So Why do Spider Mites Crash?

It  brings to mind a car crash but of course that's not what I'm talking about. I'm talking about a healthy, reproducing population of spider mites that goes into a sudden decline.

I hope you've been able to witness it happening, because it gives you a good feeling. After all, you are getting free pest control. For once, nature is being a help instead of a problem. But how does it work?

Close-up of a female spruce spider mite.

Spider mite numbers decrease for a number of reasons, but it usually revolves around the weather. Spider mites like it hot and dry. Why? The warmer it is the faster they mature and reproduce. They also prefer dry weather because wet weather keeps most of their eggs from hatching. Sometimes you'll even see the mite eggs and even the mites themselves turning black as a fungus consumes them.

But humidity also plays another role. The predatory mites that feed on spider mites do better in humid conditions. So wet favors the good bugs (predatory mites) and dry the bad bugs (spider mites).

For a view of predatory mites, click here! This publication is for greenhouse growers, but the mites are the same.

We've already been seeing spider mite numbers rise and fall this spring in Fraser fir fields in western North Carolina. There are still a few eggs present in some of these fields which might allow the spider mites to come back later on in the summer or fall. In fact, if the fall is dry that is when spider mite numbers really tend to rebound, making them a problem on harvested trees. But for some of these fields, no further action will be necessary.

So how will you know if spider mites are rebounding? You scout, of course. Scouting for mites doesn't have to be a lot of work. Just going out and checking a few trees every month or six weeks -- especially those prone for mites -- will help you keep track of their numbers. Look at some shoots with a hand lens. That's the best and easiest way to look for mites.

Scouting for spider mites with a hand lens.

For a complete review of spider mites and their control see the Christmas tree note #29: Spruce Spider Mite on Fraser Fir.