause some root die-back, so this isn't necessarily the most effective strategy versus repotting.

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 care 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 that same kinds of plants, so this may not be a challenge for your scenario! 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? Oh surprise! That's the adult form of the males—they are more capable of spreading than I had 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 can expect to find something more like tiny faux Halloween spider webbing on your plant.

The ground is lava / watch out for ants

Mealybugs live in soil, so if you leave pots on the ground outdoors any local mealybugs may find direct access to plant roots. 

Ants have a symbiotic relationship with mealybugs: they help transport the young mealies around, and in turn the ants feed off of mealybug "nectar." I learned the hard way not to keep pots on the ground where these partners-in-crime have direct access to roots. I'm not sure how far the ants are capable of transporting mealybugs, so I keep an eye out for things like ants marching up the leg of a plant table.

...and they can spread via wind

This is true of any small insect. Wind is powerful, and can transport lightweight insects long distances. The gnat-like males are capable of flying, while the waxy protrusions can serve as a sail to help whisk away female mealies 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 anyway. 

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—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 the 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.

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 it has poor nutrients and it’ll get thirsty and shrivel sometimes. In fact, one of the more effective ways to discourage mealybugs is to use an inorganic soil.