If Your Pines Turn a Weird Grey…
…they might not be dead, they might have spider mites.
Mites are sneaky little arachnids that can transform our trees while we’re busy assuming all is well, to turn around one day and think, huh, wasn’t that a darker green last week?
Pine showing spider mite damage from the year before, on the older needles with the yellow stippling
The spider mite—there are several species—often forms a visible webbing if it lives on a juniper, but the webbing is often not apparent on a pine. In both instances the mite leaves a yellow stippling of the needle or leaf if seen close up, and a graying if seen from afar. The mite sucks out the interior of a cell and causes it to lose its green color. A tree can survive repeated mite attacks for multiple years, but it does keep the tree in a weakened state and should be taken seriously. Also, bonsai will look like a dust bomb exploded nearby so aesthetically this is the opposite of ideal.
Spider mites on a fir, showing the yellow stippling on the needles
These mites are so small they are hard to see on the plant. To test for suspected mites a sheet of paper is held under the branch, which is vigorously swatted, and then the sheet is stroked with the palm. Note that vigorous swatting is used for errant apprentices as well, and a sheet of paper is often used as a flag of surrender. The techniques should not be confused.
But now pay attention to the color of the squashed mite, which will be a streak across the page.
- If green streaks occur, you likely have the naughty spider mites.
- If the streak is another color, like yellow or orange, that is likely another creature like a predatory mite, which eats the spider mite.
Abundant green streaks and you may have a problem; yellow and orange streaks and you might be ok.
A popular and effective method of controlling spider mites is a strong spray of water on the foliage from a hose rose. This overhead blasting must be repeated for some weeks to get all the generations of mites—the grandparents, young upstart mites, and those yet to be—and while it is effective, may move fine wiring somewhat. Alternatively there are miticides that control various stages of the mite life cycle (consider Avid and Forbid). Oils may be used, but be aware that these are contact controls and need complete coverage on all surfaces of the leaf/needle for them to be effective. Timing is also important…in cold winter months mites are not around.
Spruce showing spider mite damage from the previous year
Mites are more prevalent in a dry climate, and are more likely to enjoy the tree buffet that is nicely arranged on benches in the sun. The adjacent landscape may provide an unending supply of mites even if you are able to obliterate the first infestation, so prophylactic controls may well be necessary. Although mites are known to attack deciduous and evergreen broadleaf plants, spider mites do appear more frequently on conifer bonsai.
Keep a close eye out for subtle changes in how your foliage looks…and start swatting if it looks suspicious.