Small white bugs on your plant? Name the pest

Small white bugs: the usual suspects

Three pests account for most “little white bugs”. They look alike at a glance but behave very differently, and the treatment differs too.

White cast skins on new shoots belong to aphids, which shed them as they grow (UC IPM), and the yellow-orange crawlers of scale insects are tiny enough to need a lens.

How to look: a lens and a sheet of white paper

Pests hide on the undersides of leaves, in leaf joints and on new growth, so turn leaves over rather than looking down on the plant. Shaking a shoot over white paper works for thrips too, and you may spot sticky honeydew before the insects that make it (Penn State).

The treatment ladder

UC IPM’s advice for houseplants climbs from the gentlest step to stronger ones, and only goes up a rung when the one below has not worked.

1. By hand and with water. Isolate the plant, wash pests off, wipe or pick them off, and cut away infested parts. For mealybugs, a cotton swab dipped in rubbing alcohol (70% or weaker) works on a few at a time; test it on a small part of the plant a day or two first. For fungus gnats, let the compost surface dry and use yellow sticky cards.
2. Insecticidal soap or horticultural oil. These work only on pests they touch, so the spray has to reach the undersides of the leaves, and repeats are often needed. Test on a few leaves first, and UC IPM warns against spraying water-stressed plants or above 32°C (90°F).
3. Other options. Bti drenches for fungus gnat larvae, and for some pests a systemic product as a last resort, with the caveats in each pest report. Sometimes the right last step is to throw the plant out before the pests spread.

Why a re-check every 7 days

Sprays and wiping miss eggs and hidden young, which hatch over the following days. Spider mites go from egg to adult in about two weeks at 24°C (75°F) indoors, and aphids can be producing young of their own 7 to 8 days after birth (UC IPM). So treatment is a series, not a single event. UC IPM advises watching new plants weekly, and its mealybug guidance repeats treatment every week until they are gone; Clemson says spider mites often need spraying once a week for several weeks. A few pests need a faster rhythm: UC IPM repeats Bti for fungus gnats about every five days, and checks for aphids at least twice a week while plants are growing fast.

The schedule in the tool lists a check every 7 days from the date you choose. Download it as a standard .ics calendar file, which calendar apps such as Apple Calendar, Google Calendar and Outlook can import, or print the table and keep it near the plants.

New plants and holiday gifts

November and December bring new plants into the house: poinsettias, holiday cacti, amaryllis, and plants that summered outdoors coming back in. UC IPM advises inspecting plants before you buy them, roots included, and re-inspecting plants returning from outdoors. Clemson suggests keeping new plants apart for six weeks and examining them about once a week; Penn State suggests about three weeks.

Common mistakes

• Spraying before you are sure. Name the pest first; for spider mites UC IPM says to always check mite numbers before treating.
• One treatment and done. Eggs and hidden stages survive, so plan for several rounds.
• Spraying only the tops of the leaves. Soap and oil must touch the pests underneath.
• Leaving the plant with the others. Isolate it and wash your hands after handling it (UC IPM).

General information from UC IPM, Clemson, the RHS and Penn State Extension, not professional advice. Pesticide rules differ between countries and states.