Biotic resistance to Chrysanthemoides monilifera ssp. monilifera in Tasmania

Boneseed, Chrysanthemoides monilifera ssp. monilifera (Asteraceae) is concentrated in and near cities and towns on the north and east coasts of Tasmania. Its absence from intervening rural and bushland areas cannot be attributed to environmental conditions or a lack of time for dispersal from introduction points. The hypothesis tested in the present paper is that the range of boneseed in Tasmania is limited by biotic resistance through herbivory. Cafeteria experiments and field observations showed that sheep (Ovis aries), cattle (Bos taurus), Tasmanian pademelons (Thylogale billardierii), Bennett's wallabies (Macropus rufogriseus), garden weevils (Phlyctinus callosus) and native invertebrates all consumed boneseed, while common brushtail possums (Trichosurus vulpecula) did not. A boneseed population subjected to sheep grazing for 168 days suffered high mortality, while an adjacent ungrazed population survived intact. A replicated exclosure experiment showed that 75 days of grazing by cattle reduced the size of boneseed plants. Observations of a population subject to Bennett's wallaby and Tasmanian pademelon grazing over 1 year and 2 months showed consistently high leaf damage to foliage within pademelon reach and a decline in population, with high mortality rates in the driest and coldest times. Leaf loss attributable to invertebrates did not prevent a nearby population without wallabies from growing. The distributions of the taxa were consistent with biotic resistance, with those demonstrating no severe effect on boneseed individuals widespread, while those with evidence of severe effects more common in rural areas than in urban areas. Boneseed seemed unlikely to survive for very long at normal stocking levels. Macropod grazing, particularly that of T. billardierii, may also inhibit the invasion of boneseed. Thus, the recent introduction of foxes to Tasmania may not only cause the extinction of species such as T. billardierii, but also may cause an expansion of the range of boneseed.     

Identification and Phytotoxicity Assessment of Phenolic Compounds in Chrysanthemoides monilifera subsp. monilifera (Boneseed)

Chrysanthemoides monilifera subsp. monilifera (boneseed), a weed of national significance in Australia, threatens indigenous species and crop production through allelopathy. We aimed to identify phenolic compounds produced by boneseed and to assess their phytotoxicity on native species. Phenolic compounds in water and methanol extracts, and in decomposed litter-mediated soil leachate were identified using HPLC, and phytotoxicity of identified phenolics was assessed (repeatedly) through a standard germination bioassay on native Isotoma axillaris. The impact of boneseed litter on native Xerochrysum bracteatum was evaluated using field soil in a greenhouse. Collectively, we found the highest quantity of phenolic compounds in boneseed litter followed by leaf, root and stem. Quantity varied with extraction media. The rank of phenolics concentration in boneseed was in the order of ferulic acid > phloridzin > catechin > p-coumaric acid and they inhibited germination of I. axillaris with the rank of ferulic acid > catechin > phloridzin > p-coumaric acid. Synergistic effects were more severe compared to individual phenolics. The litter-mediated soil leachate (collected after15 days) exhibited strong phytotoxicity to I. axillaris despite the level of phenolic compounds in the decomposed leachate being decreased significantly compared with their initial level. This suggests the presence of other unidentified allelochemicals that individually or synergistically contributed to the phytotoxicity. Further, the dose response phytotoxic impacts exhibited by the boneseed litter-mediated soil to native X. bracteatum in a more naturalistic greenhouse experiment might ensure the potential allelopathy of other chemical compounds in the boneseed invasion. The reduction of leaf relative water content and chlorophyll level in X. bracteatum suggest possible mechanisms underpinning plant growth inhibition caused by boneseed litter allelopathy. The presence of a substantial quantity of free proline in the target species also suggests that the plant was in a stressed condition due to litter allelopathy. These findings are important for better understanding the invasive potential of boneseed and in devising control strategies.     

