Weed In Australia Research Articles

Discrepancies in Australian jurisdiction-based regulation of invasive plants

Abstract Effective regulation is essential for preventing the establishment of new invasive plants and managing the environmental, social, and economic impacts of those already established. Invasive plants are regulated by jurisdictions at a variety of local, regional, national, and international levels. Enhanced coordination of policy and regulations has been identified as a key strategy for addressing the impacts of invasive species (IPBES 2023; Pluess et al. 2012); however, coordination between jurisdictions, and even within jurisdictions, is not always considered. To review regulatory coordination in Australia, we compiled a comprehensive dataset of noxious weeds (defined as invasive plants and potentially invasive plants with controls specified in regulation) in each Australian jurisdiction (i.e., State or Territory). We found that jurisdictions on average shared c. 67% (SD = 15%) of noxious weed listings. Neighbouring jurisdictions were not more similar than separated jurisdictions in their noxious weed listings. There were significant differences in the biogeographic native ranges of noxious weeds between jurisdictions, with species native to temperate Asia being most frequently listed overall. The predominant likely entry pathway for noxious weeds in Australia was the ornamental trade. Listings were primarily dedicated to proactive control, prohibiting the cultivation of noxious weeds to avoid their naturalisation. There were 415 noxious weeds regulated in a harmonious manner across jurisdictions. However, there were 327 noxious weeds regulated by jurisdictions in a discordant manner, potentially leaving neighbouring jurisdictions vulnerable to invasion. We suggest jurisdictions reassess the regulation of these 327 discordant noxious weeds in Australia and utilise a national taxonomic standard to avoid problematic synonyms. Improved cohesion of policies could be achieved through wider adoption of existing regulatory systems and co-development of regulations between government and industry.

Leveraging Wheat Competition to Manage Seasonal Expansion of Feathertop Rhodes Grass (Chloris virgata)

Utilizing the potential of crops to suppress weeds is an important strategy for sustainable management. Feathertop Rhodes grass (FTR) (Chloris virgata Sw.) is a problematic warm-season weed in Australia that has recently expanded into colder seasons. This study investigated the growth and seed production of FTR at two planting times (May and July) and three wheat planting densities (0, 82, and 164 plants m−2) using a neighborhood design over two years. In both years, the plant height, tiller production, and panicle production of FTR were lower in the presence of wheat compared to when wheat was absent. Surrounding FTR with wheat delayed its pinnacle emergence, varying from 1 to 6 days in the first year and 4 to 21 days in the second year, depending on the planting date and wheat density. During both years and planting times, wheat’s presence caused approximately a 99% decrease in the dry matter and seed production of FTR compared to wheat’s absence. Additionally, the wheat height, an important competitive factor, was higher at both densities in the May planting compared to the July planting. The emergence, establishment, and continuous production of FTR seeds throughout the year indicate that inadequate management could result in the further spread of this weed. Our findings suggest that adjusting the date and density of wheat planting could be a viable strategy for sustainably managing this weed during colder seasons.

Life cycle, host specificity and potential impact of a gall-inducing thrips Acaciothrips ebneri, a biological control agent for prickly acacia (Vachellia nilotica subsp. indica) in Australia

Prickly acacia (Vachellia nilotica subsp. indica (Benth.) Kyal. & Boatwr.; Fabales: Fabaceae) is a Weed of National Significance and a target for biological control in Australia. Currently there are no effective biological control agents for the weed in Australia. Based on genetic and climate matching, a gall thrips (Acaciothrips ebneri Karny; Thysanoptera: Phlaeothripidae) inducing rosette galls resulting in shoot tip dieback, was identified as a prospective biological control agent from Ethiopia. No-choice host-specificity tests were conducted on 59 test plant species in a high security quarantine in Brisbane, Australia. Acaciothrips ebneri is host-specific, inducing galls and reproducing only on prickly acacia. Acaciothrips ebneri, as predicted by the CLIMEX model, is suited to hot and arid western Queensland where major prickly acacia infestations occur. The Australian Government approved A. ebneri for field release in October 2022. This is the first time a true gall-inducing thrips has ever been approved as a weed biological control agent. Field releases commenced in January 2023 and are in progress. There are early signs of field establishment resulting in shoot tip die back in the field, and field release and monitoring will continue.

