Phylogenetic placement and the timing of diversification in Australia's endemic Vachellia (Caesalpinioideae, Mimosoid Clade, Fabaceae) species

The genus Vachellia Wight & Arn. has a pantropical distribution, with species being distributed through Africa, the Americas, Asia and Australia. The relationships among the lineages from Africa and America are well understood, but the phylogenetic placement and evolutionary origins of the Australian species of Vachellia are not known. We, therefore, sequenced four plastid genes from representatives of each of the nine Australian species of Vachellia, and used Bayesian inference to assess the phylogenetic placement of these lineages, and a relaxed molecular clock to assess the timing of diversification. The Australian species of Vachellia form a well-supported monophyletic clade, with molecular-dating analysis suggesting a single dispersal into Australia 6.5 million years ago (95% range 13.9-2.7 million years ago). Diversification of the Australian clade commenced more recently, c. 3.1 million years ago (95% range 9.2-1.2 million years ago), perhaps driven by the increased aridification of Australia at this time. The closest relatives to the Australian Vachellia were not from the Malesian bioregion, suggesting either a long-distance dispersal from Africa, or two separate migrations through Asia. These results not only improve our understanding of the biogeography of Vachellia species, but also have significant implications for the biological control of invasive Vachellia species in Australia. © 2020 CSIRO

Proposed plant host test list for assessing the risk of biological control agents for Clidemia hirta (L.) D.Don.

Background The following proposed plant host list is for the testing of potential biological control agents of Clidemia hirta (L.) D. Don (Melastomataceae). Post-release evidence from other countries suggests a promising outlook for biological control of C. hirta in Australia. Seven agents have already been released in Hawaii for this purpose, with six of these becoming established (Nakahara et al. 1992). Liothrips urichi Karny (Thysanoptera: Phlaeothripidae) is the first candidate biological control species proposed to undergo testing in Australia. The thrips have been released as a biological control agent for C. hirta in Fiji (1930), the Solomon Islands (1938, 1973 and 1975), Hawaii (1953), Palau (1960 and 1972), and American Samoa (1974) (Conant 2009). Liothrips urichi has established in all but the Solomon Islands and appears to be effective in open, sunny areas (Conant 2009). Host specificity of this insect has been demonstrated both within its native range and within countries where it has been introduced as a biological control of C. hirta (Table 1)

Hydrotimetes natans as a suitable biological control agent for the invasive weed Cabomba caroliniana

The aquatic macrophyte Cabomba caroliniana A. Gray is a major invasive weed in Australia and several other countries. A classical biological control program was initiated in Australia in 2003 and native range explorations in Argentina that year led to the discovery of the aquatic weevil Hydrotimetes natans Kolbe feeding on C. caroliniana, making it the first, and so far, only potential biological control agent for the weed. However, the program was discontinued because the largely unknown biology had made rearing of H. natans difficult under quarantine laboratory conditions. We report here key aspects of the biology and reproductive behaviour of H. natans that provided significant insights to successfully establish a laboratory colony when the program was restarted in 2016. In addition, we studied the physiological host range of H. natans and determined its risks to 15 non-target plant species. The preoviposition period of H. natans was 6.50 ± 0.85 days, and the development times of eggs and pupae were 7.65 ± 0.86 and 14.27 ± 0.51 days, respectively. Egg to adult development time was 46.52 ± 0.82 days with a larval development time of 25–27 days. A single female laid 123.13 ± 23.03 eggs in 24 weeks under non-limiting laboratory conditions. Oviposition was intermittent and age-dependent with 75% eggs oviposited within 8.54 weeks after adult eclosion; percent viability of these eggs was 55.62 ± 4.61. Females oviposited mostly on the apical tips followed by on the first few nodes from the tip. Adults survived a maximum of 521 days with a mean longevity of 235.16 ± 21.16 days and females remained reproductive for 211.00 ± 35.05 days. Field surveys and laboratory host-specificity studies demonstrated H. natans is adequately host-specific to C. caroliniana. No non-target effects were observed on Nymphaea, Victoria and Trithuria species. Brasenia schreberi indicated the possibility of lifecycle completion by H. natans in choice and no-choice trials but did not sustain a population in continuation trials. The risks to B. schreberi were deemed negligible and H. natans was approved for release in Australia. © 2022 The Author(s

Genetic identity of Australian prickly acacia (Vachellia nilotica, Fabales: Mimosoideae) – assessing the target for biological control

Prickly acacia (Vachellia nilotica) has been the target of biological control programmes in Australia for over three decades, with little success. Control efforts may have been hindered by poor characterisation of the plants in Australia, and the ambiguous taxonomy of the species. Nine subspecies of this weed have been described, with only one subspecies identified in Australia (subsp. indica), though previous genetic screening identified a unique genotype in Australia that allegedly did not match any of the previously described subspecies (dubbed the “Pakistan genotype”). We used gene sequencing to characterise this weed in Australia, and to assess the native range distribution of the invasive subspecies. Two widespread ITS1 haplotypes were identified from 25 localities across northern Australia, corresponding to subsp. indica and the undescribed “Pakistan genotype”. Many plants were heterozygous at the ITS1 locus, indicating plants with the distinct genotypes are freely interbreeding. The “Pakistan genotype”, which has no defining morphological characters, was found across the native range of subsp. indica (including Ethiopia, where this subspecies has only recently been detected). The “Pakistan genotype” is not, in other words, a distinct subspecies, but simply represents genetic variation within subsp. indica. No genetic structuring was found across the native distribution of subsp. indica, so the precise provenance of the Australian plants could not be determined. Future studies should use microsatellites or genotyping-by-sequencing approaches to provide a finer-scale assessment of the provenance of the Australian plants

Biological control of Cabomba caroliniana: biology and host range of the cabomba weevil Hydrotimetes

Cabomba caroliniana Gray is a submerged aquatic weed, invasive in the waterways of Australia and several other countries. In Australia, C. caroliniana is a Weed of National Significance, and its detrimental effects include choking of waterways, reducing the water holding capacity of dams supplying drinking water and affecting native flora. During preliminary surveys and host specificity tests in the native range (Argentina and Paraguay), the aquatic weevil, Hydrotimetes natans Kolbe has been identified as a potential biological control agent to control C. caroliniana. We imported H. natans from the native range to Australia and studied its biology and host specificity. From biology studies, we found that eggs were laid on submerged leaves of C. caroliniana semi-embedded in a small divot and hatched in 7.65 ± 0.86 days. Larvae developed tunnelling through the leaves (early instar) or stems (late instar) and pupated outside the stem near the base of petioles after 25 to 27 days of development. Pupae developed into adult in 14.3 ± 2.7 days. The full lifecycle, from oviposition through to adult eclosion, took 46.5 ± 4.4 days. Host specificity trials were setup with Brasenia, Nymphaea and Trithuria species selected based on the centrifugal phylogenetic method, and data on oviposition, larval development, pupation and lifecycle completion were recorded. We found no evidence of oviposition and development of H. natans on any of the Nymphaea or Trithuria plant species tested. While Brasenia schreberi supported partial development (which was significantly lower than that on C. caroliniana), it did not support multiple generations of H. natans. In this talk, we discuss these results in light of risks of H. natans to native and other nontarget species in Australia, and its potential to be part of the integrated weed management of C. caroliniana