Aphids, mealybugs, mites, and scale insects can feed on the leaves and stems of your indoor lemon tree. Aphids often congregate at leaf joints or nodes; if they aren't taken care of in time, their population will explode. Mealybugs look like small cottony sacs while mites and scale insects appear as tiny dots all over your plant's foliage. You can control these pests by spraying soapy water around your citrus tree's foliage. Check for signs of pest activity before each spraying session. Spinosad is also effective against most indoor pests.