Pest & Disease Management - Thrips affecting ornamentals crops

These pests pose a threat to many crops worldwide.

Thrips damage on petunia - Dove Associates
Thrips damage on petunia - Dove Associates

Two types of thrips are commonly found on plants under protection - the onion or tobacco thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis), also known as WFT. But WFT attracts all the attention due to its significance as a worldwide pest of many crops, its speed of reproduction and its ability to transmit plant viruses.

WFT reached the UK in the 1980s through the international plant trade. The palm or melon thrips (Thrips palmi) is spreading in a similar manner. Notifiable, the palm thrips can attack a wide range of glasshouse crops and carry major virus diseases.

Thrips can also be a pest of outdoor crops. Gladiolus thrips (Thrips simplex) causes flecking on gladioli, but will attack some pot and bedding crops. Rose thrips (Thrips fuscipennis) causes similar damage on roses.

The tendency for thrips to inhabit growing tips and buds makes chemical control difficult. WFT has become resistant to many chemical insecticides and is the subject of extensive research. In the UK, much has centred on learning more about thrips behaviour to improve controls.

How to recognise them

At 1-2mm long, individual species are difficult to identify without specialist help. Adults are narrow bodied with fringed wings that help them to be carried by air currents. WFT adults are light-yellow to dark-brown with banded abdomen, whereas nymphs are golden yellow and wingless. Adults of the palm thrips are yellow.


Thrips lay eggs singly in flowers and leaves, which may cause surface blistering. Eggs hatch into wingless nymphs that feed on plant tissue. After two larval stages, thrips drop to pupate in soil, on capillary matting or on polythene ground cover, although some may stay on the plant. Life cycle length depends on temperature and food source but it can be completed in as little as two weeks.


Thrips adults and larvae feed on flowers and leaves, puncturing and sucking on plant cells, leading to scarring and silvery flecking. Buds and leaves distort or tear as they expand. Chrysanthemum flowers become deformed and assume a streaky appearance. Plants are further disfigured by insect frass. Heavy infestations will stunt plants.

Onion thrips and WFT can carry tomato spotted wilt virus, although WFT is considered the more efficient vector. Symptoms of virus infection include mottling, yellowing, chlorotic spotting or necrosis of leaves and delayed, distorted flowering. WFT can also transmit impatiens necrotic spot virus and was responsible for the first ever UK outbreak of chrysanthemum stem necrosis virus in 2002.

Treatment: cultural control

- Inspect bought-in plant material for signs of thrips damage.

- Monitor crops for populations using blue sticky traps - thrips can establish before damage is noticeable.

- Good nursery hygiene will help prevent carry-over to new crops.

- Regular misting may deter thrips in indoor plantings.

- Control common chickweed, sow-thistles (thrips feed on pollen) and goat willow.

- Where high numbers are present, mass trapping systems with roller traps can bring significant reductions.

Treatment: biological control

Amblyseius andersoni, A. cucumeris and A. swirskii (predatory mites) all feed on the larval stages of thrips and can survive on pollen in the absence of pests. A. andersoni will also feed on spider mites. Macrocheles robustulus (predatory mite) is applied directly to soil or growing media and will feed on thrips eggs and pupae as well as springtails, cabbage root fly eggs and sciarid flies' eggs, larvae and pupae.

For additional control, Hypoaspis miles (predatory mite) will feed on thrips pupae and the predatory bug Orius will attack all thrips life stages as well as a range of other pests.

A number of growers are using a banker plant system for Orius including cyclamen, both cut and potted chrysanthemum and gerbera and a variety of spring crops. This very aggressive predator can kill up to 80 thrips per day even though it only feeds on a few. In areas with low pest numbers it can feed and establish on plant pollen - pepper pollen is a favourite food.

The compact 'Black Pearl' ornamental pepper has become the Orius banker plant of choice. Since pepper plants germinate and grow slowly, plants need to be started well in advance to be used as banker plants. Orius can be introduced as early as late February under natural day-length conditions - extension with supplemental lighting can allow earlier release.

After Orius is released on pepper plants, nymphs can be found three-to-four weeks later. Once established in the greenhouse, Orius can be found mainly on yellow sticky traps each week and virtually no thrips. This predator can sting humans, causing a small allergic reaction.

Steinernema feltiae (parasitic nematode) applied as a foliar spray will attack nymphs of WFT. The nematodes need a film of water present for around three hours to swim through to reach their prey. Avoid use if crop foliage dries quickly.

Mycotal, Naturalis-L and Botaniguard WP are biopesticides and can be used in periods of high humidity and temperatures above 23 degsC on protected crops.

Treatment: chemical control

Active ingredient Abamectin

IRAC code 6

Formulations Various

Action(s) Selective insecticide that controls larval stages with some effect on adults. Incompatible with biological controls.

Active ingredient Beauveria bassiana

Formulation Naturalis-L (Belchim), Botaniguard WP (Certis)

Action(s) Biopesticide that requires 60 per cent relative humidity and 20-30 degsC for good effect on protected crops. Compatible with most biological controls.

Active ingredient Cypermethrin

IRAC code 3

Formulations Various

Action(s) Contact, persistent synthetic pyrethroid. Not compatible with biological controls.

Active ingredient Deltamethrin

IRAC code 3

Formulations Various

Action(s) Contact, persistent synthetic pyrethroid. Not compatible with biological controls.

Active ingredient: Lambda-cyhalothrin

Formulation Hallmark WZT* (Syngenta)

IRAC code 3

Action(s) Fast-acting, persistent, contact and residual insecticide. Not compatible with biological controls.

Active ingredient Metarhizium anisopliae strain F52

Formulation Met52 granular bioinsecticide* (Fargro)

Action(s) Effective on thrips pupae incorporated into growing media. Works best at 15-30 degsC in moist conditions.

Active ingredient Spinosad

IRAC code 5

Formulation Conserve (Dow), Tracer* (Dow)

Action(s) Selective insecticide derived from soil fungi. Incorporating sugar at 10g/100l water or Attracker (Koppert) can attract pests out from flowers and foliage. Short-term effect on some biological controls.

Active ingredient Thiacloprid

IRAC code 4A

Formulations Various

Action(s) Foliar applications not compatible with biological controls.

Active ingredient Verticillium lecanii

Formulation Mycotal (Koppert)

Action(s) Used with adjuvant Addit (Koppert), Mycotal has effect on larvae. Temperatures of 18-28 degsC and relative humidity of 70 per cent required for several days after application.

Fully updated by Dove Associates.

Use plant protection products safely. Always read the label and product information before use.

* EAMU required for use in ornamental plant production outdoors and/or under protection.

Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.

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