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