Anne: I'm not in Z10, but I know from overwintering rosemarys inside that good circulation is crucial to overall health. Perhaps they need a bit more room?

Also, it could be that your old plant came with the critters. Since nothing else was effected it could've been just bad luck.

Martie

Cold water is the best medicine. Those little buggers hate being cold and wet so a good hard spray from a nozzle at frequent intervals will discourage them completely. They probably lap up garlic.
There are some excellent chemical controls (Avid) but these are fairly pricy and if you are only worried about rosemary, totally unnecessary. I use Avid because I bring roses indoors and they are very prone to spider mite. I haven't found rosemary very susceptible and feel sure that cold water will do the trick.

You have the rosemary outside? And it still gets spider mites? spider mite around here, outside, are usually gone during the winter. I almost never get spider mites on stuff outside, unless it is not doing well for some reason. There is no reason for you to have rosemary indoors in zone 10. It would be much happier outside.

LMBO...prevent them, not in my world. They take something down every year-usually mums like this year-almost my tagetes lucida. I think the best thing is spraying them with water every so often. Spray during the day or morning when the plant will have time to dry so they don't turn around and get powdery mildew. People say that spider mites don't like humidity so that's why spraying helps...but they grow fine in FL which is the most humid place I've ever lived-sheesh, towels mold in the closets.

This may have been luck, but I was treating some other plants for powdery mildew the other day and my marjoram had spider mites so I hit it with the same baking soda/oil/ivory dish soap solution I was using for the PM. Now the spider mites are totally gone, and I had been trying to fight them with just water. I don't know if I stumbled on something but it seemed to work better than pure water and it is the P.M. cure so maybe that would be a good idea for rosemary, I know that this mixture is fine for rosemary.

You know your area best Heathen1 but her in Tx I had spider mites within the last few weeks. Here it hasn't been consistently cold enough, but they are better than during the deep summer.

Oh, watch how hard you spray, I broke 1/2 a mum plant by spraying too hard. And get that rosemary outside, it will love you for it, Heathen is right.