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I'm trying to determine if my dracaena marginata has spider mites. A little history about the plant: I got it about 5 months ago and it seemed to be an outdoor plant with the previous owner. Since bringing it home with me it now lives only indoors. I noticed some small cob webs as seen in the photos but I think those have been there since I got the plant. I recall seeing those little webs when I got it. As seen in the photos there are some small holes on the leaves but those have been there since I got the plants. I don't think the holes got bigger. I'm thinking the plant had spider mites at one point but they might be dormant now? I don't see any moving things on the plants at all. I sat and looked at a lot of the leaves but haven't seen 1 moving thing on it. This makes me think the plant once had spider mites but now it doesn't. I'm very new to houseplants so any info is appreciated underside of leaves

leaves

leaves

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Short answer: probably not.

Longer answer: Spider mite infestations present with very fine webbing coating whole areas of the plant, generally whole leaves and the stem. Within this webbing you should be able to see small ~1 mm (0.04 in) dots moving, but it does take some observation and helps if you know what you are looking for. You will be able to feel this webbing if it is present, but may not be able to see it, as it is very fine and best seen by holding up against a dark background with a light source shining on the leaves at an oblique angle.

Holes in the leaves are not indicative of spider mite activity - at least not holes of that size. Spider mites are so small that they feed off the contents of single plant cells, which range from 10 to 100 micrometres in size (that's a 100th to a 10th of a millimetre). This kills the cells, leaving a small pale yellowish dot on the leaf, but not an actual hole that you can look through.

A good example of a heavy infestation can be found on the Wikipedia page for spider mites. I've reproduced it here (file attribution: Paramecium, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons):

spider mites

In this picture, you can see the webbing between the stem and the leaves, but note the difficulty seeing it on the leaves themselves, even though it is there. You can also (if you look closely) see the damage caused by the mites themselves as the pale yellowish speckles on the leaf to the top right and middle left.

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