Status of coastal dune communities invaded by Chrysanthemoides monilifera

It has often been assumed that the introduced species Chrysanthemoides monilifera is displacing native species in invaded coastal communities in Australia but no direct evidence has been presented that this displacement is occurring and, if so, to what extent. At an invaded site near Moruya, New South Wales, we found strong negative correlations between the presence (measured as cover, frequency or volume of shrub) of C. monilifera and Acacia longifolia, the hitherto dominant native species, and of C. monilifera and Lomandra longifolia. The negative correlations are partly accounted for by differences in the amounts of each species on different zones on the dune, but the negative correlations remain even within these zones. Also a comparison with uninvaded sites indicates that C. monilifera is displacing A. longifolia from the fore-dune and mid-dune but is having less effect in the swale. In addition, there is a trend towards reduced seed production of A. longifolia when growing in proximity to C. monilifera. There is no significant difference in the presence of L. longifolia in the uninvaded and invaded sites. In invaded sites, significantly fewer C. monilifera seedlings occur in the presence of L. longifolia. Thus it appears that the observed negative associations arise from different causes. Chrysanthemoides monilifera is displacing the structurally similar, and previously dominant A. longifolia, while the tussock-forming L. longifolia is locally hindering the invasion.     

Preliminary host specificity testing of Endophyllum osteospermi (Uredinales, Pucciniaceae), a biological control agent against Chrysanthemoides monilifera ssp. monilifera

Chrysanthemoides monilifera ssp. monilifera, indigenous to the Western Cape Province of South Africa, is a serious invader of native vegetation in south-eastern Australia. The rust fungus Endophyllum osteospermi causes witches' brooms on C. monilifera ssp. monilifera in South Africa, and is associated with a reduction in growth and seed production of its host under natural conditions, as well as mortality of severely infected bushes. This rust fungus is considered to be a potential biological control agent for use against C. monilifera ssp. monilifera in Australia. Endophyllum osteospermi has a long latent period, typically between 6 and 24 months between infection and the initiation of witches' brooms. This long latent period makes the logistics of doing traditional host specificity testing, in which all test plant species are inoculated and observed for symptom development, unfeasible for this rust fungus. Germination of aecidioid teliospores and penetration by basidiospores were observed on the surface of excised leaves of 32 test plant species at 4 days after inoculation, and compared to that on C. monilifera ssp. monilifera. Germinating aecidioid teliospores aborted on 14 test plant species, whilst no penetration was attempted on a further 12 test plant species. Penetration only occurred on nine of the 32 test plant species, in addition to C. monilifera ssp. monilifera. Inoculating whole plants of nine selected test plant species confirmed the above results. Therefore, only the test plant species in which penetration occurred, or at least was attempted, need to undergo comprehensive host specificity testing. Pending these results, E. osteospermi may be suitable for release in Australia for the biological control of C. monilifera ssp. monilifera.     

Interactions between seedlings of Chrysanthemoides monilifera and Acacia longifolia

Displacement of Acacia longifolia on coastal dunes in New South Wales by the invasive species Chrysanthemoides monilifera may be linked to the greater competitiveness by the latter in the seedling stage, as demonstrated in pot experiments. This occurs despite a lower chlorophyll concentration in shoots of C. monilifera which leads to a lower assimilation rate per unit leaf area and lower carbohydrate concentrations. However, this assimilate is spread over a greater total leaf area. Such a strategy associated with‘quantity’may thus be more important than leaf‘quality’in terms of competitiveness. In A. longifolia, the production of higher quality‘leaves’but of lower total area may be well-suited in the often sparse native populations found in sand dunes, but appears disadvantageous when seedlings of C. monilifera also co-exist. The competitive advantage of C. monilifera over A. longifolia is reduced but not reversed under water stress. Under severe stress, mortality of C. monilifera is greater than that of A. longifolia in monocultures but mortality of both species is similar in mixtures. The reason appears to be that C. monilifera transpires more water per plant even though its rate of transpiration per unit leaf area is reduced under water stress because of early stomatal closure. In mixtures, faster root growth of C. monilifera ensures faster uptake of the available soil water, thus minimizing the inherent advantage in A. longifolia of its lower water use and greater efficiency.     

