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Background Story

I have a small, old cactus on my sunny, south-facing window sill in southern Germany. Since a few years, small black dots gathered around its pot. At first, I thought the dots were dust or sand and wiped them away. But the dots appeared again and again, so I had a closer look at them and saw that they were tiny insects or arachnids, probably lice or mites. We just call them arthropods from now on.

More information

The following picture shows my cactus with some of the arthropods around the pot. Click here for higher resolution.

Infected cactus

Here are some close-ups.

Arthropods around the pot

One arthropods

The arthropods seem rather dead. They do not move and most of them lie on their back. However, they are not dead. Once they are picked up and set on a paper towel they start to move. If they get on their legs (with a little help) they walk around, which can also been observed by the naked eye.

I inspected some individuals using a detached camera objective as a magnifying glass. The arthropods are brown, shiny, somewhat translucent, and have 8 legs. I could recognize a small head without a neck (like most bugs have) but couldn't recognize a mouth or eyes – probably because of the size. I couldn't take a picture, but tried to draw what I saw:

Sketch of one arthropods

Later I got hold of a cheap microscope and took a few videos. The video quality is miserable. In reality, the arthropods are not that dark, see my drawing and the photographs. Here's on individual laying on its back, flipping over, and walking away.

individual flipping over and walking away

Here's another individual walking.

Individual walking

The Strange Part

As said, the arthropods keep on re-appearing around the plant pot. But in all those years, I never saw any of them on the pot, inside the pot, or on the cactus. I also have other cacti right next to the "infected" cactus, but their pots were never surrounded by the arthropods.

Neighborhood of the infected cactus

Question

What are these arthropods, where do they live, and why do they leave their habitat?

My Theory

I suspect the arthropods to live inside the dead and cracked part of the cactus. They could walk on the spikes of the cactus and jump from the ends of the spikes to the ground. If they land on their backs, they cannot get up and stop moving after some time.

I don't think that the arthropods live inside the soil. Two reasons for that:

  • The arthropods could have drowned often. From time to time I put too much water in the pot, such that all soil is fully covered by water, often for more than 8 hours (yes I know that can't be good for a cactus *cough*).
  • In order to get to the window sill, the arthropods would have to walk quite a while. Over time, I should have seen at least some of the arthropods on their way to the outside world.
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  • $\begingroup$ There are some kind of ticks according to 8 legs and lack of segmentation. And they can live in the soil despite the drowning and damage the cactus roots. $\endgroup$ – annndrey Jul 13 '17 at 9:14
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It's hard to tell from these photos (you'll need a macro lens for good shots), but they seem indeed to look like mites (Acari).

There are mites that feed on rotting cacti, Bonomoia opuntiae for example. There is a film here about this cactus mite.

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  • $\begingroup$ Thank you for your answer! It's very interesting to see that new species are still discovered today. I don't think that the mites from the video are of the same species as the mites from my cactus. The mites from the video have long legs that are evenly spaced along the body. The mites from my cactus are darker and have not so long legs that are attached near the head. I am sorry that I cannot provide better images. $\endgroup$ – Socowi Aug 11 '17 at 18:51
  • $\begingroup$ The mites in the video are probably not the same species, but your cactus neither. Maybe each cactus species has its own specific mites? I don't know if a real entomologist is present on this site. $\endgroup$ – user35628 Aug 11 '17 at 19:04
  • $\begingroup$ In the last weeks I haven't found any of the mites. It seems like they are gone since I moved the cactus from the window sill to a shadier place. $\endgroup$ – Socowi Aug 23 '17 at 13:43

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