Literature review: Impact of Chilean needle grass ... - Weeds Australia

Nematodes are mostly minute animals that can be extremely abundant in soils and on plants, particularly on the roots. Numerous
species are associated with grasses in Australia and several have been recorded from native Stipeae. These are discussed in an
Appendix to this literature review.
Exotic invertebrates are often widespread and common in native grasslands, particularly in urban fringe areas (personal
observations). Collembola is one group with a substantial proportion of exotics, however many higher taxa including various
Coleoptera families and Formicidae are largely or almost entirely dominated by native species (New 2000). Common exotics
present in grasslands include molluscs (slugs and snails), red legged earth mites Halotydeus destructor (Tucker), insects such as
the aforementioned aphids, the African black beetle Heteronychus arator (Scarabeidae: Dynastinae), the weevils Graphognathus
leucoloma (Boheman) and Sitona discoideus (Gyllenhal) (Gregg 1997, Hill et al. 1997),the European honeybee Apis mellifera,
and possibly earthworms (Annelida: Haplotaxida). Morgan (1995b) found that H. destructor grazed seedlings of the endangered
Rudidosis leptorrhynchoides but did not significantly affect their survival, and that grazing by unidentified slugs occurred mostly
on very young seedlings and caused little damage. Exotic Chriothrips spp. (Thysanoptera: Thripidae) are found on grasses in
south-eastern Australia (Mound and Palmer 1972). The grassland daisy Senecio macrocarupus is subject to attack by red-legged
earthmites Halotydeus destructor (Tucker) and aphids (Hills and Boekel 2003). H. destructor and blue oat mites Penthaleus
major (Dugès) attack a wide range of native grassland forbs including species of Craspedia, Podolepis and Senecio (Robinson
2005). The Portuguese Black Millipede Ommatoiulus moreletti (Lucas) (Julidae) has spread into Victorian basalt plains
grasslands, and although there is no evidence of competition with native millipedes, it may influence litter decay rates (Yen
1995). Many of the invasive insect species probably have significant effects on grassland plants and the ecological functioning of
the system, but there appear to be no studies that have directly addressed particular impacts. The agricultural literature, although
restricted to few species, contains some valuable data about invertebrate impact on ecosystem processes, particularly effects on
plant biomass production (Gregg 1997) and soils.
A range of exotic molluscs are considered pests of pastures (Smith and Kershaw 1979, Kershaw 1991) and have been suggested
to be important predators of sensitive native plants such as orchids (Sydes 1994, Daniell 1994). Exotic slugs “appear to be highly
invasive of native grasslands” and were found to be equally abundant in grasslands dominated by exotic and native grasses in the
ACT (Melbourne et al. 1997 p. 366). Daniell (1994) recorded no or few species of native molluscs at four Melbourne grasslands,
and observed that exotic slugs can occur at very high densities in native grasslands. Holland et al. (2007) found only exotic
molluscs, three snail and five slug species, in extensive surveys in the Victorian volcanic plains. A few mollusc species have
been identified as threats to particular native taxa, e.g. unidentified slugs and snails are considered a threat to Diuris
fragrantissima (Webster et al. 2004), the Black-keeled Slug Milax gagates (Draparnaud) defoliates Rutidosis leptorrhynchoides
(Daniell 1994), slug predation has caused the loss of a Pterostylis nutans R.Br. (Orchidaceae) colony (Sydes 1994), while
unidentified slugs and possibly Common Garden Snails Helix aspersa (Müller) caused mortality of Thesium australe planted at
Lake Omeo (Scarlett et al. 2003). Lenz et al. (2003 p. 30) applied molluscide in a low-diversity, weedy native grassland with a
high population of the white snail Cernuella virgata da Costa and found that snail exclusion had no effect on vascular plant
species composition over 9 months. The dietary preferences of introduced herbivorous molluscs in regard to native plants do not
appear to have been investigated (Lenz et al. 2003) except for M. gagates which was found by Holland et al. (2007) to prefer
some native plants over others, with the two native grasses tested having relatively low palatability.
Grass-feeding insects
Various insect groups that avoid plants with toxic metabolites are restricted to Poaceae including some Acrididae (Orthoptera)
and Auchenorrhyncha (Hemiptera). Theoretical considerations suggest that the general absence of toxic principles should result
in a lower ratio of endophagous to ectophagous feeders in grasses than in dicotyledons, and that appears to be the case, with
external feeders being much more common than gall-makers, borers and miners. Ratios of c.30:10 and 30:1 have been recorded
in six grass species (Tscharntke and Greiler 1995). Dominance of ectophages is also apparent in Sporobolus spp. (Witt and
McConnachie 2004). Veldtman and McGeoch (2003) found five grass species with insect galls, caused by five gall-forming
insect species, in a broad survey in South Africa. Poaceae had one of the highest number of galled species compared with other
plant families. However, herbaceous plants generally have fewer gall-forming species than woody plants, possibly in part
because lignified material is more long-lasting, and is thus ‘safer’ for gall formation (Veldtman and McGeoch 2003). Most of the
chewing ectophages on grasses are oligophagous, whereas amongst the main ‘sucking’ groups (Hemiptera) Delphacidae are
monophagous while Cicadellidae has a higher proportion of oligophages (Tscharntke and Greiler 1995).
As with vertebrates, the nutritional quality of grass foliage, particularly silicate and lignin content, are important determinants of
palatability for invertebrates. Juvenile foliage has a reduced content of these structural polymers but has better chemical defenses
against herbivores. C 3 species have higher protein contents than C 4 species, so may be preferred by herbivores. Large abundant
grasses generally have larger faunas (5-12x) than rare, small species. Two variables, shoot length and life-cycle dichotomy
(annual or perennial) explain a high proportion of variance in the species richness of a particular grass. Unpredictably in the
spatial and temporal distribution of annuals, rather than any difference in the number of possible niches they offer appears to
explain this (Tscharntke and Greiler 1995).
Data on the grass food plants of insects in Australia, as elsewhere in the world, is very fragmentory. The literature appears to be
devoid of studies of whole faunas associated with Australian native grass species. According to Wapshere (1993) no arthropods
had been recorded from Nassella trichotoma in Australia, despite its long presence in the country, and nothing was known of the
invertebrate faunas of Austrostipa species (although this is no longer correct for these grasses). At attempt has been made to
draw together some of the scattered information sources on grass invertbrates in an Appendix to this Literature Review. Taxa of
Gondwanan origin such as Austrodanthonia may harbour a larger range of endemic coevolved invertebrates than species such as
T. triandra which have colonised Australia more recently from the north (E.D. Edwards cited by Driscoll 1994).
Lawton and Schroder (1977 p.137) compiled data on the insects associated with species of British plants but excluded Poaceae
“because the insect data appeared to be particularly unreliable”, the entomological literature frequently recording “grass”, rather
than particular grass species as food plants. However they found that the monocots studied had the fewest insect species
associated with them compared with shrub, perennial herb, ‘weeds and annuals’, and aquatic dicot herb groups investigated.
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