Spider mites aren’t insects

Rather, they are members of the arachnid family. Here’s what they like:

• Hot, dry and dusty conditions
• Stressed trees
• Insecticides (they kill mites’ predators)

If possible, avoid these. Left unchecked, spider mite infestations can severely damage bonsai.

Although they are too tiny so see clearly without magnification, their damage is identifiable when you know what to look for.

Spider mite damage on black pine (photos from April 2012)

Checking for spider mites can be done with a white sheet of paper.

1. Hold white paper below foliage suspected of infestation
2. Tap the branch
3. Check to see if any of the black dots are moving
4. If the dots aren’t moving, brush a finger across the paper to see if any streaks appear; green streaks identify spider mites and orange streaks identify predatory mites (the good ones) that feed on the mites that eat plants.

The paper catches debris and mites, if present

If the dots move, you may have spider mites

Spider mites reproduce very quickly. If you’re not quick to catch them, they can overcome a garden of pines in a matter of days. And even if the infestation is stopped quickly, affected trees will show damage until next year’s needles mature.

The best defense against spider mites – as is the case with all infestations – is keeping trees healthy and well-hydrated, especially during the warmest months of summer. Overhead watering in particular can be effective in keeping spider mite populations down.

Oils and insecticidal soaps are the typical first line of defense. If you have to resort to pesticides, look for miticides and check the instructions carefully to make sure they don’t exacerbate the problem.

Spider mites also provide good reason to spray lime sulfur or other dormant sprays in winter as they or their eggs can hang out during cold months under leaves or bark, patiently awaiting favorable conditions when the weather warms up.

For more info about spider mites and some non-toxic approaches to management, see the UC IPM guide on spider mites.