Thrips

What you are looking at

The damage arrives before the insect does. Patches of the leaf go silvery or bleached, sometimes with a faint metallic sheen, as though the colour has been scraped off. On a monstera or an alocasia it shows first on the newest leaf, near the point where it was rolled up.

Look closely at the silvered area for minute black dots, like flecks of dried varnish. That is thrips excrement, and silvering plus black dots is the combination that settles the diagnosis. Nothing else produces both.

The animal itself is about a millimetre long, slender and cigar-shaped, dark brown or straw-yellow depending on species and stage. Prod it and it runs, or drops. Its wings are two narrow straps fringed with long hairs, which is what the order name means, and they are only visible under magnification.

The word is thrips whether there is one or a thousand. There is no such thing as a thrip.

Why the surface goes silver

They do not pierce a vein and drink, as an aphid does. A thrips punctures individual surface cells with a single mandible, sucks out the contents and moves on, leaving an empty cell behind. Air fills the space, and air under a transparent cuticle reflects light, which is why the damage looks metallic rather than brown.

Once the cells are empty they stay empty. A silvered leaf never recovers, and the plant grows out of the problem rather than repairing it.

Flowers show it as pale streaks and flecks in the petals, and buds that will not open cleanly. New leaves that were being fed on while still furled emerge puckered, scarred and misshapen, which is the form that alarms houseplant owners most.

Half the life cycle happens in the compost

This is the part that decides what works.

A female inserts her eggs into plant tissue with a saw-like ovipositor, so the eggs are inside the leaf and nothing applied to the surface touches them. Two larval stages feed on the plant. Then, in most of the species that matter, the larvae drop off and pupate in the top layer of the compost or the soil, and the adults come back up.

So at any moment part of the population is inside the leaf, part is on it, and part is underground. Treating only the foliage leaves two thirds of them alone, which is why thrips have such a reputation for coming back.

Work the leaves and the compost at the same time. A predatory mite on the foliage, and a soil predator or a nematode drench underneath it.

What the damage amounts to

Cosmetically it is severe and permanent on the leaf it affects. Practically, an established plant loses photosynthetic area and keeps going, and it grows out of a light infestation once the population is dealt with.

The serious risk is viral. Western flower thrips is the principal vector of tomato spotted wilt virus and impatiens necrotic spot virus, both of which are untreatable and both of which mean destroying the plant. That is the reason to take an infestation on a houseplant collection seriously rather than tolerating it.

What to do about them

Find out whether you have them. Hang a blue sticky trap. Thrips respond to blue far more strongly than to yellow, and the yellow traps most people already own are the reason so many infestations go unnoticed for months.

Raise the humidity, take off the spent flowers, and shower the plant weekly, paying attention to the crevice where a new leaf is unrolling.

Then treat the leaves and the compost together. Amblyseius mites in sachets hung on the plant deal with the larvae on the foliage, and Stratiolaelaps mites or Steinernema feltiae nematodes in the compost deal with the pupae. Under glass in summer, Orius takes the adults in the flowers.

Remove the worst leaves. They will not green up again, and they are carrying eggs inside the tissue.

Why no spray is listed here

Western flower thrips is among the most insecticide-resistant pests in world horticulture, carrying resistance to several chemical groups simultaneously in commercial populations. A treatment that kills most of a colony selects for the survivors.

The life cycle makes it worse. Eggs inside the leaf and pupae in the compost are both out of reach of anything sprayed on the foliage, so a spray addresses the one stage it can see and the population rebuilds from the two it cannot.

Contact products also kill the predatory mites, which are the control that does work.

What it gets mistaken for

Red spider mite produces a fine even pale stippling rather than larger silvered patches, leaves no black excrement dots, and in a heavy infestation spins fine webbing at the shoot tips. Thrips never spin anything.

Leaf scorch and sun damage bleach a leaf in a broad area on the side that faced the light, with no black specks and no distortion of new growth.

Hard water and foliar feed residues leave white deposits sitting on the surface that wipe off with a damp cloth. Thrips damage is in the leaf.

Fungus gnats are dark and about the same size, and they fly around the pot rather than feeding on the plant. No leaf damage means they are not the cause of anything.

Downy mildew on some plants produces greyish patches, but on the underside and with angular yellow blotches above bounded by the veins.