Thrips imaginis
(plague thrips)

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.

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).

Introduction on flowering plant material is the most likely route of entry to other countries. The eggs, larvae and adults within flowers can easily be overlooked. The species is frequently abundant so it is likely to be intercepted in flowers exported from Australia. It is also possible that the larvae or pupae could be transported in soil or growing medium accompanying plants in flower.

T. imaginis is common in a wide range of habitats, breeding on many native plants and non-native crops. Large numbers can be caught in traps well away from host plants and the species appears to be carried on the wind between habitats.

During outbreaks, the species is so abundant that the adults can be recorded on virtually any species of plant. However, the species also has a very wide host range. It breeds on a wide range of native and non-native plants, including species in the Asteraceae, Fabaceae, Mimosaceae, Myrtaceae, Proteaceae and Rosaceae (Steele, 1935; Kirk, 1987a). The adults land in response to colours, and white, yellow or blue flowers tend to attract more thrips than red flowers (Lloyd, 1973; Kirk, 1984).

Genetics

Genetic variation within the species has not been studied. However, the mitochondrial genome has been sequenced and shows several unusual features (Shao and Barker, 2003).

Physiology and Phenology

The adult females vary from pale-yellow through to dark-brown, but are typically golden-brown. Darker forms tend to be larger than the paler ones and are associated with cooler weather (Evans, 1932; Steele, 1935; Kirk, 1984). The colour variation is probably a result of the temperature during the immature stages, but genetic variation cannot be ruled out. The adult males are less variable and are typically pale-yellow. There has been no detailed study of the variation within the species.

Reproductive Biology

The egg stage is followed by two larval instars: one propupal instar and one pupal instar and the adult stage. Populations include both females and males. Fertilized eggs are diploid and produce females, whereas unfertilized eggs are haploid and produce males by arrhenotoky.

The eggs are inserted into the plant tissue, mainly in and around the flowers. A few eggs are laid each day throughout most of the life of the adult females (Andrewartha, 1935). The larvae usually live within the flowers and feed on plant tissues, including pollen. In bright sunlight, they retreat within the flowers and are hard to see, but in dim light and at night they will crawl over the flower and are much more exposed (Kirk, 1984). The mature second-instar larvae drop from the plant and pupate in the soil (Andrewartha, 1934). The propupal and pupal stages are spent in the soil. Following emergence, the adults fly to flowers, where they feed on plant tissues, including pollen. The adults also tend to avoid bright sunlight when in flowers and are less obvious than the adults of Thrips tabaci. However, they fly readily in hot weather when shade temperatures exceed approximately 25°C (Evans, 1932). Although the larvae are usually found within flowers, they can also complete their development on tender leaves (Evans, 1932).

The development rate increases with temperature (Andrewartha, 1936). At 20°C, the egg, larval and pupal stages take 4.4 days, 5.7 days and 5.0 days respectively, whereas at 25°C they take 2.6 days, 3.8 days and 3.1 days. Neither eggs nor larvae develop at temperatures below approximately 8°C. Many generations are completed in a year. The adults live longer at lower temperatures. At 23°C, the adult females live for 23 to 66 days. They do not lay eggs at temperatures less than approximately 8°C (Andrewartha, 1935).

The adults and larvae require a high humidity to survive and soil moisture content is critical to the survival of the pupal stages. For example, at 23°C there is high pupal mortality when the soil moisture as a percentage of field capacity is below 25% or above 85% (Andrewartha, 1934).

The life cycle and population dynamics were studied over many years in Adelaide, South Australia, following large outbreaks in the 1930s (Davidson and Andrewartha, 1948a, b). The species is active throughout the year in the Adelaide area, but is claimed to spend the winter as inactive adults in cooler parts of Australia (Andrewartha and Steele, 1934). The population increases greatly in the spring and early summer (September to December) when many host flowers become available and the soil moisture is suitable for the pupal stages. A population crash in the summer (December to January) follows the peak, when hot, dry weather causes the disappearance of the abundant spring flowers and the soil dries out. There is a small peak in the autumn when the rains return before the numbers drop again over winter.

The size of the population peak in spring is variable, but in some years the species becomes so abundant that the adults are present on virtually all plants. Over 4000 thrips have been recorded in a single rose flower (Zeck and Noble, 1932). These massive outbreaks are the reason for the common name of 'plague thrips'. In these 'thrips years', susceptible crops flowering in the spring may be severely affected and even crops on which T. imaginis does not breed, can be infested by many adults. The size of outbreaks has been predicted accurately from four measures of rainfall and temperature in the preceding autumn and spring. Outbreaks occur across large geographical areas and appear to be particularly favoured by an early start to the growing season of the wild host plants in the previous autumn. The detailed interpretation of the 'Andrewartha-Birch equation' that was used to predict the size of the population peak in the Adelaide area (Andrewartha and Birch, 1954), has proved extremely controversial in the field of population dynamics, but the accuracy of the prediction has not been disputed (Kirk, 1997).