Grower attitudes and practices toward area-wide management of cropping weeds in Australia

ContextThe mobility of weeds, use of biological controls and spread of herbicide resistance mean that weed management is a landscape-scale problem. Area-wide management (AWM) presents one approach for land managers, industry and government representatives to collaborate to manage weeds across public and private properties. Such an approach has been successfully used for other landscape-scale problems, such as managing animal and insect pests. ObjectiveThis study aims to identify what individual, community and institutional factors are associated with growers’ participation in collaborative weed management in three cropping regions of Australia. MethodsSurvey responses from 604 cropping growers from the Riverina (n = 218), Sunraysia (n = 200), and Darling Downs (n = 186) regions of eastern Australia were recorded between July and September 2021. Questions were designed to collect information on: socio-economic characteristics; the nature of farming operations; weed management concerns and beliefs; and individual and collaborative weed management practices, which constitute area-wide weed management (AWWM). Fisher’s Exact test was applied to assess differences between growers who do and do not work together with others to manage weeds. P < 0.05 was considered statistically significant. The Boruta random forest function was used for feature selection and a random forest regression including the selected variables was then applied to determine the accuracy of the model that explains whether land managers collaborate with others on weed management. Results and ConclusionAlmost all (95%) growers agreed that each land manager has a responsibility to the whole region to control weeds and 84% agreed that effective control of weeds requires land managers to work together. Yet only 24% of growers currently work with other land managers on weed management. Growers who are less likely to work with others to manage weeds are those who: are less concerned about herbicide resistant weeds spreading to neighbouring land; are unlikely to share information with other land managers about weeds; or who are unlikely to attend meetings about local weed issues. Supporting greater uptake of AWM of weeds in the future will require increased awareness and education about the spread of herbicide resistance, building of new networks among growers and other key stakeholders, and development of AWWM activities that are accessible to all land managers regardless of time and financial constraints. SignificanceThis research demonstrates for the first time the limited extent of AWM of cropping weeds among growers in eastern Australia, and that AWWM is hindered by knowledge, network and access constraints.

Phylogenetic analyses reveal multiple new stem-boring Tetramesa taxa (Hymenoptera: Eurytomidae): implications for the biological control of invasive African grasses

Many native South African grass species have become invasive elsewhere in the world. The application of biological control to invasive grasses has been approached with trepidation in the past, primarily due to concerns of a perceived lack of host specific herbivores. This has changed in recent times, and grasses are now considered suitable candidates. The Tetramesa Walker genus (Hymenoptera: Eurytomidae) has been found to contain species that are largely host specific to a particular grass species, or complex of closely related congeners. Very little taxonomic work exists for Tetramesa in the southern hemisphere, and the lack of morphological variability between many Tetramesa species has made identification difficult. This limits the ability to assess the genus for potential biological control agents. Species delimitation analyses indicated 16 putative novel southern African Tetramesa taxa. Ten of these were putative Tetramesa associated with Eragrostis curvula (Schrad.) Nees and Sporobolus pyramidalis Beauv. and S. natalensis Steud., which are alien invasive weeds in Australia. Of these ten Tetramesa taxa, eight were only found on a single host plant, while two taxa were associated with multiple species in a single grass genus. The Tetramesa spp. on S. pyramidalis and S. africanus were deemed suitably host-specific to be used as biological control agents. Field host range data for the Tetramesa species on E. curvula revealed that the wasp may not be suitably host specific for use as a biological control agent. However, further host specificity testing on non-target native Australian species is required.

Germination ecology and growth phenology of cowvine (Ipomoea lonchophylla) as influenced by environmental parameters

AbstractCowvine (Ipomoea lonchophylla J.M. Black) is a native and widely spread summer broadleaf weed in Australia. It contains glycoresins, which are toxic to livestock. However, limited information is available on seed germination ecology and growth phenology of this species. A series of experiments were conducted to determine the response of I. lonchophylla to different environmental conditions. Results showed that the primary dormancy exhibited by I. lonchophylla is due to the physical impediment of the hard seed coat. The seed germination percentage was the highest at the constant temperature of 27 C and alternating temperatures of 35/25 C. Germination of I. lonchophylla was not stimulated by light, suggesting that this species is non-photoblastic. Ipomoea lonchophylla germination was intolerant of a medium to high level of salt stress, and germination was completely inhibited at 250 mM NaCl. The emergence of I. lonchophylla was not restricted by seeding depth up to 8 cm, but only 5% emergence was recorded when seeds were planted at a 16-cm depth. The germination percentage was also drastically reduced by 90% to 100% after exposure to either 3 mo in silage, 48-h digestion in steers, or silage plus digestion treatments. The growth and reproductive phenology of I. lonchophylla was affected by emergence time. Plants that emerged in late spring (November 15) were able to produce more berries per plant than those that emerged in midsummer (January 15) in southern New South Wales. Information gained in our study concerning high soil salinity, ensiling, and digestion will help to develop more sustainable and effective integrated weed management strategies for controlling and reducing the spread of this weed.