Epidemic increase of Endophyllum osteospermi (Uredinales, Pucciniaceae) on Chrysanthemoides monilifera

The epidemiology of the rust fungus Endophyllum osteospermi was investigated. This rust fungus is considered to be a candidate biological control agent for Chrysanthemoides monilifera ssp. monilifera, which is an invasive alien weed of native vegetation in south-eastern Australia. Between 10 and 20 plants of C. monilifera were marked at each of five sites in the Western Cape Province of South Africa, where both organisms are native. The infection levels and number of witches' brooms were determined every 2 months over a 2-year period. Additionally, at three of these sites, the infection levels and number of witches' brooms of all bushes in the host population was determined annually over 4 years. The increase in number of witches' brooms per bush ranged between 0 and 282 within 1 year, with an average increase per bush of 28 (SE±4.8) in 1993, and 39 (SE±9.2) in 1994. The average r_s for all bushes during 1993 was 0.015 month^-1 (SE±0.0041, n=72) and 0.0098 month^-1 (SE±0.0073, n=43) during 1994. When host bushes that either died back or died during the course of each year were excluded, then the average r_s during 1993 was 0.023 month^-1 (SE±0.0048, n=45) and 0.0348 month^-1 (SE±0.0106, n=20) during 1994. Under suitable conditions in South Africa, E. osteospermi undergoes epidemic increase within its host plant's populations. This rust fungus should therefore be considered as a suitable candidate biological control agent for use in Australia against C. monilifera ssp. monilifera.     

The Impact of the Weed Chrysanthemoides monilifera ssp. rotundata on Coastal Leaf Litter Invertebrates

In coastal areas of Australia, there are extensive infestations of the environmental weed Chrysanthemoides monilifera ssp. rotundata (bitou bush). This study looked at the impact of long-term infestations on the abundance and assemblage composition of leaf litter invertebrates. Assemblages were compared in weed infested and native shrublands along the New South Wales coastline over 12 months. The total abundance was not significantly reduced in the weedy habitat but the abundance of mites, thrips, spiders, ants, and centipedes was reduced at many sites. The invertebrate assemblages also differed between habitats, with the C. monilifera supporting a lower diversity of beetles. However, the millipedes, amphipods, earthworms, pseudoscorpions and isopods appeared to respond positively to the invasion, occurring in higher abundance and detected more frequently in the weedy areas. This has been partially attributed to a change in microclimate within the C. monilifera infestations. It is generally moister and darker, which these invertebrates tend to prefer. Secondly, C. monilifera produces less leaf litter of higher quality, and possibly higher palatability than the native sclerophyllous vegetation, which may encourage species that consume litter.     

Selection of Cassida spp. from southern Africa for the biological control of Chrysanthemoides monilifera in Australia

Four species of tortoise beetle (Chrysomelidae, Cassidinae) (Cassida spatiosa Spaeth and three undescribed Cassida species, labelled 1. 2 and 3) from South Africa, were assessed as potential biological control agents against Chrysanthemoides monilifera monilifera (L.) T. Norl. and C. m. rotundata (DC.) T. Norl. These southern African plants are important weeds of conservation areas in Australia. In South Africa, Cassida spatiosa was found feeding on C monilifera subcanescens (DC.) T. Norl., Cassida sp. 1 on Chrysanthemoides monilifera pisifera (L.) T. Norl. and C m. monilifera, Cassida sp. 2 on C m. pisifera and Chrysanthemoides incana (Burm.f.) T. Norl. and Cassida sp. 3 on C. m. rotundata. The life cycle of each species was completed in about three months on the leaves of the host. Cassida species 1, 2 and 3 were tested against eight species of plant and only fed and completed development on Chrysanthemoides species and the related species. Calendula officinalis L. Cassida species 1 and 3 showed no preference between Calendula officinalis and Chrysanthemoides species for oviposition. Cassida species 3 was selected for further assessment as a biological control agent based on match to the host subspecies and target climate in Australia.     

