The most common genera of root mealybug in tropical and house plant cultivation are Rhizoecus sp. Smaller than a grain of rice, they usually do not get bigger than 1.5mm (1/16 in). While they prefer dryer conditions, they are adaptable and thrive in even very humid conditions. Feeding occurs directly at the roots, and if left untreated an infestation can eventually kill plants. Life cycles range from 24-40 days. Females deposit eggs in a waxy-covered sac in the soil, often right along roots. Crawlers are considerably smaller, and more mobile than adults. Adults are slow moving but readily infest nearby plants through drainage holes, dripping water, and shared growing equipment. If you received a plant with a minimal amount of root mealies, it is possible it takes more than one or even two generations for infestations to be noticeable, which means a very loooong quarantine.
Golden Nightmare (Chryseococcus arecae)
I have not found documentation of this species being found in the US yet, but, would guess it’s just a matter of time (if they aren’t here already) as they were found in the UK in 2012. The originate from New Zealand and do not necessarily present additional challenges for tropicals, but they are particularly terrifying for gardeners in areas that normally freeze because they are freeze tolerant (Rhizoecus are not). For those with outoor plants, perhaps keep an eye out for these in garden plants, easily distinguished from Rhizoecus by their golden color.
Sourcing ~ sometimes I read articles on Integrated Pest Management (IPM) that recommend getting plants from reputable sources – yes, of course do this, but my experience has been this is not enough to prevent pests from entering a collection. Many (and perhaps even most) growers and collectors are unknowingly distributing root mealy. They are so easy to miss, and I do not think they have become widespread in the houseplant and terrarium plant community until more recently so lots of people are just now finding out about them.
Visual Inspection ~ inspecting a new plant thoroughly by gently removing the root ball from the pot and inspecting with the naked eye for any signs of pests, and then under magnification if anything suspicious is found can be helpful. Looking at all the roots with solely a hand lens and not with the naked eye first is a good way to make yourself paranoid.
The main limitation of this is if the plant is infested enough that you will see signs upon first receiving it, the grower would hopefully not have sent in the first place, which is why most root mealy infestations only become apparent months after the plant has received. For nurseries that regularly use pesticides, I think the populations are kept low, if this protocol is abandoned infestation becomes apparent in the months following.
Closed Systems & Long Quarantine ~ I think this is the only real way to ‘prevent’ Rhizoecus from becoming a chronic problem in collections. By closed quarantine I mean a protocol that minimizes in every way the possibility of cross-contamination from the quarantine space to areas where the rest of the collection is stored. This means keeping plants in containers with minimal ventilation, ideally in a separate room, with no shared tools and strict hygiene in regards to the person moving from space to space. It means prepping pots and bins with substrate in advance of handling new plants so as not to contaminate media with potential cooties. It also means being conscientious about coming into a clean grow space from outdoors, not wearing outdoor shoes in the grow space, etc. The period of time it has taken for root mealy to become easily detectible is anywhere from 6-18 months. So, a quarantine that allows for the upper limit to be reached is ideal.
Clear pots and Regular Root Checks ~ having pots where you can see root growth without having to unpot is tremendously helpful in keeping tabs on Rhizoecus infestations. Even on plants that have ‘graduated’ from quarantine, I check roots regularly for signs of infestation.
If an infestation is new, you will often just see signs of root mealybug. The waxy secretions can appear almost blue, and are most readily spotted in potted plants on the outside of a root ball. Often if you pull the plant out of the pot, you will see white wax residue left on the sides of the pot. In other cases, detection requires pulling apart the root ball. In yet other cases, there are no immediate signs and you just have to wait. The plant can appear healthy for more than a year while still hosting Rhizoecus. A 30-60x jewelers loupe is a really helpful tool to help in identifying pests.
Things that can resemble root mealy
Dead Isopods ~ if you have smaller species of isopod in with your plants and their numbers become very populous, they might try and migrate out of the container to find more resources. They often do not make it, and their bodies can dry out and stick (even if no condensation is present) to the sides of a container. Over time grow lights can cause the bodies to lose some of their color and fullness, leaving a mostly white chitin exoskeleton which can resemble in shape and size, Rhizoecus.
Springtail Molts ~ often appear as elongated silvery filaments that will move easily if you blow gently on them. They don’t look alot like root mealy under magnification but without a jewlers loupe if they are concentrated, can be mistaken for root pests.
