Following from the previous post about dealing with houseplant pests.
Some common indoor pests include:
Aphids: little green, yellow, red, orange, or black critters that suck the sap out of the plant and cause wilting and internal drying of the plant. Female can reproduce parthenogenically (they clone themselves without any need of the males), so their numbers can increase ridiculously quickly. They like to cluster around the tenderer parts (new shoots, leaves, buds) and I find it immensely satisfying to run my finger and thumb up the stem to crush them (before spraying). Outdoors, ladybirds and hoverflies are the classic aphid predators, but these are not practical to use in a home; greenhouse keepers will probably find them far more useful. Spray with soap or garlic.
Whitefly: these look like tiny little white moths, smaller than a pinhead. In medium to bad infestations, you might see a cloud of them flying off if you disturb the plant. Little white flecks stuck to the undersides of the leaves are another symptom (I’m not sure what these actually are; I think they’re either dead whiteflies or possibly the remnants of moulting). Spray with soap or garlic every two or three days for at least two weeks; they lay eggs in the soil that will survive, so you have to get the adults as they complete their life cycle. You can also get sticky traps for whitefly.
Red spider mite: tiny red arachnids (mites are arachnids); they do spin webs that one could be forgiven for mistaking for those of real spiders. They are also capable of parthenogenesis. Symptoms include stippled yellowing leaves, a general sickly look to the plant, and the webs. Remove the webs, burn them, and spray the plant with soap. They generally take hold when humidity is very low, so increasing the humidity can help control future infestations.
Mealybug: these look like tiny greyish-whitish-pinkish pillbugs, often covered with white fluff and/or having two long tails (a particular species). If there are just a few, try swabbing them with rubbing alcohol (or crushing them…); otherwise spray with soap.
Scale insect: these appear as small scales or shells of various shapes and colours stuck to the stems and leaves. These are in fact mature females that live inside the scales, sucking the juices from the plant. If there are just a few, scrape them off gently with your fingernails and swab the area with rubbing alcohol. Less mature females have softer shells that can be penetrated by soap. Larger infestations might warrant pruning the affected parts off and burning them. You can also try spraying with rubbing alcohol first (test first to make sure it doesn’t damage the plant), then with soap, or a mixture of a spoonful of vegetable oil to a cup of water. For long-term control, soap solution will kill males and immature females, which are mobile and soft, but you’ll probably never see or notice them.
Fungus gnats: these aren’t too pestiferous really; they’re more of an unsightly nuisance than a real threat to healthy houseplants. They’re little black flying or walking insects that might be mistaken for fruit flies at first glance. The larvae feed on soil fungi and plant roots, but seldom to a harmful extent unless the infestation is really bad. Sometimes they can be a symptom of overwatering. They’re probably most dangerous to seedlings, because they can spread fungi on their feet – such as the sort that causes damping off. I’ve never found it necessary to control these by the usual methods; my carnivorous Pinguicula do a nice job of it. They’re also attracted to water, in which they have a tendency to drown. Oddly enough, once I left some leftover sesame seed oil in a saucer on the kitchen counter and the next day there were a bunch of fungus gnats drowned in it. Whether they were attracted to the oil itself, or the shiny surface, or something else, I couldn’t say. I really like sesame oil, so I don’t care to waste any by conducting tests.