Environmental Requirements

The environmental determinants of the distribution of T. imaginis have not been studied. The species is abundant in parts of south-east Australia with mild winters and hot, dry summers, but it has also been found across the arid centre of Australia wherever Acacia or Maireana were in flower (Mound, 1997). It may not be able to survive cold winters.

Associations

T. imaginis is often found in flowers with other species of flower thrips, such as Thrips australis, Thrips tabaci and Frankliniella schultzei. Since the arrival of the western flower thrips, Frankliniella occidentalis in Australia in 1993 (Malipatil et al., 1993), this species has also started to be found in flowers along with T. imaginis (Steiner and Enkegaard, 2002).

T. imaginis can be abundant in the flowers of many native plants in Australia, but its role in the beneficial pollination of native plants is not known (Armstrong, 1979; Kirk, 1984; Williams et al., 2001). During outbreaks, it has been claimed to be so abundant in flowers that it reduces the availability of nectar and pollen for bees and so causes problems for beekeepers (Gosford, 1931; Evans, 1932; Andrewartha and Kilpatrick, 1951).

T. imaginis has been noted for apparently having very few natural enemies (Davidson and Andrewartha, 1948b). However, more recent studies in Australia have shown that the indigenous parasitic wasp, Ceranisus menes attacks the larvae and outbreaks of pathogenic fungi, such as Entomophthora sp., have been recorded in the field (MY Steiner, National Centre for Greenhouse Horticulture, New South Wales, Australia, personal communication, 2003). It is likely that predators in Australia include bugs (Campylomma and Orius), thrips (Desmothrips and Haplothrips), mites and ants (Iridomyrmex sp.) (Kirk, 1984).

Thrips adults and larvae can be detected by tapping flowers onto the palm of the hand or over a dark background, which shows up pale larvae. A more thorough search can be done by submerging the buds and flowers in 70% alcohol and then dissecting them under a stereoscopic microscope.


The adults respond in flight to colours and scents (Kirk, 1984; 1987b), so flying adults could be detected by white, blue or yellow sticky traps.

To the naked eye, T. imaginis looks very similar to the other species of thrips that can be found in Australian flowers, such as Thrips australis, Thrips tabaci, Frankliniella occidentalis and Frankliniella schultzei. Identification keys are available for the separation of adults (Mound and Walker, 1982; Mound and Gillespie, 1997; Mound and Kibby, 1998) and larvae (Kirk, 1987a; Milne et al., 1997). These keys require specimens to be mounted on microscope slides.

The adults can be readily separated from other similar pest species by a few characteristics that can be seen with a good stereoscopic microscope. The presence of seven antennal segments, as opposed to eight, separates T. imaginis from F. occidentalis and F. schultzei. A forewing first vein with a wide gap in the row of setae, as opposed to an evenly spaced row of setae, separates T. imaginis from T. australis, F. occidentalis and F. schultzei. In live or freshly killed specimens, T. imaginis has red ocelli, whereas T. tabaci has grey or brown ocelli.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Phytosanitary Measures

T. imaginis is on the EPPO Alert List because it may present a phytosanitary risk for the EPPO region.

Cultural Control and Sanitary Methods

T. imaginis normally migrates into a crop from outside, so cultural control or sanitary methods will have little or no impact. However, the control of flowering weeds in and around orchards may help by removing potential host plants.

Host-Plant Resistance

Flowering time in relation to peak numbers can affect the amount of damage. Some varieties of apple have been found to be more resistant than others (Evans, 1932).

Biological Control

No biological control measures have been developed against this species.

Chemical Control

T. imaginis is still easily controlled with insecticides, such as dimethoate, tau-fluvalinate or spinosad, and several are approved for T. imaginis or thrips in general on a range of crops in Australia.

The postharvest disinfestation of waxflowers was achieved by washing flowers in water with the addition of a non-ionic surfactant or petroleum oil and shaking off the excess. The addition of deltamethrin, fluvalinate or bifenthrin enhanced the control (Seaton et al., 1993).

Early Warning Systems

No early warning system is currently in operation. However, an equation was developed in the 1940s for the Adelaide area of South Australia that accurately predicted the size of outbreaks from four measures of rainfall and temperature in the preceding autumn and spring (Davidson and Andrewartha, 1948b).

Field Monitoring/Economic Threshold Levels

Thrips can be easily monitored in the field by tapping a flower against the palm of the hand and counting the number of thrips that are dislodged. More than 20 thrips per flower are needed to cause damage to apple blossom in the cooler, moist highland districts of New South Wales, whereas only four or five per flower can reduce setting in dry inland districts (Lloyd, 1973). A spray threshold of six to eight per flower following warm dry weather during the pink to full bloom period has been recommended in New South Wales (Johnson et al., 1980). In strawberries in Western Australia, serious damage occurs when there are more than ten thrips per flower (Houlding, 1995). Insecticide treatment when thrips reached a level of 40% of flowers with ten or more adult thrips, kept strawberry fruit damage to below economic threshold levels (Steiner and Enkegaard, 2002). Waxflowers (Chamelaucium spp.) should be treated once the number of thrips exceeds ten per bunch (Seaton and Woods, 2003).

IPM Programmes

No IPM programmes have been developed.