Identifying and Treating Mealybugs on Bonsai

White Cottony Patches on Your Bonsai? Likely Mealybugs

A bonsai with fern-like foliage and reddish leaves showing white, cotton-like masses on leaves and bark is almost certainly infested with mealybugs—common sap-sucking insects that thrive in warm, dry indoor conditions.

Immediate Treatment

1. Physical removal: Dip a cotton swab in 70% isopropyl alcohol and dab each visible mealybug. The alcohol dissolves their waxy coating, killing them on contact.
2. Insecticidal soap spray: Apply a ready-to-use insecticidal soap (e.g., Safer’s) thoroughly to all plant surfaces, including leaf undersides and bark crevices. Repeat in 7–10 days to catch newly hatched crawlers.
3. Isolate the plant: Prevent spread to other plants.

Follow-Up

Monitor weekly for recurrence. Mealybugs often hide in root zones; if infestation persists, consider a systemic insecticide (e.g., imidacloprid) as a last resort, following label instructions carefully. Improve air circulation and avoid over-fertilizing with high nitrogen, which encourages soft growth susceptible to pests.

Identification Note

Accurate diagnosis requires knowing the tree species. If unsure, consult a local bonsai club or extension service. Many “bonsai” sold in stores are unidentified species, making specific care challenging.