Fungus ~ mycorrhizae and other fungi can appear at first glance like root mealy wax but generally have more of a webbed appearance, and the anatomy of the structure is more filamentous.
Calcium carbonate (chalk, bone meal), and various powder supplements ~ when initially applied and until thoroughly wet these can resemble the chalky secretions produced by Rhizoecus and other Mealybug species.
Are they really that bad?
Agricultural crops have been significantly affected by various root mealy species, as well as multiple commercial African violet growers in the NE. In my experience, they are an order of magnitude worse than thrips, aphids, fleas (I have big hairy dogs), or foliar mealybug.
A study conducted from 1958-9 concluded that as a result of three Rhizoecus species present on Alfalfa in CA, 50% of the Alfalfa stands were dead one year later. They were found feeding at depths up to 38″ below the soil surface. Soil samples were collected from various depths, of 12, 24 and 36 inches. The greatest quantity of specimens were found at the 36 depth (23), compared with only 3 at the 12 inch depth. Prune trees some 8 miles from the study site were found to host Rhizoecus as well, and infected plants presented with Zinc and Potassium deficiency. Rhizoecus were found 6 feet below the soil surface here, and it was estimated that 50 acres of prune trees were showing signs of damage from Rhizoecus.
If you are not planning on distributing plants to other growers, and are comfortable regularly (read: indefinitely) applying insecticides Rhizoecus present less of a problem. In locations that do not freeze, one should be careful not to inadvertently spread Rhizoecus outside.
Heat treatment ~ the University of Hawaii at Manoa has an excellent study published on this method, found here. In summary, a 120F hot water soak of the potted portion of Rhapsis palms to allow the root ball center to reach 115F for 15 minutes is 100% effective at killing all life stages of root mealy. I have been wondering if there is something magic about the fifteen minutes, and if shorter soaks are also effective. If you have tried this, please let me know. Phytotoxicity is likely much lower at shorter intervals.
In high humidity, root mealy will often migrate above what would normally be the ‘potted part’ of the plant, to aerial roots above the surface. For this reason, I am skeptical that for plants with aerial roots soaking the root base is sufficient. This presents a problem, as targeting only aerial roots above the root base requires also soaking delicate foliage. In any case, do not use heat treatment on Begonia darthvaderiana, Sinningia, Sonerila- anything delicate. Do not use heat treatment on anything not especially delicate (by terrarium plant standards) that you can’t afford to lose or do not have backup of. Most aroids (even miniature species), Hoyas, and Orchids, tolerate heat treatment very well.
To adapt this methodology for terrarium plants, a sous vide cooking instrument is a very useful tool for this. I never would have thought of this, being very ill-equipped in the kitchen, but some friends recommended it, and it works wonderfully. An 800-volt unit in a 7-gallon tub usually takes 10-20 minutes to heat to the proper temperature even though the instructions say to not heat more than 1-2 gallons at a time. Because of the lesser water volume required for small plants, calibrating the sous vide to 115F is sufficient and results in less foliage damage.
Because the plants will be directly in the water and too much substrate could damage the sous vide, I remove all media from the roots. I have wondered if putting the plants into one of the sous vide cooking bags would also work, and potentially provide protection for the foliage. If anyone ends up trying this, please let me know!
Alcohol ~ many Hoyas, aroids, and Orchids tolerate this fairly well, but, overall it seems harsher than the hot water method. A soak in 70% isopropyl alcohol for a few minutes kills mealy, and also most plants. There is perhaps a happy medium to be found with shorter soaks, or maybe even a dip, and increased plant survivability, but to my knowledge, this has not been trialed.
Less toxic controls
Neem Oil ~ might work if applied to bare root plants in high enough concentrations. I soaked some Begonias in a strong neem solution and the root mealy was not evident 3-4 months following treatment, however, the plants never recovered and were eventually tossed. Due to the harshness of the treatment required and undetermined efficacy, I opt for either the alcohol or heat methods.
Cuttings ~ taking small cuttings from infected plants is perhaps the safest way to save the largest number of plants. It is not uncommon for Rhizoecus to migrate up a plant stem in search of aerial roots, so there is no guarantee in taking cuttings that one or two have not already reached that part of the plant, however, for sensitive plants this is often a very good option. The smaller the cutting/surface area, the less chance of contamination, and best to take it from new growth/as high up as possible. Then you wait! And treat it like it’s going through quarantine all over again.
