What are the white fuzzy spots on my plants?

This is the most common houseplant pest call I get, and nearly every time the answer is mealybugs. They are soft-bodied insects that coat themselves in a white waxy secretion, so what you see is not the animal but its covering, which is why they read as fuzz or lint rather than as bugs.

Mealybug or powdery mildew? The distinction that decides the treatment

Get this one right first, because the two look similar at a glance and nothing that treats one does anything for the other.

• Cottony, three-dimensional masses tucked into leaf axils, stem joints, and leaf undersides → mealybugs. An insect. They sit where leaf meets stem and in tight crevices, they can be picked up on a fingertip, and they move slowly if you watch one. The clusters are lumpy, not flat.
• Flat white powder spread across the upper leaf surface → powdery mildew. A fungus. It coats the leaf blade rather than hiding in joints, it looks dusted on, and it smears when you rub it. It thrives where humid air sits still: crowded shelves, plants against walls, poor airflow. Insecticidal soap and alcohol do nothing to it; the answer is airflow, spacing, light, and a fungicide where warranted.

The two secondary signs that confirm mealybugs

• Honeydew. Mealybugs excrete a sticky sugary residue that lands on the leaves below them and on the floor, table, or shelf under the plant. If the surface under your plant is tacky, you have a sap-feeding insect, and mildew is ruled out.
• Sooty mold. A black, soot-like fungus that grows on the honeydew rather than on the plant itself. It does not attack the plant, but it blocks light from the leaves and it is a reliable sign that a sap-feeder has been at work for a while. It wipes off once the insects are gone.

The other causes, and how to tell them apart

• Cottony, sticky fuzz that moves when poked → mealybugs. They cluster where leaf meets stem and under leaves, excreting sticky honeydew that invites black sooty mold. They commonly arrive on new nursery plants and thrive indoors.
• Hard or waxy bumps that don't wipe off → scale insects. Armored little dots on stems and leaf undersides. On sagos and other cycads, a spreading white crust is cycad scale, the most destructive version, capable of killing a mature sago.
• Flat white dust that smears → powdery mildew. A fungus, not a bug. Thrives where humid air sits still: crowded beds, shaded corners, plants against walls.

Why Florida makes these worse

Year-round warmth means year-round pest generations; there is no winter kill here. Humidity feeds mildew ten months of the year, and the constant flow of nursery plants into Gulf Coast homes keeps reintroducing mealybugs and scale. The practical upshots: quarantine every new plant for a couple of weeks before it joins the collection, and treat infestations in repeated rounds, because a single spray never catches every generation.

How to treat mealybugs, step by step

1. Isolate the plant today, in a different room rather than a different shelf. Mealybugs crawl between touching plants and ride on hands and watering cans.
2. Spot-treat with 70% isopropyl alcohol on a cotton swab. Touch each cottony cluster directly; the alcohol dissolves the waxy coating and kills the insect underneath on contact. This is genuinely the most effective thing you can do in the first hour, and it is oddly satisfying.
3. Then treat the whole plant with insecticidal soap or horticultural oil, covering leaf undersides, stem joints, and the soil surface. Contact products only work where they land.
4. Repeat every five to seven days, at least three or four times. This is the step that decides the outcome. Eggs and juveniles hide in crevices, in the top of the soil, and even on the roots, and a single treatment always leaves a generation behind. Houses that treat once are still finding mealybugs a month later.
5. Wipe away the honeydew and sooty mold with a damp cloth so you can see whether new deposits appear.
6. Check the neighbours, and keep checking for a fortnight after you think you have won.

Test any soap or oil on one leaf first and wait a day: some ferns, succulents, and thin-leaved tropicals mark. Never apply oils to a wilted plant or one in direct sun.

When to call a professional

Light infestations respond well to patient home treatment. Call when it has spread through several plants, when a valuable specimen is heavily infested, or when you have already treated and it came back. That last one usually means the schedule slipped rather than the product failed. A pest management visit means identification under magnification, one coordinated treatment cycle across the whole collection instead of plant by plant, and pet-appropriate product choices where there are animals in the house. A photo through the booking form is often enough for a first identification.

Related: spider mites are the other pest that thrives in AC-dried air, and fungus gnats are what a chronically wet pot produces.