Aloe mite: The hidden scourge of the Mediterranean succulent garden
One of the worst critter infections I have had to deal with in my plant collection is of this evil little creature, Aceria aloinis, a member of the mite family Eriophyidae. Acerias are microscopic mites that attack certain members of the Aloe family (Aloaceae or Asphodelaceae, depending on which source you refer to) including Aloes, Haworthias and Gasterias. I think these latter two a bit more resistant as I have not seen any of mine infected yet, and I have a lot of them out in the garden near infected aloes. I talked to one Dave's Garden member who says he's seen this infection on an Agave, too. I had not seen one of these mites until I wrote this article. I took an infected leaf with galls with me to work where I made a 'skin' scraping much as I would do with a dog or cat looking for mange mites. And sure enough, with the aid of a microscope (10X) I was able to discover that the leaf was covered with this dinky monsters.
Mites are arachnids (related to spiders) and not insects. The reason this is important to know is when you are trying to eradicate mites from your plants, some products that only kill insects will be useless (for example, Imdacloprid is a popular insecticide thanks to its wide margin of safety in mammals and birds, but is completely useless for treating arachnids.) Mites, like spiders, usually have 8 legs (4 pairs), but this genus has only two pairs of legs. They actually look more like stubby caterpillars with legs only at the front end; in fact, similar looking mites exist in mammals and cause a form of mange. They are slow movers and hardly do anything but crawl millimeters at a time. However, they are small enough to be blown about by the wind, and that is thought to be their primary mode of spreading, although probably insects (such as ants) or other animals, including humans, also aid in their spread.
this Black Widow is a typical ararchnid (note: 8 legs); photos taken of mite off an aloe with Aloe mite (all legs at top end with mouth)
The disease these mites cause is called Aloe cancer or Aloe gall. Personally I think Aloe cancer is a good name since the mites secrete a chemical substance while feeding that alters the normal growth pattern of the tissues, causing them to grow haphazardly and bizarrely, a lot like the way many cancers grow in people and animals. And often the results look very cancer-like as well. This chemical is actually a growth hormone regulator and it tends to induce a gall (solid mass) around the mites as they eat and this gall probably protects the mites from the dangers of the ‘outside world'. The mites not only continue to feed inside the gall, but then lay their eggs as well. The actual damage to the plant done by the feeding of the mites is insignificant, and even the gall formation itself rarely ends up killing the plant (though it may weaken the plant severely in massive infections and then something else will come along and kill the plant). However these cancer-like galls are permanent disfigurements of the leaves or flowers and unless removed, they will often continue to grow abnormally even after the mites have died or are long gone. For some reason, the mites are attracted to rapidly growing tissues, so galls tend to form at growth centers (crowns of the rosettes and flowers), though they can form anywhere.
Shots of aloe cancer between leaves, at tip of lower and below living leaf crown
more aloe cancer shots. Second two photos are of my own plants, Aloe greatheadii with early changes on flower, and Aloe marlothii showing very 'cancer-like' tumors on stem
flowers are the most severely and easily infected. First two photos are of the flower stalk showing early and later signs of inection. Third photo is of another deformed flower
flower in lower right of first photo is of a flower gall on Aloe barberae, the huge tree aloe; second photo is of a flower so deformed it is barely recognizable as such
Spread of this infection is very unpredictable. Some plants are naturally resistant, and sometimes it's simply luck, as it jumps from one plant to another, skipping several closer plants in the process. I have noticed some species are far more likely to show up with this than others. For example, Aloe arborescens and spotted aloes are very commonly infected plants.
The control of this infection is labor intensive, and the elimination of such a scourge from a collection is nearly impossible unless one is really diligent and can effectively quarantine all infected plants. The mites are easy to kill with most toxic products (probably even simple soaps and diluted bleach will kill them). But the problem is getting to the mites. Safe in their galls, very little in the way of pesticides will thoroughly penetrate these solid structures and so the structures themselves have to be removed to kill these beasts. In other words, all infected looking tissue needs to be carved away and immediately tossed in the trash or burned. Instruments used in carving these galls off the aloes need also to be thoroughly cleaned after each use. Some severe infections require one to remove so much tissue that the plant occasionally may be unable to survive such radical surgery. These plants should probably be removed from the collection completely. Once the offending cancer is removed, the remaining tissues still need to be bathed in some disinfectant or pesticide repeatedly. Note that just about all pesticides are potentially toxic to us as well, can be very toxic to pets and most certainly will be toxic to beneficial insects. Sometimes systemic pesticides will penetrate the galls and kill the mites... but the galls have to be removed anyway, so one probably should not rely on this approach exclusively. Also, it is possible some strong systemic products may not be too good for the aloe itself. There are no systemic that are designed to kill aloe mite (and not kill aloes, too), but some have luck with a variety of systemic used on roses and other ornamentals. Use at your own risk is all I can add to that. Once the aloe has been treated, one may also need to treat for ants if that is the assumed method of contagion. Ants, as many know, are very hard to control as well.
My Aloe longistyla last year with a beautiful, perfect flower. But this year, this tumor showed up as soon as the flowering bud appeared. Third photo is of plant with tumor carved out
Aloe longistyla flower cancer after removal. Note the thick, solid gall on cut surface. The mites are hiding in this solid mass somewhere.
I have no idea how you would control this parasite in a large botanical collection (and as far as I can tell, no one is putting much effort into doing it.) Probably it would require simply digging up and tossing out hundreds of plants, some which might be difficult to replace. Or with the judicious use of systemic pesticides, perhaps. If anyone knows of any success stories, please leave comments at the end of this article.
some aloe collections are worth a fortune and just unreplaceable
Many of my own aloes that have been damaged by Aloe mite, but apparently recovered since treatment, now have branched profusely and quite uncharacteristically for those species. It seems the infection damages the growth center of the plant causing it to go crazy in an almost desperate act to create a mass of new individuals. Other individuals have returned to their normal look and shape. Either way I am always vigilant with these treated plants and try to keep them from mingling with the rest of the population needlessly (in other words, I don't move them among non-infected plants) as every once in a while, even several years later, hints of returning infection can be seen.
my Aloe imalotensis after about a year- perfect/pristine. Then early in summer second year the first signs of infection show up as rouch edges along the leaves
infection now causing massive 'suckering' behavior as several 'new plants' start growing out beteen the leaves of the original one. Last photo is of aloe over a year later, apparently 'cured' of the infection, but now a bizarre multiheaded version of what is supposed to be a solitary species.
Similar story with a newly acquired Aloe fievetii- rough spots show up, then color change and 'cancer fuzz' grows, and lastly an aggressive dichotomizing behavior takes place
Before long I have a multiheaded Aloe fievetii. This second photo is of an Aloe thraskii at a friend's home showing similar manic suckering behavior in response to a different infection (this one mealy bugs and their protectors, ants).
Unfortunately the only way to tell if a plant is infected is to see the abnormal tissue growth. The mites are all but impossible to find, even with a magnifying glass. So you are taking a chance when you buy new aloes and add them to your collection. I was upset the first few times an apparently ‘clean' aloe ended up with mites soon after, but I don't know realistically how an aloe grower can absolutely insure their aloes are not infected. It is just something I have accepted and will continue to battle...but it has kept me reluctant to share any of my aloes with others, fearing I will infect their collections. So don't ask me for any aloes unless you are willing to deal with their unseen inhabitants.
I got this Aloe spinossisima at a local nursery, planted it, and the following year it flowered. It looked perfectly healthy and had a normal flower.... until just before the top of the flower developed, THIS happened! I just disposed of this plant immediately as it was near some rare aloes I did not want to become infected.