Many of the worst pests found on houseplants are in the soil itself or had already infected the plant when the plant was purchased from the nursery. Many of these pests can be picked off the plant or blasted with soapy water. Some insects are harder to kill, and insecticides are required to eliminate them. However, homeowners can't simply use agricultural insecticides, since many of these insecticides are harmful to humans.
Chewing insects are usually killed using stomach poisons, which are poisons that kill the insect when the poison is swallowed. There are stomach and contact poisons also available. Sucking insects penetrate into the plant and suck the internal parts of the plant. Because of this, stomach poisons do not work and contact or systemic poison needs to be used. Systemic poison is poison that circulates through the plant and is sucked up by the sucking insect, getting the poison inside the insect's stomach.
Ants can be difficult to kill since they live deep within the soil. They usually need to be killed with ant traps that contain bait that lures the ants into the trap. Aphids are common on plants. They are usually killed by picking them off by hand or by washing them with soapy water. Caterpillars and cutworms can also be picked off of the houseplant.
Some bugs, such as fungus gnats and mealy bugs, are difficult to kill because of the way they infest the plant. Fungus gnats often need to be killed by soaking the soil, pot and plant with the insecticide. Mealy bugs usually need a forceful stream of insecticide in the infected area in order to kill.
Mites sometimes infect plants and need to be killed off with miticide. Some mites can be killed off by insecticide as well. They can also be stopped by destroying the webs that they create around the infected plants. Slugs usually can be killed by picking them off of the plants. Severe infestations of slugs might require slug bait.
Some pests rarely need to be controlled on plants, such as springtails and psocids. These two species are so small that they are often not noticed. These pests feed on decaying matter and only sometimes harm the houseplants that they infect. If necessary, they are easily killed with aerosol insecticides.
Insecticides are often not approved for use within the home since the insecticides can be toxic to humans in large quantities. Also, many homeowners have pets that might chew on the plants that contain the insecticides, causing them to become sick. There are a small number of insecticides that are approved for use in the home and many insects can also be effectively killed using alcohol.