Green, grey or black bugs clustered at the growing tips ( these are aphids).
Almost invisible webs which when flicked over a piece of white paper drop little brown specs that crawl (red spider mites).
Leaves that curl down and pucker (mites, aphids or thrips).
Hard raised little dots on leaves and stems (scale).
White fuzzy balls of cotton (mealybugs).
Little black things that jump (fungus knats).
Little black things that don't jump (the droppings of tiny cyclamen mites).
The first challenge is identifying these critters, which are often on the underside of leaves. Use a magnifying glass.
Treatment is not too difficult but requires persistence, persistence and more persistence. Permanent cures are almost impossible, which is why one must keep watch and treat problems right away and weekly thereafter. Insects are happy plant jumpers and spread easily. One respected mail order nursery now recommends using only water sprays, or insecticidal soap. When they don't work, they suggest getting rid of the plants (and they will sell you replacements).
Some things to do that lower the insect populations, most particularly giving plants a good shower every few weeks. For plants inside the house, we don't like to use toxic poisons. Some are less toxic, and there are new bio-pesticides which might be safer.
Precautions When Using Pesticides Always remember, substances that kill bugs can't be good to breathe in or get on our skin. Wash hands after using any pesticides, even safe ones. And, ideally spray outside. When spraying indoors, a good safety measure is to put a big plastic or paper bag over the plant with holes in the sides into which to spray. Leave it on for several hours until the spray has settled. Then air out the room because chemicals are absorbed easily by the lungs. (Keeps dried spray spots off the walls, too.)
Treatments for Particular Insects Mites: These are quite difficult to control. The National Arboretum in Washington D.C. says that mites can be washed off with a strong stream of water although it has to hit the leaf joints and under the leaves. The old fashioned way was to put them under the shower as needed. A common chemical spray called Klethane is specific for mites, and works quite well, but has to be repeated as they reappear. A newer substance called Avid is absorbed by the leaves. Safer for humans, Avid is made from a natural soil organism and takes about 4 days for the mites to die.
Aphids, Mealybugs, and White Flies: Frequent repetition is the key to control. A strong water spray or under the shower drench is recommended currently. The old way was to remove the critters with a cotton swab dipped in alcohol, including under the leaves, followed by a good washing. There are other less time consuming options though. Commercial insecticidal soap spray is one. Or make your own using a half teaspoon of Ivory Dishwashing Liquid in a gallon of water. (I have found the usually recommended one teaspoon per gallon too strong for many indoor plants. Causes brown damage on tender leaves.) Or consider a spray of water and mouthwash (with alcohol) which smells pleasant, like peppermint. The bugs don't like the alcohol or the smell. Pyrethrum, the safest chemical, which is in most house plant sprays, is the classic treatment for these critters. For non edible plants, spray with light horticultural summer oil, which lasts longer than all of the above. It also may cause brown leaf damage so test it first on a small area.
Fungus Knats: In the soil, the immature, l/8 inch long maggots eat the roots. The old remedy was to soak the soil weekly, 3 times, with a solution of the chemical malathion. The new safe control is Gnatrol, a new formulation of Bacillus thuringiensis Israeliensis, which is the same ecologically safe material used for mosquito control in our swamps. This particular BT (there are many varieties) was discovered in 1976 by an Israeli scientist, Yoel Margolit, who noticed that mosquito larvae were dying in a particular farm pond. He isolated the insecticide, which is now used worldwide to kill mosquitoes, black flies and flies that cause river blindness in Africa. Yoel Margolit, who was born in Europe, spent his childhood from age 5 to 12 in a Nazi concentration camp, and went to Israel as an orphan.
Scale: This is a tough problem, scale proliferates wherever nitrogen is available. They suck it out of the plant's juices. The soft insects protect themselves with a hard, waxy coat and under which they just sit and suck and multiply. More fertilizer makes bigger scale families. Outdoors, lady bugs eat them. The old way was to scrape the scales off with one's fingernail, plus an insecticide to get the soft bodied young crawlers before they make their protective shells. Now one can use a systemic insecticide.
Systemic Insecticides: These are substances which are watered into the soil, taken up by the roots and gradually spread throughout the leaves. As sucking and chewing insects try to feed, they are killed. It saves any spraying of pesticides in the air. There are several chemicals, some of which are restricted, so be sure to read the labels carefully. Needless to say, these can never be used on edible plants or where children or cats may play in the soil and could be poisoned. The chemical, acephate (Orthene) is most readily available systemic, but indoors, should be used only as a soil drench, never a spray. A new, safer bio-pesticide called Imidacloprid is an insect hormone that keeps the bugs from molting so it acts slowly. However, it attacks many pests (leaf miners, weevils, scale, aphids, mealy bugs and white flies). Sold under the name of Marathon, it's exactly the same stuff that's in Merit which is used for grub control on lawns. These newer bio-pesticides, lately dubbed "naturalytes" by the companies that are developing them are safer than the chemicals we are familiar with. However, we don't know their long term biological effects so it's wise to treat them with the same precautions we use for any chemicals. After all, DDT was for years considered the panacea for all the insect problems. Let's hope these new biological insecticides work better in the long run than did DDT.
How to Buy Clean Houseplants No one can resist buying seasonal flowers to make the indoor garden a thing of beauty. But before buying, it's worth a minute to carefully check for insects or diseases. Look at the stems for discolorations or rot. Examine the leaves, top and bottom, for rot, spots or dead insects or eggs. If the leaves are curled or puckered, suspect tiny insects. Ditto if there are tiny discolored spots on the leaves. If anything flies when you move a plant, give it a wide berth. When in doubt, pass up the plant, or ask the store to spray it before you take it home.
Even healthy plants may carry eggs of one insect or another (particularly the ubiquitous white fly) so keep an eye on your new plants. As soon as you see any problem, isolate the plant, and treat it. It takes just one bad plant to spread problems to the whole indoor garden. Plants bought at local nurseries are more likely to be carefully observed by a professional horticulturist and treated as problems arise. Plants on sale at large national chain stores or giant supermarkets may have been shipped in bulk from giant commercial growers all over the world, so may not be as carefully watched and treated while on sale.