Movement and modelling of buried seed of the invasive perennial Chrysanthemoides monilifera in coastal dunes and biological control

Movement in the soil of seeds of the invasive shrub Chrysanthemoides monilifera was studied in relation to the success of proposed biological control treatments. Marked seeds were buried at depths from 0 to 6 cm and recovered after periods of 3–24 months. Survival declined with time but was higher for more deeply buried seeds. The average movement of seeds was 0.7 cm per year downwards which enhances their survival but also tends to remove them from the zone of highest likelihood of germination. A model was constructed to predict the size of the pool of potentially germinable seed under treatments including pre-dispersal predation and burning. It indicated that satisfactory control with a pre-dispersal seed predator will be achieved only if there is a < 95% reduction in seed-fall to the soil surface over the entire year. A control agent that is 100% effective for most of the year will probably be unsatisfactory if its rate of predation drops to > 90% for a part of the year (e.g. during winter when seed-fall is abundant). The effectiveness of the control could be enhanced by using intense fires to reduce the soil seed-pool and to kill many of the mature shrubs. However, the feasibility of this combination of treatments will depend on the reaction of the biological control agent to fire.     

Development of a Pesticide Exclusion Technique for Assessing the Impact of Biological Control Agents for Chrysanthemoides monilifera

Seven biological control agents have been released on the environmental weed Chrysanthemoides monilifera in Australia and two are widely established on infestations of C. monilifera subsp. rotundata in New South Wales. Five pesticides were screened for their impact on seed production of C. monilifera and two on the survival of the agent Comostolopsis germana, a shoot tip-feeding lepidopteran. The insecticides carbaryl, carbofuran, dimethoate, fluvalinate and the fungicide benomyl had no significant effect on seed production of C. monilifera when applied over a period of four months. Numbers of C. germana larvae were readily suppressed after three months by applications of fluvalinate or a mixture of carbaryl, carbofuran and dimethoate. Shoot growth was not affected by applications of carbaryl, dimethoate and benomyl. It is concluded that exclusion by pesticides of biological control agents is a valid method of measuring the impact of agents on C. monilifera.     

Evidence for allelopathy as a mechanism of community composition change by an invasive exotic shrub, Chrysanthemoides monilifera spp. rotundata

Chemical interference is increasingly suggested as a mechanism facilitating exotic plant invasion and plant community composition. In order to explore this further, we employed a comprehensive extract-bioassay technique that facilitated detection and demarcation of phytotoxicity, direct allelopathy and indirect allelopathy of bitou bush (Chrysanthemoides monilifera spp. rotundata) compared to an indigenous dominant of the invaded system, acacia (Acacia longifolia var. sophorae). Extracts of the leaves and roots of both species exhibited phytotoxic effects against five indigenous plant species. Evidence for allelopathy between co-evolved indigenous plants was detected between acacia and Isolepis nodosa. Allelopathy between bitou bush and four indigenous plant species was also detected. Therefore we propose that both the acacia and bitou bush have the potential to chemically inhibit the establishment of indigenous plants. Eventual dominance of bitou bush is predicted, however, based on more ubiquitous effects on seedling growth.     

Impact of the biological control agent Mesoclanis polana (Tephritidae) on bitou bush (Chrysanthemoides monilifera subsp. rotundata) in eastern Australia

The seed fly Mesoclanis polana (Diptera: Tephritidae) was released in Australia in 1996. Its impact on seed production of bitou bush (Chrysanthemoides monilifera subsp. rotundata) was monitored at eight sites along the New South Wales coast from 1996 to 2004. Peak flowerhead production occurred in autumn (March to May); therefore, samples collected in May of each year were used to compare abundance and impact of M. polana across sites and between years. Latitude had a significant effect on abundance and impact of M. polana. By May 2004, 99.6% of flowerheads at the five most northern sites contained at least one egg, while 64% of flowerheads from the two most southern sites contained at least one egg. In May 2004, mean numbers of M. polana eggs per flowerhead were between 13 and 17 at four of the six northern sites and below two at the two southern sites. In May 2004, average seed destruction by M. polana was 58% at the four most northern sites and 11% at the two most southern sites. The highest level of seed destruction recorded was 86% at Kingscliff in May 2003. Percentage seed destruction in May provided a reasonable estimate of seed destruction for the whole year. Parasitoids were found throughout the range of M. polana in Australia. Their attack rate on M. polana was less than 10% at all sites, except at the two most northern sites where parasitism of up to 27% was recorded. Parasitism results are compared with predictions made following an earlier study. The release of Mesoclanis magnipalpis, a species suited to cooler environments, is recommended for the southern range of C. monilifera subsp. rotundata.     

