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10 Neat Things About Houseplant Pests
by Shauna Dobbie
March 29, 2015

1. Fruit flies. The little guys who colonize your fruit basket in the summer do not infest houseplants. Drosophila melanogaster (a fascinating, if annoying, little creature) feeds on fermenting fruit material, which you don’t (or shouldn’t!) have in your houseplants. The things you may find flying around your houseplants are either Sciaridae or Mycetophilidae, also known as fungus gnats.

2. Fungus gnats. Unless you have an awful lot of them, fungus gnats themselves don’t pose much threat to healthy, mature houseplants. The larvae live in the soil feeding mostly on fungus, though they will also go after small rootlets and tender seedlings. Fungus gnats serve a useful purpose to the indoor gardener: they indicate that a plant is being left too wet for too long. This issue might be cleared up by adjusting your watering schedule (with most houseplants, the soil should dry out completely between waterings), or you might discover a more serious problem that is preventing your plant from taking up water.

3. Spiders or mites? Spider mites are mites, not spiders. Zoologically speaking, both are arachnids, but spiders (for one thing) feed on insects and these mites feed on the juices in your plants.

4. Webs. A telltale sign that you have spider mites (Tetranychus urticae) is the coating of sticky webs they leave on your plants. They spin these as a protective covering for their eggs. Fungus gnats will also leave silk from their cocoons behind, but those look like tiny cocoons rather than a fine web. You can also tell you have spider mites by the damage they leave on the foliage (dead patches) and, if you look very closely, you may be able to see the tiny red mites.

5. Prevention and control of spider mites. Spider mites do not care for a moist environment. Since you don’t want to have your plants in soggy soil, inviting a fungus gnat infestation, try misting the leaves with water. If you already have spider mites, you can spray insecticidal soap on the plant at 10-day intervals (since it will only kill adults, not eggs, you have to repeat applications to get new adults who were eggs the previous time you sprayed), making sure you get to all parts of the foliage. This is best accomplished in the shower. Leave it for about an hour and then rinse the plant in tepid water. The optional but optimal finishing step is to wipe the plant with a soft, damp cloth.

6. Scales. These become most noticeable as hard scabs on a plant that may look like some kind of fungus. In fact, that scab is a waxy covering over an adult female Coccocidea, a plant-sucking insect commonly called scale. This type of scale insect is particularly interesting because the adult female is immobile. The juveniles crawl, and the male adults have wings and can fly.

7. Dealing with scales. Because of the waxy coating, the damaging adult females are particularly difficult to target. Some horticultural oils may suffocate the insect under the shell. I used to know a fellow who sprayed diluted isopropyl alcohol on his plants to attack the adult females. These insects, however, are best controlled at the juvenile stage with soaps or, if you prefer, chemicals. The adult males are harmless to the plant; they live only about two days and do not feed, but if you hit one with a spray of soap, you might get to it before it mates.

8. Mealybugs. Mealybugs are also scale insects, but the adult females do not encase themselves in a shell. Most of them retain their legs and can move, though they tend to attach to a plant, sucking the juices and going nowhere. These ladies secrete a powdery wax all over their bodies that makes them look kind of like a half-centimetre cigarette ash or bit of bird poop; that mealy coating makes them just about as difficult to target as their immobile sisters.

9. Whiteflies. (Aleyrodidae.) You know you have whiteflies if your plant is rapidly dying and, when you give it a shake, a cloud of tiny flying insects appears. These are enormously destructive insects that attack a plant the way most of the pests do: they pierce the outer tissue and suck out the juices. Whiteflies are particularly effective at killing a plant for several reasons: their saliva is toxic, they occur in massive numbers, and they excrete honeydew (like aphids) that promotes mould growth.

10. Controlling whiteflies. These are difficult to control because they are extremely adept at developing resistance to insecticides. At the agricultural level, Imidacloprid, which is the chemical that might be at the root of the very serious colony collapse disorder affecting honeybees, is sometimes used. Beneficial insects such as ladybugs and lacewings might prove effective. Of course, you always have the option of simply composting the plant.

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