Thrips of Australia

Resource utilisation by thrips

As a result of extensive field work over the past 50 years, various patterns of resource utilisation by thrips in Australia have become clearer, including flowers, leaves, grasses, fungi and predation on other arthropods.

Thrips and flowers

In much of the southern half of Australia the flower-habitat is dominated by the plague thrips, Thrips imaginis, although north of Brisbane this species is replaced by the closely related Thrips safrus. In the eastern rainforests the most common flower thrips is Thrips setipennis, but in the flowers of Eucalyptus across much of the continent Thrips australis predominates. The genera Cranothrips, Odontothripiella and Thrips have each developed a range of host-specific species, although some species in each of these genera are oligophagous. Similarly, a few of the species of Pseudanaphothrips seem to be host-specific, but the biological significance of some of the named species in this genus remains in doubt due to lack of field work on host specificity and structural variation (Mound & Palmer, 1981). Cycadothrips includes three described species that breed only in the male cones of Macrozamia cycads in eastern, western and central Australia, and the genus Anaphothrips and its relatives involve at least 50 species that live in flowers from a wide range of plant families (Mound & Masumoto, 2009). Similarly, the flowers of native Poaceae support an interesting diversity of specialist thrips taxa (Mound, 2011b). Kirk (1997) summarised the available information on pollination by thrips species and indicated that the importance of these insects as pollinators had probably been overlooked. Since then other studies have emphasised the significance of thrips, including Williams et al. (2001) for a rainforest tree, Wilkiea huegeliana, in eastern Australia, also Mound & Terry (2001), Terry et al. (2004) and Brookes et al. (2015) for Macrozamia cycads, and in Southeast Asia Macaranga trees are pollinated by thrips (Moog et al., 2002). The close association of particular thrips species with particular flowers, coupled with the mobility of these insects, suggests that pollination by thrips is more general than has been proven experimentally. For example, Thrips knoxi is abundant in the flowers of various Lomandra species, Thrips wellsae is abundant in the flowers of certain Epacridaceae in the south-eastern mountains (Mound & Masumoto, 2005), and Oxythrips australopalmae is abundant in the flowers of two palm trees in northern Queensland, Normanbyia and Arch¬ontophoenix (Mound & Tree, 2011). However, precise demonstrations of pollination by Thysanoptera, such as that on Macrozamia cycads, and by Thrips antiaropsidis on Antiaropsis decipiens (Moraceae) in New Guinea, remain unusual (Zerega et al., 2004).

Thrips and leaves

In the moist tropics and eastern coastal regions of Australia, many species of Phlaeothripidae are known to feed on green leaves or apical buds (Mound, 2008). Moreover, there are many such species in the dry areas, particularly on the foliage of Casuarina and Acacia species (Mound, 1970; Crespi et al., 2004). Leaf-feeding Phlaeothripidae are often host-specific, and thus members of this family are rarely considered pests. In contrast, some leaf-feeding species of Thripidae are polyphagous, with many introduced species being pests of non-native cultivated plants. Particularly noteworthy is the absence of any Thysanoptera from the leaves of Eucalyptus species, the solitary exception being a polyphagous panchaetothripine species, Australothrips bicolor. Members of the Thripidae subfamily Panchaetothripinae are all leaf-feeders, but these are primarily tropical in distribution, with few species occurring in southern Australia (Mound, 2009). Members of another thripid subfamily, the Dendrothripinae, are also leaf-feeding, and there are several endemic species and genera (Mound, 1999; Mound & Tree, 2016). The fourth thripid subfamily, the Sericothripinae, is also well represented in Australia, with both native and introduced species feeding primarily on leaves (Mound & Tree, 2009b), with one significant in the biological control of the noxious weed, Ulex europeaus (Ireson et al., 2008). Minimising water loss is an essential survival strategy for small insects in dry areas, and many Phlaeothripidae species achieve this by inducing galls. One native species of Thripidae, Cyrilthrips cecidis, is particularly interesting as it is one of the few members of this family worldwide that has been demonstrated to induce leaf galls (Tree & Mound, 2009).

Thrips and grasses

Plants of the family Poaceae, and to a much lesser extent Cyperaceae, support a particularly wide diversity of Thysanoptera. Among the Aeolothripidae, Desmothrips reedi and Gelothrips cinctus are predators on small arthropods at the base of grasses. But it is amongst the Thripinae that Poaceae have been particularly adopted as hosts (Mound, 2011b). Most Thripinae that breed on Poaceae feed on leaves, with only a few breeding in the florets, such as members of the genus Chirothrips. Several species associated with pasture grasses in the South of the continent are clearly European immigrants, whereas in the North many species are shared with Southeast Asia. However, there are several endemic radiations of Thripinae on native Poaceae, and a number of endemic genera are now recognised.

Thrips and fungi

Fungus-feeding thrips in Australia are all members of the Tubulifera family Phlaeothripidae. The abundance of such thrips is higher in dry sclerophyll leaf litter than in rainforest leaf litter, and Eucalyptus leaf litter appears to support a richer thrips fauna than Acacia leaf litter (Tree & Walter, 2012).

Thrips as predators

A few species of thrips, in all three major families, act as predators, and the range of prey includes thrips larvae, mites and their eggs, scale-insects and whitefly nymphs (Palmer & Mound, 1991). These thrips are generally facultative predators, taking prey as well as feeding on plant tissues (Mound & Teulon, 1995). This behaviour is found in various species of Aeolothripidae and also the pest species, Thrips tabaci and Frankliniella occidentalis. However, several groups of thrips are obligate predators, such as members of the Thripidae genus Scolothrips that feed on mites (Mound, 2011a), including Scolothrips ochoa that feeds on Raoiella mites on Eucalyptus leaves (Mound et al., 2010).