My houseplants had been looking fine all summer, but now they were obviously ailing. No leaves were drooping, no obvious critters were chomping on the leaves. It was more of a general sense of decline—and a dappled, grayish pallor to the foliage.
Closer inspection revealed that many of the V-shaped joints between leaf petiole and stem were filled with minute webbing. My skin crawled. My plants were infested with spiders! To be more accurate, my plants had spider mites. These tiny bugs are not insects. They are arachnids, just like spiders, scorpions, and ticks. Like spiders, they have two body parts and eight legs. Unlike spiders, all of whom are predators, spider mites are more like vampires. They suck plant juices.
If we only had to contend with one or two, these almost-invisible mites wouldn’t be a threat to our greenery. Unfortunately, spider mites live in huge colonies—hordes of hundreds all determined to suck our plants dry. Eeek.
The speed with which the mites conquered my houseplants was downright scary. Apparently, a newly hatched spider mite can become an adult in only seven days. Then the mated females lay several hundred eggs over the next couple of weeks. No wonder they seemed to come out of nowhere!
Because they are not insects, most insecticides do not work on spider mites. In fact, some bug sprays can make matters worse by killing all the good insects that eat these nasty pests. Yet, we have to do something—left to themselves, spider mites will eventually kill their hosts, all while fingering the next victims.
Outdoors, spider mites are usually kept in check by voracious predators: Minute Pirate Bugs, Big-eyed Bugs (I am not making these names up), lady beetle larvae, and the like. In the house, control is up to us.
The first thing you need to do is survey the damage. Remove any leaves or stems which are heavily infested, seal them into plastic bags, and throw them in the trash. If the whole plant is severely bug-ridden, harden your heart and toss it as well. Now turn your attention to the surviving plants.
There are a number of miticides (pesticides that kill mites) on the market. Some are for commercial use only—you need a special license to apply them. Others are only marginally effective. Additionally, given that these plants live in your home, chemicals may not be appropriate. Insecticidal soap can help, especially if it is applied every week or two, but it won’t kill them all.
If you want something to spray, the best choice is horticultural oil, which physically smothers the pests. You can buy these special oils at a garden center.
There are other options for the determined mite murderer. The best for indoor use is to change the microclimate surrounding the affected plants. Mites thrive in low humidity—just what we have at this time of year. Increasing the water vapor around your plants is an important first step. Try setting your pots on a pebble-filled tray. Then add water to the top of the pebbles. The rocks will keep your plants from becoming waterlogged, while the evaporating water annoys the mites. You can also make regular use of a mister, or try one of those small decorative tabletop fountains. Your plants will thank you as well. (Another benefit for outside plants—increased humidity encourages those predatory insects.)
Just like the wicked witch in Oz, mites succumb to a strong jet from the hose or shower head. If your plants are small, restrain their potting mix with your fingers, turn them upside down, and swish their foliage around in a sink full of warm water. Not only will you wash away a host of mites, you will also damage their webbing, annoying the heck out of them. Finally, wipe off the stems and leaves with a soft damp cloth. Take that, you mites!
Because all those spider mite eggs will continue to hatch, you’ll need to repeat the washing-squirting-wiping routine every week until you get the upper hand.
It’s sad, but in spite of all your efforts, your plants might still succumb. Spider mites are extremely difficult to control. To an enthusiastic gardener, that’s just plain scary!
For more information, see the Colorado State University Extension publication on Spider Mites.
Photo credits (from top): Whitney Cranshaw, Colorado State University, Bugwood.org; Louis Tedders, USDA Agricultural Research Service, Bugwood.org; J. Holopainen, Wikicommons.