Several insect pests inhabit the soil of tropical houseplants at varying stages of their life cycles. The larvae of some species live in the soil, while adult insects may feed stems and roots beneath the soil, damaging the plant and leaving few signs of their existence. When you identify an insect pest on your houseplants or notice reduced vigor, carefully inspect the plants and treat for insects.
Several plant bug species spend part or all of their lives in tropical houseplant soil, according to the University of Minnesota Extension. Mealybugs can dig under the surface to feed on the stem and roots. Springtails occupy the soil full-time, feeding on decaying roots and fungi. The larvae of fungus gnats live in the soil of potted plants. The pupae of thrips also take up residence in houseplant soil.
Mealybugs produce large quantities of sticky honeydew, and females produce white cottony webbing when laying their eggs. According to the Colorado State University Extension, most mealybug infestations occur out of sight, below the soil, but the bugs will periodically move to visible parts of the plant. Springtails and fungus gnats are nuisance pests that cause little or no damage to the plant. Springtails hop when the soil is disturbed, and fungus gnats congregate on the soil surface. Thrips can be damaging and are usually diagnosed by the damage they cause: large, irregularly shaped silver spots flecked with feces.
If the soil and roots of your tropical plant are badly infested with bugs and you don't want to discard the plant, the Clemson Cooperative Extension recommends taking a root cutting to start a new plant. Only plant tropical plants in potting soil, as soil from your garden can harbor pests and diseases. Pesticides may help to control populations above the surface. Select only pesticides approved to treat the particular pest and follow all instructions on the label.
Some bugs can be controlled through cultural changes. For example, fungus gnats infest damp soil, so watering the plant from the bottom or moving it to a drier location may clear up the problem. Removing infected plants during treatment ensures that the infestation will not spread to nearby tropical plants.
Most houseplant pests are introduced into the home on new plants purchased or received as gifts, notes the Colorado State University Extension. It recommends quarantining any new tropical plants for three weeks and inspecting them closely for insects or damage before putting them near other plants.