YARD AND GARDEN: Controlling the “bad guys” on your houseplants | Home & Garden

Yesterday, when I watered my plants, a small cloud of minute black insects ascended, reminding me of a scene from a Harry Potter movie. If you have those black things hovering ‘round your plants, see fungus gnats below.

Healthy houseplants usually don’t have problems with insects, but I don’t know a single indoor gardener who hasn’t had problems, so feel no angst. A good organic potting soil, the proper amount of light and water, and regular feedings with a good liquid houseplant food will go a long way toward keeping your plants healthy. NOTE: Winter is resting time for plants, so don’t fertilize then. Check new plants for insects before bringing them into your home. Inspect regularly for yellowing, discolored or sticky leaves. If you find insects, isolate the plant and treat the problem.

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Insect/disease control indoors — sprays

Never use an insecticide inside your home unless the product label recommends it for household plants. Insecticidal soaps and pyrethrum sprays (made from chrysanthemums) can control most common indoor pests.

There are several effective insecticidal soaps, or you can use a mild, unscented non-detergent soap. Dilute 2 to 3 teaspoons of soap per quart of water. Solutions must be applied thoroughly to all surfaces, repeatedly and persistently (weekly for a month or more) to get good control. The bathtub is a handy place to perform this operation.

However, there is the potential of plant injury with homemade remedies so caution is recommended. Test the soap-detergent spray on a small area a day or two before an extensive area is treated.

Spiders are great predators of other insects. One of our U of I instructors during the Master Gardener training expressed the opinion that spiders inside indicated a healthy house!

However, spider mites are a different kettle of fish. Smaller than the period at the end of this sentence, they have tiny mouthparts for piercing individual plant cells and removing the contents. The foliage takes on a yellow or bronzed cast. Heavy infestations can kill plants.

Finding spider mites early is important. Take a piece of white paper, strike some foliage on it; look for the tiny mites walking on the paper. Another method is to mist the foliage, which highlights the hard-to-see webs.

CONTROL: Regular spraying with a forceful jet of water from a hose or shower spray can keep spider mites under control. Insecticidal soaps are also useful (see above).

These sap-feeding insects don’t even have the decency to look like insects. They affix themselves to the plant under a tan protective shell-like covering ranging in size from 1/16 to 1/4 inch. Scale insects feed by sucking plant sap, causing poor, stunted growth and eventually death. They excrete a large quantity of a sweet sticky liquid called honeydew which produces a sticky, shiny mess on the plant, furniture and floors. If your plant leaves or floor feel sticky, you’ve got scale. This, in turn, creates favorable conditions for black sooty mold fungus. The mold blocks photosynthesis, further weakening the plant.

CONTROL: You can rub the scales loose from the leaves and stems, a laborious chore on large plants, or use insecticidal soap or horticultural oil. But it’s a productive task to do during TV commercials. Another possibility is using the non-toxic sprays noted above.

I once had a huge, scale-infested schiffelera on which I experimented by pruning off all the leaves so all that remained was a stump with 6-inch twigs. I sprayed and repotted the stump in new soil. The leaves quickly re-grew into a shapely uninfested plant. Unless the plant is particularly valuable, you might find it best to toss it before the pests spread to other houseplants.

Adult fungus gnats are about 1/8 inch long with dusky wings, dark hairy bodies, and look a little like mosquitoes. Larvae are 1/16 inch long and clear bodied with a black head. Eggs are microscopic. Each adult can lay up to 200 eggs and in moist conditions, the eggs transform to adults in 10-14 days. Strong healthy plants are not affected, but younger plants are. Not only do the larva eat roots, but they spread disease, such as black root rot and Pythium wilt.

CONTROL: Avoid over watering, especially young plants. Add a top layer of one half inch of sand to your pots. Gnats don’t burrow that far and the eggs are open to drying out because sand desiccates quickly. Yellow sticky cards work. Bt H-14 is an organic control in the form of Gnatrol. Use it weekly for 2 or 3 weeks.

Whiteflies, related to scale insects, are tiny, snow-white insect pests that look like flying dandruff. Look for eggs on the undersides of leaves in a circular or crescent-shaped pattern. The adult whitefly emerges from the pupal case and flies to other host plants to lay eggs and begin the cycle again. Both adult and nymph stages suck plant juices. Heavy feeding produces mottled leaves, then yellowing and eventually death. Like scale, whiteflies also exude sticky honeydew (see above).

CONTROL: Control measures are complicated because the insects cling to the underside of leaves, making them difficult to reach with sprays. In heavy whitefly populations of mixed life stages, two to three applications per week of insecticidal soap may be necessary.

Mealy bug females lay eggs in cotton-like pouches from which the first nymphal stage emerges. They cannot be controlled with soap sprays because a repellant waxy covering protects the adult. The mature female mealy bug is soft bodied, pink / gray in color and about 5 mm long. They feed by sucking large amounts of sap from the stems or leaf joints. The excess sap is excreted as honeydew. (Sound familiar?)

CONTROL: You need to either use an insecticidal oil to suffocate them, or daub them with a 70{d4d1dfc03659490934346f23c59135b993ced5bc8cc26281e129c43fe68630c9} solution of rubbing alcohol in water with a cotton swab. Reapply every week or so until you’ve broken their life cycle.

All infested plants should be quarantined and heavily infested plants should be discarded. Remember to use only indoor plant insecticides labeled for use on your particular plant. Be sure to follow all directions on the product label.

Good luck in your crusade!

If you have questions about your garden or landscape, contact a master gardener at the University of Illinois Extension office in Mattoon at 217-345-7034.