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Scale and spider mites are hitting houseplants hard. Here’s how to check yours.

  • Author: Jeff Lowenfels
    | Alaska gardening
  • Updated: February 15
  • Published February 15

Spider mite parasitizes on sick and dry grapes leaves, isolated on black background.

A couple of months ago I warned folks that they would be seeing lots of articles about NASA concluding that plants clean the air. It is old news — about 15 or 20 years old. Still, sure enough, the RSS feeds are replete with articles on the subject, literally hundreds and hundreds of them. I just had a feeling, and it sure proved out. I wonder what the next botanical "fact" will be that catches the media's attention.

If all things were Alaska-oriented, that next thing might be a proliferation of articles on scale and spider mites. I have been getting way too many questions about these houseplant pests. We can only hope that the Lower 48 isn't having as bad a problem with them.

Let's start with scale. These are curious little plant-sucking insects. They start out in a crawling phase, but soon become sedentary and coated with a covering that often makes them look like they are 1/8 inch or so bumps on the plant that actually look like they may be part of the plant.

As noted, these insects suck plant sap. When their numbers become high enough, they can damage the plant. Leaves turn yellow and soon drop off. In addition, they produce a sticky substance that can attract ants (though rarely here in Alaska) and a fungus that causes "black mold," a sooty substance that detracts from the plant's appearance and can kill it.

In addition to houseplants, scale often attacks shrubs and landscape trees, particularly those that bear fruit, though this is less of a problem in Alaska. Right now, it is the indoor plants that are getting hit.

You can try to get rid of scale. If you catch it while the insects are still mobile, you can wash them off or kill them with a light spray of rubbing alcohol. For these and especially for scale that's immobile and has the covering, you should use a toothbrush or Q-tip dipped in the alcohol.

Sometimes only part of the plant will be hit. In that case, you may be able to prune the infected part of the plant off. Or, you can apply dormant oil to smother the scale insects. Neem oil, insecticidal soaps may work to kill them, but other than that, I don't recommend pesticides. If things are so bad that organic methods don't work, toss the plant out.

Next, spider mites. Some eat pathogens, but we are interested in those that eat vegetative matter. They are tiny little animals, 1/150 to 1/50 inches and come in various colors from red to green to yellow. They have eight legs, but lack antennae and they, too, suck juices out of plants, but they chew leaves as well.

Symptoms include spotted or speckled leaves that may turn bronze or lose color altogether. Eventually leaves curl and you can find a fine web on the bottom of leaves. You can take a piece of paper and put it under an infested plant and shake it. Some of theses small, slow moving insects will appear on the paper.

Neem and insecticidal soaps, hard washing and hand squishing may help reduce populations. keep infected plants away from healthy ones. If you have forced air heating, forget about it and toss the plants.

Finally, this is the time of year we bring nursery starts into the house. Always, always inspect plants before you do. And, of course, check out the ones you already have. Truly, the complaints to me about scale and mites are high enough to warrant a check of your collection of houseplants.

Jeff's Alaskan gardening calendar

Attention: It is almost time for the 11th annual Alaska Botanical Garden spring conference. Do not delay getting tickets for this all-day, March 3, event at alaskabg.org

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