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

Predators and pathogens
Evans (1991 p. 60) considered that the natural enemy complexes (invertebrates and fungal diseases) of all the world’s worst
grassy weeds to be “largely unknown”. This paucity of information extended to grasses in general, in part as the result of failures
to consistently identify grasses found to be under attack (Evans 1991 p. 53).
Wapshere (1990) found natural enemy species-specificity to be rare amongst grasses, compared with genus-specificity. He
considered it likely that Australian grass genera with many species would have large groups of specific or near-specific predators
and parasites. The corollary of this is that Nassella, with many South American species, should have a large herbivore fauna and
a a wide variety of diseases.
Wapshere (1990) determined from published records the main invertebrates and fungi that attack grasses in Europe: Noctuidae
(cutworms, armyworms, etc., Lepidoptera), microlepidoptera (various families), Cecidomyiidae (gall midges, Diptera),
Brachycera (flies, Diptera), Aphididae (aphids, Hemiptera), Ustilagines (smuts) and Uredinales (rusts). As an indication of host
specificity, he also determined the number of species in each group recorded from a single grass species, a single grass genus, 2-
3 genera and 4 or more genera. Cecidomyiidae and Ustilagines showed the highest levels of species- and genus-specificity
amongst these groups. High levels of specificity was also apparent with leaf-miners (insects) and particularly with gall makers
(arthropods, nematodes and fungi).
Predators
Very little appears to be known about the animals that attack N. neesiana, a similar situation to that for N. trichotoma, for which
Wapshere (1990 p. 71) noted an absence of “any readily available knowledge concerning the arthropods … in its home range”
and (1993 p. 344) no arthropods recorded from it in Australia. Herbaceous monocots have often been viewed as having relatively
impoverished invertebrate faunas compared to other groups of plants. Their simple architecture and decreased structural
complexity (hence lower niche diversity) is often cited as the cause (e.g. Lawton and Schroder 1977). However, as Waterhouse
(1998 p. vi) noted in relation to the paucity of grasses as targets for classical biological control, “it would be surprising if coevolution
and co-adaptation have never led to effective and highly specific natural enemies of at least some individual species in
the Poaceae, as it has in members of other plant families”.
Molluscs
Published information about slug and snail utilisation of grasses indicates they may often be avoided in preference for other
plants, but the data is equivocal and it is clear that some species are recognised pests of cereals and grass fodder, and that grasses
in general are not unpalatable (Barker 2008). Holland et al. (2007) found that Milax gagates Draparnaud (Milacidae) consumed
two native grasses including T. triandra in laboratory tests and that the palatibility of the grasses was within the palatibility range
of native dicot species tested. Newly emerged seedlings appear to be at most risk of damage by molluscs. No information
appears to be available on mollusc attack on N. neesiana.
Insects
Evans (1991 p. 52) referred to a seeming “general absence of host specific insects associated with grassy weeds”. He found from
a literature survey that insects specific to grass species were unknown, and that grass-feeders were generally polyphagous,
attacking a range of grasses, often including cereals. The relatively uniform structure of Poaceae supposedly “promotes
polyphagy” (Evans 1991 p. 53) and reduces the evolutionary pressures for monophagy (Briese and Evans 1998).
Evans’ (1991) generalisation was largely, but not entirely correct, as presaged by Waterhouse (1998) and demonstrated by the
studies of Wapshere (1990), Witt and McConnachie (2004) and others. Wapshere (1990) found host-specificity at species or
genus level for grasses in Europe to be particularly common in Elachistidae (Lepidoptera), Chloropidae and Cecidomyiidae
(Diptera), and Tetramesa (Hymenoptera: Eurytomidae). He noted that all Tetramesa spp. in the USA are relatively
monophagous, being recorded only from single grass genera. In Australia the genus is represented by three species that are
phytophagous in grass seeds or internodes, according to Naumann (1991), although Boucek (1988) stated that no Australian host
records were known. Tetramesa spp. “develop as phytophages feeding on the inner tissues of the internodes or of the seeds and
places attacked, specific for each species, swell some to some degree, sometimes considerably, so that a characteristic gall is
formed” (Boucek 1988 p. 95). De Santis and Loiácono de Silva (1981 1983) found that the stem-boring Tetramesa adrianae De
Santis was the main natural enemy of the stipoid Amelichloa brachychaeta in the provinces of Buenos Aires, La Pampa and
Entre Rios, Argentina, and discussed programs to breed and disperse it more widely as part of an integrated control program.
This species causes gall-like stem deformations that affect seed production (De Santis and Loiácono de Silva 1983). Possibly this
wasp also affects N. neesiana: Gardener et al. (1996b) reported that botanical specimens in the Instituto Darwinion Herbarium
had galls of undetermined origin on the flowering stems which appeared to prevent flowering. The Palaearctic species T.
cylindrica (Schlechtendal) and T. punctata Zerova attack the flowers of Stipa capillata L. and S. lessingiana Trin. and Rupr. (De
Santis and Loiácono de Silva 1983). A. brachychaeta is also attacked by a cecidomyiid, Contarinia sp. (De Santis and Loiácono
de Silva 1983).
Some Eurytoma spp. “can complete their development feeding solely on the plant tissues in stems of grasses, but because their
eggs are laid only in places where a genuine phytophagous eurytomid of the genus Tetramesa is developing (and causing growth
of plant tissues), they normally devour the larva of Tetramesa before reaching maturity” (Boucek 1988 p. 107). Australian
Eurytoma are poorly known and no grass hosts were mentioned by Boucek (1988). The Australian eurytomid genus Giraultoma
Boucek is also associated with grasses, while the biology of some other Australian genera is unknown (Boucek 1988, Systematic
Entomology Laboratory USDA 2001).
Because Australia has a wide diversity of Austrostipa species, closely related to Achnatherum, Australian species from these
insect taxa may be the most likely candidates amongst the host-specific or narrowly-oligophagous species to ‘host shift’ on to N.
neesiana in Australia. Exchange of insect species (“acquisition of a herbivore guild on an evolutionary timescale”) between
closely related plants is hypothetically more likely because of closer biochemical and structural similarities between related
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