Biological control of weeds in Australia: the last 120 years

AbstractThe development of the field of biological control of weeds in Australia is described, from the first attempts in 1903 to the present day. The interest sparked by the obvious success of prickly pear program, apparent from 1930 to 1935, resulted in several programs during the next 20 years, followed by a decline in activity until the 1970s when activity increased enormously following the success of the skeleton weed program and the effective use of a plant pathogen for the first time. This momentum was maintained until the beginning of the present century with several successes and was marked by several important advances in genetic profiling, host‐specificity testing, economic evaluation, conflict of interest resolution and the ecology of insect/plant interactions, including evaluation of the effectiveness of individual introductions. Biological control has proved to be a valuable and effective approach to weed management in Australia with 39% of all programs considered to produce complete or near‐complete control, 30.5% partial control and an average benefit–cost ratio of 23:1. Funding for research has been variable with a decline from the late 1990s but with a significant increase again since 2020.

Sensitivity of herbicide-resistant rigid ryegrass (Lolium rigidum) populations to cinmethylin, a new herbicide site of action

AbstractRigid ryegrass (Lolium rigidum Gaudin) is the most problematic weed in Australia, with evolved resistance to multiple herbicide sites of action. Selection pressure by cinmethylin (Group 30, a fatty-acid thioesterase inhibitor) has been limited, because few populations have been exposed to the herbicide since its introduction in 2019. In this study, we examined the sensitivity of L. rigidum populations to this new herbicide. From a screening of almost 500 field populations in 2020, 28 potentially resistant populations were further investigated in a dose–response experiment. Seedlings from five populations surviving treatments of 250 or 375 g ai ha−1 cinmethylin were grown to maturity and seeds were harvested. The level of resistance found among the five putative-resistant parental populations of L. rigidum was negligible. In one population, one round of selection with cinmethylin resulted in a 2-fold increase in the lethal dose causing 50% mortality in the progeny population, although this dose was still only one-sixth of the recommended field rate of cinmethylin. Having a unique site of action, cinmethylin is a viable preemergence herbicide option to control existing multiple-resistance populations of L. rigidum. Comprehensive field monitoring and recurrent selection studies under controlled environmental conditions are needed to better ascertain the risk of L. rigidum evolving a high level of resistance to cinmethylin, although current data suggest that this risk is relatively low.

Effect of selected environmental factors on the seed germination of the invasive species Polygala myrtifolia (Polygalaceae) in Australia

Context Polygala myrtifolia L. has become a significant environmental weed in Australia, where it has invaded coastal ecosystems in temperate regions and there is a high risk of extensive further spread. Knowledge of seed-germination behaviour is essential to understand the potential future impact of this species. Aims We investigated the effects of selected environmental factors and dormancy on P. myrtifolia seed germination and emergence to improve management strategies. Methods Seeds were exposed to light, temperature, pH, salinity, osmotic potential and burial depth treatments to assess germination responses, dormancy and viability. Key results Non-dormant seeds readily germinated to high percentages (93.0–95.0%) under specific day–night temperatures of 25°C–15°C regardless of light conditions and across all soil pH (75.0–100.0%). Salinities were tolerated up to 100 mM NaCl (70.0% germination) before sharply declining. Germination reduced from 98.3% to 40.0% at osmotic potentials of −0.4 MPa and −0.6 MPa respectively. Emergence was greatest on the soil surface (48.33%) and absent from depths of 8 cm. Conclusions Germination was high during autumn, winter and spring conditions and across soils of any pH. The species is unlikely to invade areas of high salinity; however, it is moderately tolerant of low soil moisture during germination. Implications There is high risk of P. myrtifolia further invading sensitive ecosystems because of its high germination success. Preventing seed dispersal and ensuring seedlings are controlled on emergence are critical to reducing its impact. We recommend that the cultivation and sale of this species should be prohibited across Australia to prevent further spread.