Predatory Insects ~ certain easily obtained predatory insects such as soil nematodes and rove beetle can help keep populations in check. It is possible that if plants come in with small numbers of Rhizoecus, applying either or both of these early on will be sufficient to get rid of them entirely, but I haven’t seen any studies on this.
I do not recommend any of the following treatments, but wanted to give an overview of the types of approaches you might come across if you do a google search for root mealybug treatment.
The following methods are proven to work in controlling root mealybug populations. These are measures to reduce a population are unlikely to be effective for long-term management if used on their own. Usually what is meant by control in an open grow space such as a greenhouse is that the application will stop or help slow the spread of the target pest, but will not eradicate pests on already infected plants. I do not personally recommend using any of the chemicals listed below due to their general environmental toxicity and long half life.
Bonide Systemic Granules applied to pot ~ these seem to be the favorite pesticide of choice for many growers, likely due to the wide range of insects it treats and ease of application. The granules can be sprinkled on the surface of the growing medium and are watered in. The active ingredient is imidacloprid. Its half-life in soil is anywhere from 26-224 days. Imidacloprid is a neonicotinoid and is toxic to invertebrates, water health, pets, and humans/kids if touched or injested.
Aria and Altus ~ documented in this study, both of these pesticides achieved good control over a three month period. I haven’t found any long term studies, and taking into consideration the long reproductive cycles of most Rhizoecus species, a three-month time period is in my point of view not close to enough time by which to evaluate its effectiveness.
Before correctly identifying root mealybug as such, and thinking the waxy white secretions were a kind of fungus, I was treated with potassium bicarbonate. A few months following treatment, I noticed no signs of infestation. Potassium bicarbonate is not effective against any sort of mealybug. So, even known ineffective treatments can superficially appear effective.
Systemic Soak ~ alot of cacti hobbyists seem to use this method, where the roots are cleaned of any substrate, and the plant is soaked in systemic solution. Marathon, Bifen, and Imidacloprid are all suggested. All of these have very long half-lives should *NOT* be put down the sink drain/any sort of drain. It is hazardous waste.
Things you might hear
The following are treatments I have read hobbyists recommending in facebook groups and forums, and that in my opinion should be regarded as ineffective for treating tropical plants with Rhizoecus.
Hydrogen Peroxide ~ although it is readily suggested by hobbyists there is no evidence to suggest it works and I personally have found it to be no better than water for treatment, which is to say- it superficially ‘cleans’ the plants.
Washing and Repotting ~ unless the plant is thoroughly cleaned and washed (read: soaked) in hot water, the root mealy eggs will remain. Even if it ‘looks clean’ after a few months- you will see them again.
Growing in LECA or PON or any particular media~ does not prevent them, or prevent their spreading to other plants. They need roots, and maybe (?) are more populous in soil-based mediums, but will happily proliferate in any media.
Growing in high humidity/moisture ~ does not prevent root mealy. They prefer drier conditions but are adapted to handle 100% humidity and lots of wetness. They will even breed on aerial roots of plants grown hydroponically.
Diatomaceous Earth ~ works on contact to kill many pests due to its sharp, cutting consistency. As soon as DE gets wet though, it loses its sharpness. For tropical plants this makes an impractical solution. For drought-tolerant plants it might help a little (?), at least, until it’s time to water. Likely a decent control method in a dry greenhouse.
Growers Talks has some of the best up-to-date open-access studies available on root mealybug. The second of a two-part study on Rhizoecus treatment was published in 2020 and trialed ten different treatments over a 36-day period – unsatisfyingly short, but- the results were interesting. Multiple novel organic methods were trialed, including an entomopathogenic nematode, entomopathogenic fungi, and a novel bioinsecticide. In summary, after 36 days, there were no Rhizoecus observed on plants treated with Gradevo SG, Beauveria bassiana, or Steinernema carpocapsae, compared with 9 in the control group. In the summary, it is mentioned that all of these applications performed well over a 1+ year time period, however, there is no data provided as to what exactly that looks like. In any case, exciting that there might be some effective organic controls that would be practical for managing Rhizoecus in greenhouses and outdoor grow spaces in tropical areas.
Resources and References
Behind a paywall and potentially informative