Quick answer: Check leaf joints, leaf undersides, stems, and the soil surface with a flashlight, a magnifier, and white paper. Then treat each pest for what it is. Let the soil dry out for fungus gnats, dab mealybugs with rubbing alcohol, rinse the plant and spray the undersides for spider mites, and scrape scale before spraying. Repeat weekly until you see no pests for several weeks.
Identify the pest in 60 seconds
| Pest | What you see | Confirm with | First step |
|---|---|---|---|
| Fungus gnats (Bradysia spp.) | Small black flies rising from the soil when you water; slow growth or wilting in seedlings and cuttings | A yellow sticky card in the soil. Adults stick to it. | Let the soil dry out between waterings |
| Mealybugs (Pseudococcidae) | White cottony clumps in leaf joints and along stems | Crush a clump on a tissue. A reddish smear means it was alive. | Dab each bug with rubbing alcohol on a cotton swab |
| Spider mites (Tetranychus urticae) | Fine yellow or white speckling, dull leaves, fine webbing where leaves meet stems | Tap a leaf over white paper. Moving specks are mites. | Rinse the plant, including the undersides |
| Scale (Coccoidea) | Brown or tan bumps that don’t wipe off, mostly on stems and leaf undersides | Run a fingernail over a bump. A hard shell over a fixed body is scale. | Scrape adults off, then spray soap or horticultural oil |
Fungus gnats: dry the top layer, then treat the larvae
Adult fungus gnats are weak fliers and mostly a nuisance. The larvae live in the top inch or two of moist soil and feed on fine roots and fungi, so seedlings and cuttings suffer most.
- Catch adults. Put a yellow sticky card in the soil. UGA Extension says sticky cards can help reduce the next generation of larvae.
- Dry the surface. UGA Extension says letting the soil go dry between waterings “will help reduce fungus gnat populations and often solves the problem.” Empty saucers and remove standing water.
- Treat larvae if they persist. Use a soil drench labeled for fungus gnat larvae and follow the label. UC IPM notes that Bti does not persist indoors and may need repeated applications about five days apart.
Skip this: spraying insecticide on the leaves. UGA Extension says insecticides are often ineffective against adults, and leaf sprays do not reach larvae in the soil.
Mealybugs: alcohol on a swab, weekly, until none are left
Mealybugs hide in leaf joints and under leaves. Their white wax makes them easy to miss, so check with a flashlight.
- Test first. Put rubbing alcohol on one leaf and wait a day. SDSU Extension advises testing alcohol mixtures on a few leaves first, since some plants mark.
- Dab each bug. Dip a cotton swab in rubbing alcohol and dab each visible bug and egg mass. Iowa State Extension describes this as satisfactory for lightly infested plants.
- Repeat weekly. Eggs and newly hatched crawlers survive the first round.
- Keep checking for about four weeks after the last bug you see.
Soap or oil products also work, but check that the label lists mealybugs and the plant you are treating. Iowa State Extension recommends throwing away a plant that is heavily infested before the insects spread to others.
Spider mites: rinse the plant, then go after the undersides
Spider mites can reproduce within days in a warm spot. SDSU Extension notes they are most common in winter, when indoor air is dry. Damage starts as fine speckling, so check the undersides before you see webbing.
- Confirm. Tap a leaf over white paper with a magnifier. Moving specks are mites.
- Rinse. Rinse the plant in a sink or shower, including the undersides.
- Spray. Use insecticidal soap or horticultural oil labeled for houseplants, coating the undersides. SDSU Extension says these products work well and to follow the label.
- Repeat weekly until no moving mites appear. Cut off leaves that are heavily webbed or bronzed.
Scale: scrape the adults, spray the crawlers
Adult scale has a hard waxy shell that protects it from spray. SDSU Extension notes that the crawler stage is the easiest to kill.
- Confirm. A hard shell over a body that doesn’t move is scale. Crawlers, the tiny moving stage, show up as specks on white paper.
- Scrape. Remove adults with a soft toothbrush or an alcohol-dipped swab.
- Spray. Apply soap or horticultural oil to all surfaces, including the undersides. Repeat until no crawlers appear.
- Check new plants before they join your others.
A weekly check that takes two minutes
- Tilt each leaf and check the undersides and leaf joints with a flashlight.
- Look at the soil surface for flies and check any sticky card.
- Tap one leaf over white paper to spot mites early.
- Write down anything you find, so you can tell whether the treatment is working.
Frequently asked questions
Do I need an insecticide for fungus gnats?
Often no. Drying the soil and using sticky cards is the first step. UGA Extension says insecticides are often ineffective against adults. A soil drench labeled for larvae is the option if the problem continues.
What is the fastest way to kill mealybugs?
There is no single fast fix. Rubbing alcohol on a swab kills the bugs you touch, but the eggs survive. Repeat weekly for about four weeks.
Should I throw out an infested plant?
Iowa State Extension recommends discarding a heavily infested plant before the pests spread. A lightly infested plant can usually be saved with the steps above.
Sources
- Morgan, J. and Suiter, D. (2022). Fungus Gnats: A Tiny Nuisance of Houseplants. UGA Extension Circular 1250.
- Bethke, J.A. and Dreistadt, S.H. (last updated 2013). Pest Notes: Fungus Gnats. UC ANR Publication 7448, UC Statewide IPM Program.
- Graper, D. (updated 2018). Check Your Houseplants for Pests. SDSU Extension.
- Iles, L. (reviewed 2023). Mealybugs. Iowa State University Extension and Outreach.