10 Neat Things About Spider Mites
by Dorothy Dobbie
by Dorothy Dobbie

The Local Gardener magazines, Ontario Gardener, Manitoba Gardener and Alberta Gardener, are published by Pegasus Publications Inc.

Drawing on her 30 years' experience as a senior executive in the magazine publishing industry, Dorothy launched Manitoba Gardener in 1998, initially running the business out of her home. Two years later, Dorothy's daughter Shauna, living in Ontario, jumped into the fray with Ontario Gardener. And two years after that, they started Alberta Gardener. Visit us at and register for our "Ten Neat things" newsletter. Watch Shaw TV for garden tips and Listen to CJOB for the Gardener Sundays at 9:08

October 8, 2017

1. A tangled web.

If you see webs on your houseplants, it's probably already too late to deal with the pesky 2-spotted spider mite, Tetranychus urticae, the scourge of house plant lovers everywhere. The webs are there to deter predators and protect eggs laid on the underside of leaves.

2. Bug or spider?

These pesky animals are not insects. Although the nymphs have six legs, the adults have eight. They are closely related to actual spiders and to ticks and scorpions. Don't bother looking to count the legs unless you have a really good magnifying glass. Spider mites are the size of a pin-head - 1/50th of an inch long.

3. Floating on the breeze.

If you detect spider mites on a plant, quarantine it immediately. The tiny eggs and animals can float on air currents or be carried on your clothes or shoes or even by your pets. Mites on one plant threaten your whole indoor garden.

4. How did I get them?

Chances are you recently bought a new plant or a friend gave you some cuttings. Infested plants often come from box stores who don't have the time or the expertise to care properly for their plants. But no matter where you obtain them, new additions to your indoor greenhouse should be quarantined for the first couple of weeks.

5. Many generations.

Spider mites produce about eight generations a year. It takes just three days to hatch the 20 eggs a female will lay in a 24-hour period and only five days until the hatchlings become sexually mature. The adult mother goes on laying eggs for her whole life - anywhere from two to four weeks. But these wily animals can also go dormant when threatened so there is no safe rule of thumb in control, except keep it up and be vigilant.

6. Seeing the little suckers.

Suck is what they do. They insert their miniscule mouth parts into your plant and suck it dry. One of the first symptoms of an infestation is stippled leaves - the plant may look like it has had a thousand little pinpricks - which it has - or it might even look "dusty". Eventually, the leaves turn yellow and drop. Turn the leaf over and you may see the buildup of mites and their eggs on the underside.

7. Killing the little suckers.

Because they reproduce so quickly, they also just as quickly develop resistance to insecticides. And anyhow, pyrethrums kill adults but not the eggs. The best line of defence is to clean the plant with water. You can also use an insecticidal soap. A house plant can get a good wipe down plus a bath every two to four days for a couple of weeks - then hope for the best. Continue to keep conditions humid by spritzing the plant frequently - spider mites prefer hot dry environments indoors.

8. Oh no, my spruce needles are dropping.

The spruce spider mite, Oligonichus ununguis, is one of the more than 1,200 spider mite species around the world. It enjoys sucking the life out of spruce, cedar, juniper, fir and larch. If you look closely, you will see an infested tree with stippled needles that are flecked with yellow. Severe infections will display webbing. Often the homeowner won't notice anything until the inner branches of his tree become bare of needles. The infestation starts at the branches closest to the trunk. Spider mites can kill a tree and are especially active in cool springs and falls.

9. Help! How can I deal with them?

It is a good idea to give your trees a thorough wash with a sharp spray from the garden hose a few times a year. This can also help get rid of other insects and their eggs. Natural predators include lady bugs which will eat spider mites if aphids are not available; they eat nine mites an hour. Phytoseiulus persimilis is a natural spider mite predator but they are fussy about living conditions, being most effective at 20 to 27 C (6 to 81 F) and 60 per cent or greater humidity. This makes it tricky because spider mites like it hot and dry! You can also spray trees with 20 per cent neem oil or cottonseed oil .

10. Try the shake test.

One way to check evergreen trees for mites is to hold a sheet of white paper under a branch. Shake the branch, then run your hand over the paper. Spider mites will show up as coloured streaks, usually red, but they could be yellow, orange, brown or grey!

Dorothy Dobbie Copyright© Pegasus Publications Inc.

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