Fungus gnats are 1/8-inch, fruit-flylike pests that hover around houseplants and scurry across the soil when you water plants. Adult fungus gnats don't harm indoor plants, but their larvae feed on plant roots and crowns.
Treat fungus gnats by letting the top 3 to 4 inches of soil dry out before watering, which will kill off larvae and stop the cycle. If this isn't possible, apply a soil drench product containing Bacillus thuringiensis var. israelensis. Avoid future infestations by not overwatering.
Mealybugs are 1/8-inch to 1/4-inch white, cottony pests that tend to congregate on the underside of foliage and where leaves meet stems. This pest sucks plant sap, excreting a sticky honeydew substance as it feeds. Mealybugs weaken plants and cause leaf loss.
Remove mealybugs by washing them off with a strong spray of water, wiping them with a swab dipped in isopropyl alcohol or spraying the pest with insecticidal soap or pyrethroid spray.
Scale are brown or gray 1/16-inch to 1/8-inch, bumplike insects that appear on plant stems and leaves. These slow-moving pests suck plant sap and excrete a honeydew substance. Their feeding leads to reduced vigor and yellowing and dropping of leaves.
Control scale by washing or physically removing them. Insecticidal soap and various insecticides are also effective.
Spider mites are tiny yellow or green insects that thrive in the warm, dry conditions often found indoors. These destructive pests suck plant sap, causing mottled, faded foliage and leaf loss. When infestations are severe, plants become covered in a spiderlike webbing.
Spider mites thrive in dry conditions, so your best defense against this pest is to increase humidity around the plant and wash the plant with a strong spray of water on biweekly basis until the infestation is under control. Horticultural oil spray is another effective treatment option.
Thrips are 1/16-inch, slender insects that scrape houseplant foliage with their mouthparts and suck the resulting fluid that comes out. Leaves become covered in silvery patches and dotted with tiny black spots of excrement.
Control thrips by washing plants with a strong spray of water and applying insecticidal soap, pyrethrins or neem oil.