Thrips
Also known as Western flower thrips (Frankliniella occidentalis), Thunderflies, Thysanoptera
In short
Thrips are slender insects about 1.3 mm long or less that scrape leaf and petal surfaces and suck up the contents. Damaged leaves look silvery or dull and are dotted with dark, varnish-like droppings; the western flower thrips also spreads plant viruses.
How it looks
Adults are less than about 1.3 mm (1/20 in) long, slender, with two pairs of narrow wings edged with long fringes of hair. Colour ranges from translucent white or yellowish to dark brown or black. Larvae are wingless, slender and pale yellow or whitish. They occur on both leaf surfaces, usually more on the undersides, and in flowers. Tapping a shoot or flower over white paper shows them as tiny moving slivers.
Signs
Stippling and silvery to brown patches on leaves, with small dark specks of excrement; leaves become dull, distorted, curled or galled. Flowers may show pale or dark streaks (colour break) or open distorted. Damaged tissue does not regain its colour even after the thrips are gone.
Life cycle
Egg, two feeding larval stages, then two non-feeding stages (prepupa and pupa), usually in the soil, leaf litter or plant crevices, before the adult emerges. In warm conditions egg to adult can take as little as two weeks, and outdoors there can be up to about eight generations a year.
What favours it
Warm weather speeds development. Excess nitrogen fertiliser favours them, and because the pupae drop into potting mix, sprays aimed at foliage miss part of the population.
Prevention
- Inspect and quarantine new plants, including their flowers.
- Monitor with yellow or blue sticky traps, or by tapping foliage over white paper.
- Water adequately and avoid excess nitrogen fertiliser.
- Increase ventilation in greenhouses and conservatories.
Treatment
Start with the first step and move down only if the problem persists.
- Prune off infested shoot tips and spent or damaged flowers; avoid shearing, which encourages flushes of tender growth.
- Use biological controls: predatory mites, the predatory bug Orius laevigatus, the rove beetle Atheta coriaria, and Steinernema nematodes against stages in the potting mix; minute pirate bugs and lacewings also eat thrips.
- Contact sprays of insecticidal soap, horticultural oil or pyrethrins, repeated for several applications; follow the product label.
- Spinosad is generally more effective but is toxic to bees, so do not use it on plants in flower; follow the product label.
- Systemic neonicotinoid-class products are a last resort and harm pollinators; follow the product label.
Pros and cons
Cons
- Western flower thrips spreads tospoviruses to many ornamentals and vegetables.
- Damage is permanent on affected leaves and flowers.
Look-alikes
- Spider mite damage
- Pale stippling with fine webbing and no dark droppings.
- False spider mite damage
- Scabby brown patches along the midrib and no insects with fringed wings.
Affected plants
A wide range of houseplants, orchids, flowering ornamentals and vegetables.
Related
Sources
- Thrips — UC IPM Pest Notes
- Indoor plants: sap-feeding insects — RHS
- Orchid Insect and Mite Pests in South Florida (ENY2114) — UF/IFAS
Written in our own words from these sources. Check a product label before using any treatment.