Unsuccessful introduced biocontrol agents can act as pollinators of invasive weeds: Bitou Bush (Chrysanthemoides monilifera ssp. rotundata) as an example

The extent of self‐compatibility and reliance on pollinators for seed set are critical determinants of reproductive success in invasive plant species. Seed herbivores are commonly used as biocontrol agents but may also act as flower visitors, potentially resulting in pollination. However, such contrasting or potentially counterproductive interaction effects are rarely considered or evaluated for biological control programs. We investigated the breeding system and pollinators of Bitou Bush (Chrysanthemoides monilifera ssp. rotundata), an invasive species in Australia that has been the subject of biocontrol programs since 1987. We found the species to be obligate outcrossing in all six populations tested. From 150 video hours, we found 21 species of potential pollinators, including Mesoclanis polana, the Bitou Seedfly, native to South Africa and released in Australia as a biocontrol agent in 1996. Mesoclanis polana transferred pollen to stigmas and was the most common pollinator (52% of pollinator visits), followed by the syrphid fly Simosyrphus grandicornis (9%) and introduced honeybee, Apis mellifera (6.5%). Fruit‐to‐flower ratios ranged from 0.12 to 0.45 and were highest in the population with the greatest proportion of Mesoclanis polana visits. In an experimental trial, outside the naturalized range, the native bee Homalictus sphecodoides and the native syrphid Melangyna viridiceps were the primary pollinators, and fruit‐to‐flower ratios were 0.35, indicating that Bitou Bush would have ready pollinators if its range expanded inland. Synthesis. Invasive Bitou Bush requires pollinators, and this is effected by a range of generalist pollinators in eastern Australia including the Bitou Seedfly, introduced as a biocontrol agent, and the major pollinator detected in this study. Fruit‐to‐flower ratios were highest when the Bitou Seedfly was in high abundance. This study underscores the importance of evaluating the pollination biology of invasive species in their native ranges and prior to the introduction of biocontrol agents.     

Integrating revegetation with management methods to rehabilitate coastal vegetation invaded by Bitou bush (Chrysanthemoides monilifera ssp. rotundata) in Australia

Two multi‐year field experiments investigated the effects of integrating revegetation with invasive plant management methods to rehabilitate coastal dune and woodland vegetation invaded by Bitou bush (Chrysanthemoides monilifera (L.) Norl. ssp. rotundata (DC.) Norl.) in New South Wales, Australia. The revegetation technique used was to sow directly seeds of three native species common to coastal habitats. Management treatments involved combinations of prescribed fire, manual removal of Bitou bush and an application of herbicide. Addition of native seeds significantly increased density of native species in both habitats. The benefits of manually removing Bitou bush were observed only where densities of native species were at their lowest. Fire increased densities of some native species in the woodland, but decreased those of others in the dune. Densities of Acacia longifolia ssp. sophorae (Labill.) Court (woodland) and of Banksia integrifolia L.f. (woodland and dune) were significantly reduced within 4 months of herbicide application, alone or in combination with other treatments. The majority of these effects, however, did not persist. Manual removal in both habitats and addition of seed in the woodland were most effective in reducing Bitou bush densities when applied post‐fire. Herbicide treatment on its own or in combination with other treatments did not significantly reduce Bitou bush densities by the end of the experiments. We conclude that restoration of coastal ecosystems invaded by a major invasive plant species requires a whole‐of‐system approach involving revegetation in combination with known management methods to assist recovery of native species in the longer term.     

Identification of volatile compounds released by roots of an invasive plant, bitou bush (Chrysanthemoides monilifera spp. rotundata), and their inhibition of native seedling growth