Obnoxious Plants and Pestiferous Growths: how figurative language reinforces the management of weeds in Victoria, Australia

ABSTRACT The militaristic metaphors common in public discourses about invasive species have been criticised for promoting combative management approaches and constraining policy responses. But are they really to blame for entrenching a command-and-control approach to managing weeds in Australia? Since 2000, almost every state and territory has introduced new biosecurity legislation encouraging ‘shared responsibility’. Yet, this term remains noticeably absent from new legislation in Victoria. We aim to examine whether public discourses around invasive plants have remained unchanged to better understand how invasive plants have been framed and whether this can provide insight into the lack of engagement with ‘shared responsibility’ in Victorian legislation. This study investigates figurative language used in Victorian newspapers from 1885 to 2020 to describe three invasive plants and their management: Bathurst burr, blackberry and gorse. The figurative language reveals limited acknowledgement of humans' role in the spread of weeds and emphasis on the diverse impacts of weeds on humans. Militaristic metaphors have existed for over 130 years, but are neither the most predominant nor community-mobilising. Overall, figurative language has focused on individual efforts to control weeds, without critical analysis of the inter-relationships between humans and weeds nor having created opportunities for caring or collaborative weed management.

Changing Seasonality of Lolium rigidum (Annual Ryegrass) in Southeastern Australia

Lolium rigidum (annual ryegrass) is the most costly winter weed in Australia and has recently been observed growing in summer. The occurrence of L. rigidum in winter is well documented, but there is no research on populations of this weed that germinate and grow in summer. Herein, we discuss how the potential cause of this seasonal expansion may be related to a change in dormancy duration. Dormancy can affect germination patterns but the growth and reproduction of a plant species depend on other factors, such as temperature and photoperiod. Therefore, dormancy alone cannot be the sole variable responsible for the presence of L. rigidum in summer crops, as typical summer temperature conditions are not favorable for the growth of this weed. Additional changes to the growth and development of summer populations may allow for the adaptation and infestation of L. rigidum in summer conditions.

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.

Characterisation of a 4A QTL for Metribuzin Resistance in Wheat by Developing Near-Isogenic Lines.

Wheat (Triticum aestivum L.) production is constantly affected by weeds in the farming system. Chemical-based weed management is widely practiced; broad-spectrum herbicides such as metribuzin have been successfully used to control weeds in Australia and elsewhere of the world. Breeding metribuzin-resistant wheat through genetic improvement is needed for effective control of weeds. Quantitative trait loci (QTLs) mapping efforts identified a major QTL on wheat chromosome 4A, explaining up to 20% of the phenotypic variance for metribuzin resistance. The quantitative nature of inheritance of this QTL signifies the importance of near-isogenic lines (NILs), which can convert a quantitative trait into a Mendelian factor for better resolution of the QTL. In the current study, NILs were developed using a heterogeneous inbred family method combined with a fast generation-cycling system in a population of Chuan Mai 25 (resistant) and Ritchie (susceptible). Seven pairs of NILs targeting the 4A QTL for metribuzin resistance were confirmed with a molecular marker and phenotyping. The resistant allele from the resistant parent increased metribuzin resistance by 63–85% (average 69%) compared with the susceptible allele from the susceptible parent. Segregation analysis in the NIL pairs for thousand grain weight (TGW) (g), biomass per plant (kg), tillers per plant, plant height (cm), yield per plant, and powdery mildew visual score (0–9) indicated that these traits were linked with metribuzin resistance. Similarly, TGW was observed to co-segregate with metribuzin resistance in most confirmed NILs, signifying that the two traits are controlled by closely linked genes. The most contrasting NILs can be further characterised by transcriptomic and proteomic analyses to identify the candidate genes responsible for metribuzin resistance.

