New plants are often the source of insect infestation.
Houseplant pests can be brought home and spread to other plants when a new plant is purchased and not thoroughly checked for insects. Another common way insects infest houseplants is when plants are taken outdoors and brought back inside without proper treatment. Sometimes pests are present in potting soil and healthy plants become infested during repotting. When new plants are brought home, the leaves, soil, and stems should be examined for insects. The same article recommends examining plants that have been outside before bringing them back indoors and around other plants.