Plague thrips refers to the species Thrips imaginis, a native Australian insect that can become a significant agricultural pest during outbreaks.

Also refered to as Northern plague thrips, Apple blossom thrips, Apple thrips, Australian thrips.

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Scientific Classification

• Common Name: Plague thrips

• Scientific Name: Thrips imaginis

• Order: Thysanoptera

• Family: Thripidae

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Identification

• Size: Adults are tiny, around 1.2 mm long

• Color: Dark brown to black, slender with fringed wings

• Nymphs: Pale yellow or orange, wingless

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Host Plants

Thrips imaginis infests and feeds on a wide range of crops, including:

• Lettuce

• Beans

• Tomatoes

• Strawberries

• Cucurbits

• Stone and pome fruits

They are most commonly found during the flowering period of these plants.

• Grapevines

• Citrus

• Stone fruit

• Vegetables like tomatoes and beans

• Ornamentals and native flowering plants

They are particularly attracted to flowers, where they feed on pollen and plant sap.

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⚠️ Damage

• Distorted flowers and fruit

• Brown or silvery scarring on petals and leaves

• Reduced pollination success

• Nuisance swarms (they’re attracted to bright colors and can bite people)

• Feeding by plague thrips can lead to:

  • Reduced pollination

  • Damage to flowers and young fruit

  • Potential transmission of tospoviruses, such as Tomato Spotted Wilt Virus (TSWV)

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️ Control & Management

• Monitoring: Use sticky traps and regularly inspect plants, especially during the flowering period.

• Biological control: Introduce natural predators like the predatory mite Neoseiulus cucumeris and the predatory minute pirate bugs Orius tantillus.

• Chemical control: Targeted insecticides during peak populations, rotating modes of action to prevent resistance.

• Cultural practices: Remove weedy hosts and manage flowering stages to avoid overlap with peak thrips populations

   

The Northern Plague Thrips (Thrips imaginis) is a native Australian pest primarily affecting crops in the southern half of the country. It is especially prevalent during mid to late spring and early summer, coinciding with the flowering stages of many plants.

If you need assistance with identification, monitoring techniques, or control strategies for plague thrips, feel free to ask!

 

Summary of Invasiveness

T. imaginis is highly polyphagous, can breed fast and can be carried long distances on the wind. Its habits mean that it can remain hidden with flowers and so can easily go undetected in quarantine. It therefore has the potential to be a major pest and to spread rapidly by means of horticultural trade. It is surprising that outside Australia it has so far only managed to establish in New Caledonia. A possible reason for it not spreading is that it is still easy to control with insecticides. However, the arrival in Australia of Frankliniella occidentalis, which is highly insecticide resistant, means that T. imaginis may be jointly exposed to more insecticide treatments and the selection pressure for resistance may be increased. T. imaginis is on the EPPO Alert List because it may present a phytosanitary risk for the EPPO region.

Notes on Taxonomy and Nomenclature

T. imaginis was first described by Bagnall in 1926 (Bagnall, 1926). The adults are highly variable in colour and size, and this has led to many forms being incorrectly described as separate species. These have now been synonymized (Mound and Houston, 1987). However, the tropical populations may be biologically distinct (Mound and Gillespie, 1997). There has been no detailed study of the variation within the species.In the past in Australia, some reports of crop damage by thrips appear to have confused or misidentified the species that were abundant in flowers, such as T. imaginis, Thrips australis, Thrips tabaci and Frankliniella schultzei. As a result, some early reports of damage may have been attributed to the wrong species. Thrips species are often not distinguished by growers and are collectively described as 'thrips' or 'thrip'. When these species occur together it is not easy to distinguish which is responsible for any damage.

Description

The eggs are oval or kidney-shaped, white to clear and approximately 0.3 mm long. They are inserted into plant tissue. As the egg develops, two red eyes become visible within it.On emergence, the bodies of the larvae are white to clear with red eyes. The bodies quickly turn yellow to orange. Microscopic features of sclerotization and the lengths of setae can be used to distinguish the species and separate the two instars (Kirk, 1987a; Milne et al., 1997). The larvae grow to a length of approximately 1 mm.The propupae are whitish with short, forward-pointing antennae and short wing buds. The pupae are also whitish, but the antennae point backwards over the head and the wing buds are longer. Both are approximately 1 mm long.The adult females vary from pale-yellow through to dark-brown, but are typically golden-brown. The adult males are less variable and are typically pale-yellow. The body size of the adult females is variable, but is typically just over 1 mm long, whereas the males are just under 1 mm long. Like most thrips, the adults have two pairs of strap-like fringed wings. T. imaginis has antennae with seven segments and a wide gap in the row of setae on the first vein of the forewing. In live or freshly killed specimens, the ocelli are red.

Distribution

T. imaginis is common to abundant in the non-arid parts of Australia, but it has also been found across the arid centre of Australia wherever Acacia or Maireana were in flower (Mound, 1997). The distribution has not been mapped in detail. Only a few specimens have been recorded from Fiji and New Zealand (Mound, 1983). Outside Australia, the species has only established in New Caledonia (Mound and Kibby, 1998). It is not clear why the species has not established more widely, considering that other similar species have become cosmopolitan pests (Mound, 1997; Kirk and Terry, 2003). It has not been possible to locate the original specimens or substantiate a record from Papua New Guinea (Palmer, 1992), it is therefore possible that this is an error.The distribution map includes records based on specimens of T. imaginis from the collection in the Natural History Museum (London, UK): dates of collection are noted in the List of countries (NHM, various dates).