Germination ecology of four African mustard (Brassica tournefortii Gouan) populations in the eastern region of Australia

Abstract African mustard (Brassica tournefortii Gouan) is a problematic winter annual weed in Australia. Germination ecology of B. tournefortii may change in response to the maternal environments or habitats in which the plants grow. A study was conducted to evaluate the effect of environmental factors on germination and emergence of four populations of B. tournefortii that were collected from different fields. Averaged over populations, germination was stimulated by dark and was higher at 25/15 C (92%) compared with 15/5 C (76%) and 35/25 C (45%). Averaged over light/dark regimes, at the lowest temperature regime (15/5 C), population A had higher germination than population D; however, at the highest temperature regime (35/25 C), population D had higher germination than population A. Populations B and C had higher germination in the temperature range of 25/15 C and 30/20 C compared with 15/5 C, 20/10 C, and 35/25 C. Seeds germinated at a wide range of alternating day/night temperatures (15/5 to 35/25 C), suggesting that seeds can germinate throughout the year if other optimum conditions are available. Population A was more tolerant to water and salt stress than population D. The sodium chloride concentration and osmotic potential required to inhibit 50% germination of population A were 68 mM and −0.60 MPa, respectively. Averaged over populations, seeds placed at 1-cm soil depth had the highest emergence (54%), and burial depth of 8 cm resulted in 28% seedling emergence. Averaged over populations, wheat residue retention at 6,000 kg ha−1 resulted in greater seedling emergence than the residue amount of 1,000 kg ha−1. The results suggest that B. tournefortii will be favored in no-till systems and that the seedbank of B. tournefortii could be managed by tillage regimes that bury its seeds below 8-cm depths and restrict seedling emergence and growth of new plants.

Broad and narrow host ranges in resolved species of Cintractia limitata s. lat. (Anthracoideaceae, Ustilaginomycotina) on Cyperus

Cintractia (Anthracoideaceae, Ustilaginomycotina) is a widespread genus of smut fungi that parasitizes species of Cyperaceae. Specimens of Cintractia spp. on species of Cyperus s. lat. were examined to resolve their taxonomy, including a species on Cyperus aromaticus, which is an invasive weed in Australia. A phylogenetic species concept was used to show that Cintractia limitata s. lat. contains three different species, Cintractia limitata, C. tangensis, and a new species C. kyllingae. The proposed taxonomy is based on phylogenetic analyses of the ITS rDNA region and on the position of the sori in diverse parts of inflorescences examined in approximately 100 specimens of Cintractia. A morphological synapomorphy in C. tangensis is sori with firmly agglutinated spore masses around the base of the peduncles, rarely in the spikelets. Cintractia limitata forms powdery spore masses between flowers and is known only to occur on Cyperus species with spikelets in capitate heads. Cintractia kyllingae was found on Cy. aromaticus and may have potential as a biological control agent in Australia.

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.

Reallocating budgets among ongoing and emerging conservation projects.

Conserving biodiversity and combating ecological hazards require cost-effective allocation of limited resources among potential management projects. Project priorities, however, can change over time as underlying social-ecological systems progress, novel priorities emerge, and management capabilities evolve. Thus, reallocation of ongoing investments in response to shifting priorities could improve management outcomes and address urgent demands, especially when additional funding is not available immediately. Resource reallocation, however, could incur transaction costs, require additional monitoring and reassessment, and be constrained by ongoing project commitments. Such complexities may prevent managers from considering potentially beneficial reallocation strategies, reducing long-term effectiveness. We propose an iterative project prioritization approach, based on marginal return-on-investment estimation and portfolio optimization, that guides resource reallocation among ongoing and new projects. Using simulation experiments in 2 case studies, we explored how this approach can improve efficacy under varying reallocation constraints, frequencies, costs, and rates of project portfolio change. Periodic budget reallocation could enhance the management of stochastically emerging invasive weeds in Australia and thus reduce the overall risk by up to 50% compared with a static budget. Reallocation frequency and the rate of new weed incursion synergistically increased the conservation gains achieved by allowing unconstrained reallocation. Conversely, budget reallocation would not improve the International Union for Conservation of Nature conservation status of threatened Australian birds due to slow rates of transition among conservation states; extinction risk could increase if portfolio reassessment is costly. Although other project prioritization studies may recommend periodic reassessment and reallocation, our findings revealed conditions when reallocation is valuable and demonstrated a structured approach that can help conservation agencies schedule and implement iterative budget-allocation decisions cost-effectively.

