Plant enemy part 2: Aloe Mites - Satanic hydra

Early in 2022 I was shopping for trees for my backyard at a local garden store. While perusing the succulent section I saw an aloe plant with a gnarly flower gall.

These galls, sometimes called Aloe cancer are caused by Aceria aloinis, more commonly known as aloe mite. These mites belong to the Eriophyid family of mites. I've seen mealie bugs, scale, spider mites but I have never seen an aloe mite. Since they are now capable of infecting Haworthia plants, which I grow, I felt I should see what they look like for myself. I returned to the garden center the following week and purchased the infected plant so I could see what these pests look like. I was extremely cautious with this plant and made sure it NEVER got close to my plants or garden and I disposed of it carefully after taking pictures and hosed down my microscope area with alcohol afterward and showered as soon as I could.

These pests are too small to be seen by the naked eye. I used a dissecting microscope to view them. First, I scanned the outside of the gall. I didn’t find any. Then I cut the gall open. There were several of them crawling around the inside of the gall. They did not look at all like the kind of mites I have seen in the past. These look nothing like the spider mites most of us have seen. They look like hydra with fat stems and stubby tentacles. You could see lots of round eggs around them.  It was mesmerizing to watch these evil creatures. I was surprised how fast they moved.

What makes these mites concerning is that their host range is expanding. Just as the COVID virus has evolved new variants with different qualities, Aloe mites have evolved variants capable of effectively infecting Haworthia and other succulents. It’s very important that growers are on the lookout for them and treat infestations before they get out of hand. My friend Matt has experience with dealing with these noxious creatures and has written an excellent article about them for the Haworthia journal (Aloe mite, a Haworthiad problem). He has given me permission to distribute the article to those interested. Send me an e-mail if you want a copy. Matt and I have been talking to growers and researching what is known about them for a review paper we are writing for the CSSA journal.

Here is more information about aloe mite infestations in Haworthia:

Aloe mites will usually not kill Haworthia, but they cause cosmetic damage on the leaves and stunt growth. Here is a picture from Matt's paper showing an infected Haworthia plant:

In Haworthia, aloe mites will generally not form the flowers galls seen in aloe plants. The damage to Haworthia leaves is caused because when aloe mites feed, they inject toxins into the leaves that cause unsightly spotting and scarring. This damage can resemble damage caused by thrips and other pests or nutritional deficiencies such as lack of calcium.  Here are the symptoms to look for: 1) Bubbly lines on the windows that run perpendicular to window lines 2) rust colored or black or orange spots or scars or lines on the leaves.

The good news is that aloe mite is not difficult to treat. Matt’s paper has guidance for treating these mites and we will have more information in the article we are writing. Abamectin and Carbaryl are effective at killing aloe mites (Imidoclaprid is not recommended and may actually make infestations worse according to a US aloe grower).  As is the case with other pests, repeated sprayings are necessary to kill newly hatched pests. With aloe mites, symptoms may persist and even worsen after treatment. A study of miticides used to treat aloe mites in Aloe found that two months after treatment with miticides, the damage was still apparent even in cases where plants were mite-free (Curative and Preventive Control of Aceria aloinis (Acari: Eriophyidae) in Southern California). Because the mites live at meristem-leaf junctions deep inside the plants, you will continue to see damage emerge long after the mites are dead. Don't be discouraged if this happens.

Like COVID, aloe mites are unwelcomed pests here to stay. Be vigilant.  They have evolved variants that can infect Haworthia and it's likely they are capable of evolving variants that can infect a wide range of succulents. I have already heard reports of them infecting Adromischus and Stapilia. I always recommend quarantining new purchases and I recommend quarantining any new Haworthia, Aloe, Gasteria you acquire. Keep them far from your collection until you are sure the plants are free of pests. You can see pests like mealies and spider mites. You can't see aloe mites unless you have a microscope. But you can see signs of the damage they cause. Keep a lookout for weird bubbly lines and rusty scars on your plants. 

Lastly, new threats are always on the horizon. There is a new mite, red orchid mite, that has jumped from orchids to aloe in Asia. And reports are that it can infect related succulents. It has already made it into the US. Beware, unlike Aloe mite, red orchid quickly kills plants. There may be no time to eradicate them before they kill the plant. 

 More reading here:

Tiny Monsters: Aceria aloinis

Curative and Preventive Control of Aceria aloinis (Acari: Eriophyidae) in Southern California

Eriophyid Mites on Ornamental Plants

First record of Aceria aloinis (Eriophyidae) on Aloe arborescens in central México

Selected Miticides for Use on Ornamental Plants