Allelopathy has been suggested as a mechanism promoting the monoculture formation of some invasive exotic plants. Previous studies have shown that hydrophobic extracts of the roots and soil of exotic bitou bush (Chrysanthemoides monilifera spp. rotundata (DC.) T. Norl.) inhibited the seedling growth of five Australian native plants, including the dominant acacia (Acacia longifolia var. sophorae (Labill.) F. Muell.). Based on this finding, we compared the hydrophobic root and soil chemical profiles of bitou bush and acacia to determine whether bitou bush roots release allelopathic compounds that are novel to the invaded system. We detected three compounds that were exclusive to the bitou bush root and soil, and seven compounds that were common to the bitou bush and acacia roots but only present in the bitou bush soil. The compounds unique to the bitou bush invaded soil were all sesqui- and diterpenes. Several of these compounds were found to inhibit the seedling growth of a native sedge, Isolepis nodosa (Rott.) R. Br. Of particular interest are the sesquiterpenes: β-maaliene, α-isocomene, β-isocomene, δ-cadinene, 5-hydroxycalamenene and 5-methoxycalamenene which were found in high concentrations in the bitou bush root and soil extracts and exhibited phytotoxic activity. Therefore, we present evidence to suggest that bitou bush exudes low molecular weight volatile compounds into the soil which inhibit native plant seedling growth. The reduced establishment of native plants via allelopathy is likely to create space and contribute to the invasion of bitou bush on the eastern Australian coast.     

“Barcoding” and ISSR data illuminate a problematic infraspecific taxonomy for Chrysanthemoides monilifera (Calenduleae; Asteraceae): Lessons for biocontrol of a noxious weed

Chrysanthemoides monilifera Tourn. ex Medik is a noxious weed in Australia and New Zealand. It is a widespread species in southern Africa, where it shows considerable morphological variation that has resulted in a confusing infraspecific taxonomy. We use DNA sequence data from the nuclear Internal Transcribed Spacer (ITS) region from 78 samples of this species from its indigenous distribution range and 10 samples from Australia and New Zealand to determine the regions of origin of the invasive plants. These data are augmented by a smaller study using ISSR markers. Bayesian Inference analysis was somewhat resolved, with many weakly supported nodes. Clades with support tended to correspond to infraspecific taxonomic entities, and were geographically coherent. In contrast, a neighbour-net analysis was not as well resolved and indicated considerable reticulation. All analyses of ITS data retrieved two major clades corresponding to Western and Eastern distributions, with some overlap. Samples from New Zealand and Australia correspond to the taxon C. monilifera subsp. monilifera, and are resolved as most closely related to samples from the greater Cape Town area. Biological control agent populations for C. monilifera subsp. monilifera should be sourced from this region in order to avoid host plant incompatibility problems.     

Evaluation and host specificity of two seed flies Mesoclanis polana and M. magnipalpis (Diptera: Tephritidae): biological control agents for Chrysanthemoides monilifera (Asteraceae) in Australia

Larvae of the South African tephritid flies Mesoclanis polana Munro and M. magnipalpis Bezzi feed in the developing seeds of Chrysanthemoides monilifera. Host specificity evaluation using 109 plant species from 25 families indicated that complete development was restricted to their natural host C. monilifera. Minor feeding and limited development was detected on 18 species, but was of no ecological or economic significance. Mesoclanis polana and M. magnipalpis have been released in Australia and M. polana has established and dispersed widely. Mesoclanis magnipalpis has not yet become naturalized. Parasitism of M. polana in Australia by several species of Hymenoptera has been detected, but is not expected to limit the establishment and impact of these flies.     

Soil‐mediated effects on germination and seedling growth of coastal wattle (Acacia sophorae) by the environmental weed,bitou bush (Chrysanthemoides monilifera ssp. rotundata)

Bitou bush (Chrysanthemoides monilifera ssp. rotundata; Asteraceae) is a major woody weed that competes with the native legume Acacia sophorae in coastal ecosystems of eastern Australia. Three glasshouse experiments examined whether litter or soil from beneath bitou bush or Acacia plants could influence seed germination and seedling growth of A. sophorae. The presence of litter decreased seed germinability and this effect was greater for bitou bush litter than for Acacia litter. Shoot growth was increased by the addition of Rhizobium after 40 days, irrespective of soil type. After 78 days, shoot and root biomass were significantly lower for seedlings grown in bitou bush soil than for those grown in Acacia soil. There was a non‐significant trend towards a lower median population of Rhizobium in the soil beneath bitou bush than in that beneath Acacia. The results demonstrated a slight effect of bitou bush on the growth of A. sophorae, which could, however, be overshadowed by the judicious use of herbicides or fire for weed control and revegetation.