The role of soil temperature and seed dormancy in the creation and maintenance of persistent seed banks of Nassella trichotoma (serrated tussock) on the Northern Tablelands of New South Wales

A large and persistent soil seed bank characterises many important grass weeds, including Nassella trichotoma (Nees) Hack. ex Arechav. (serrated tussock), a major weed in Australia and other countries. In the present study we examined the effects of constant and alternating temperatures in regulating primary and secondary dormancy and the creation and maintenance of its soil seed bank in northern NSW, Australia. One-month-old seeds were stored at 4, 25°C, 40/10°C and 40°C, in a laboratory, and germination tests were conducted every two weeks. Few seeds germinated following storage at 4°C, compared with seeds stored at 25°C, 40/10°C and 40°C. Nylon bags containing freshly harvested seeds were buried among N. trichotoma stands in early summer, and germination tests conducted following exhumation after each season over the next 12 months. Seeds buried over summer and summer plus autumn had higher germination than seeds buried over summer plus autumn plus winter, but germination increased again in the subsequent spring. Seeds stored for zero, three, six and 12 months at laboratory temperatures were placed on a thermogradient plate with 81 temperature combinations, followed by incubation at constant 25°C of un-germinated seeds. Constant high or low temperatures prolonged primary dormancy or induced secondary dormancy whereas alternating temperatures tended to break dormancy. Few temperature combinations resulted in more than 80% germination.

Biological control under a changing climate: The efficacy of the parthenium weed stem-galling moth under an atmosphere enriched with CO2

Parthenium hysterophorus (Parthenium weed) is a highly invasive species that has become a major weed in Australia and many other parts of the world. The present study reports on the effect of atmospheric CO2 enrichment upon the performance and effectiveness of one of its biological control agents, Epiblema strenuana (stem-galling moth). Parthenium weed plants, when grown under an elevated CO2 concentration (550 µmol mol−1), produced a significantly greater dry biomass (38%), produced more branches (35%) and seeds per plant (37%), than plants grown under an ambient CO2 concentration (380 µmol mol−1). Galls produced by E. strenuana significantly reduced the height (46%) and dry biomass (45%) of parthenium weed under the elevated CO2 concentration. In the presence of E. strenuana, the total number of cypselae produced by plants was 60 or 32% less at ambient or elevated CO2 concentrations, respectively. X-ray revealed that E. strenuana had a significant negative impact upon cypsela fill under elevated CO2 concentration with about 50% not being filled. Gall induction by E. strenuana stimulated lateral branching and this was observed in plants grown both under elevated and ambient CO2 concentrations. Under elevated CO2 and in the presence of E. strenuana, net photosynthesis and water use efficiency were decreased by 25 and 28%, respectively. Despite parthenium weed producing more biomass and seed, this study indicates that the efficacy of E. strenuana as a biological control agent of parthenium weed is likely to be retained in a future climate with an elevated CO2 concentration.

Production of pyrrolizidine alkaloids and shikonins in Echium plantagineum L. in response to various plant stressors.

Echium plantagineum, a native of Europe and Africa, is a noxious invasive weed in Australia forming monocultural stands in pastures and rangelands. It produces a complex mixture of bioactive secondary metabolites, including toxic pyrrolizidine alkaloids (PAs), that protect the plant from insect and livestock herbivory and naphthoquinones (NQs), which suppress competition from weeds, insects and pathogens, and also influence invasion success. However, the extent to which allelochemical production is impacted by environmental factors, thereby influencing plant defense against pests, remains unclear. Following plant stress induced by drought, herbivory and high temperature, extracts of E. plantagineum shoots and roots were subjected to metabolic profiling by UPLC-MS-DAD- QToF mass spectrometry. Abundance of NQs, especially deoxyshikonin, shikonin and dimethylacrylshikonin, rapidly increased in roots exposed to elevated temperatures. Water withholding initially increased NQ abundance, but prolonged drought resulted in reduced total PAs and NQs. Intraspecific competition elevated the production of NQs, whereas simulated herbivory had no initial effect on NQs. Following herbivory, the abundance of the PA 3'-O-acetylechimidine-N-oxide in seedling shoots was increased. Differential accumulation of defense metabolites by E. plantagineum following exposure to various stressors suggested stress-dependent biosynthetic regulation, particularly with respect to NQ production, which was rapidly induced following drought, intraspecific competition and high temperature treatment, thereby positively impacting resistance or defense against herbivores, weeds and pathogens. We propose that trade-offs between above- and below-ground metabolism in E. plantagineum may facilitate allelochemical production in response to stress, rendering plants with an enhanced ability to defend against other neighboring plants, insects and microbes, with allelochemical production further facilitated by catabolic recycling following lengthier exposure to stress. © 2019 Society of Chemical Industry.