Northern Territory Weed Risk AssessmentReport: Erect spiderling … · 2019-08-14 · Annual to short-lived perennial erect herb with a strong tap root, up to 1 m in ideal conditions

DEPARTMENT OF ENVIRONMENT AND NATURAL RESOURCES

www.nt.gov.au/weeds

Northern Territory Weed Risk Assessment Report: Erect spiderling (Boerhavia erecta)

August 2018

Erect spiderling (Boerhavia erecta)

August 2018 Page 2 of 23

Please cite as:

Northern Territory Government (2018). Northern Territory Weed Risk Assessment Report: Erect spiderling (Boerhavia erecta), Northern Territory Government, Darwin. Rangelands Division Department of Environment and Natural Resources PO Box 496 Palmerston Northern Territory 0831 © Northern Territory of Australia 2018 ISBN 978-1-743501-75-7 This product and all material forming part of it is copyright belonging to the Northern Territory of Australia. You may use this material for your personal, non-commercial use or use it with your organisation for non-commercial purposes provided that an appropriate acknowledgement is made and the material is not altered in any way. Subject to the fair dealing provisions of the Copyright Act 1968 you must not make any other use of this product (including copying or reproducing it or part of it in any way) unless you have the written permission of the Northern Territory of Australia to do so. Report written by Louis Elliott (NT Department of Environment and Natural Resources). Latest version: August 2018. Online resources are available at https://nt.gov.au/environment/weeds which provides information about the NT Weed Risk Management System including an explanation of the scoring system, a fact sheet and user guide. Cover photographs: erect spiderling (Boerhavia erecta) in Darwin: erect habit of plant (top-left); seedlings (top-right); fruiting stalk with characteristic ribbed fruits (bottom) (DENR).

Acknowledgments The NT Weed Risk Management (WRM) System was jointly developed by Charles Darwin University (CDU) and the Weed Management Branch, Department of Environment and Natural Resources (DENR); our thanks to Samantha Setterfield, Natalie Rossiter-Rachor and Michael Douglas at CDU. Project funding for the development of the NT WRM System, obtained by Keith Ferdinands and Samantha Setterfield, came from the Natural Heritage Trust. Our thanks to the NT WRM Reference Group for their assistance in building the NT WRM System and the NT WRM Committee for their role in building the system and their ongoing role in weed risk assessments.

https://nt.gov.au/environment/weeds



Contents

1 Executive Summary ......................................................................................................................... 4

2 Background Information ................................................................................................................. 5

3 Summary – Weed Risk .................................................................................................................... 8

4 Summary – Feasibility of Control ................................................................................................... 9

5 Management Matrix ....................................................................................................................... 10

6 General Management Recommendations .................................................................................... 11

7 Determinations – Weed Risk ......................................................................................................... 12

8 Determinations – Feasibility of Control ....................................................................................... 13

9 Evidence Used – Weed Risk ......................................................................................................... 14

10 Evidence Used – Feasibility of Control ........................................................................................ 19

11 References ..................................................................................................................................... 22

12 Appendix – Summary of Method .................................................................................................. 23



1 Executive Summary This report summarises the results and information used for the weed risk assessment of erect spiderling (Boerhavia erecta) in the Northern Territory (NT), Australia, as determined by the NT Weed Risk Technical Committee using the NT Weed Risk Management System.

The following provides the context to the weed risk assessment of erect spiderling:

• Erect spiderling (Boerhavia erecta) has been listed as a target species by the Northern Australia Quarantine Strategy since at least 2001. This target list includes organisms that have a significant negative impact on agricultural systems overseas, are not generally present in Australia and have the potential to impact agriculture here.

• Erect spiderling is a declared Class C weed (not to be introduced to the Northern Territory).

• In September 2017, erect spiderling was detected on a single property in the Darwin rural area. It was the first record of this species in Australia.

• An eradication program for erect spiderling was initiated by the NT Weed Management Branch with the assistance of the landholder in October 2017.

The weed risk assessment of erect spiderling was determined as:

• Low weed risk • Very high feasibility of control.

The output from the weed risk management matrix was:

• monitor • assist interested parties.

The NT Weed Risk Technical committee provided the following general management recommendations:

• Noting the highly restricted nature of the species in the NT, the significant impacts of this species on crop yields overseas, and the fact that this is the first reported incursion in Australia, maintain the existing Boerhavia erecta eradication program, so long as it remains feasible to eradicate.

• In collaboration with the Northern Australia Quarantine Strategy, conduct follow up surveys in the vicinity of the eradication site to determine:

o Whether the species has spread to adjacent or nearby properties; o Raise awareness among local landholders in relation to the species; and o Provide some confidence that extent of the incursion has been delineated.

• Ensure relevant NT Government officers are made aware of the species and are equipped to

identify and report new infestations.

• Reassess the viability of the eradication program in the event that Boerhavia erecta be detected at another location in the NT. At this time, if a decision is made to continue the eradication program, then it is recommended that consideration be given to raising a public alert in the immediate region.



2 Background Information

Common name erect spiderling1

Other common names erect tar vine erect boerhavia spindlepod

Scientific name Boerhavia erecta

Scientific synonyms none

Family Nyctaginaceae (Bougainvillea family)

Lifeform Herb

Environment Terrestrial

Legal status Declared Class C2

Origin and Distribution Erect spiderling (Boerhavia erecta) is native to the Americas, from southern USA through to north-western South America, but has become widespread in other tropical and subtropical regions (Fig. 1). It is now present in sub-Saharan Africa, India and parts of Asia and the Pacific and continues to expand its range, appearing for the first time in Taiwan in 2003. It has also established in Indonesia, Timor Leste, Solomon Islands and Papua New Guinea and was detected in Australia for the first time in 2017.

Figure 1. Worldwide distribution records of erect spiderling (Boerhavia erecta). Source: Global Biodiversity Information Facility. GBIF.org (12 June 2018) GBIF Occurrence Download https://doi.org/10.15468/dl.hfa547.

1 ‘Spiderling’ reportedly refers to the appearance of species with numerous long, slender flower stems, interlocking in a manner suggestive of a spider's web. 2 Not to be introduced to the Northern Territory.

Erect spiderling growing in garden bed, Darwin.

https://doi.org/10.15468/dl.hfa547



Description

Annual to short-lived perennial erect herb with a strong tap root, up to 1 m in ideal conditions (DAFF 2001; Holm et al. 1997) but otherwise up to about 60 cm tall (Chou et al. 2004; B. Lukitsch pers. comm.). Leaves are opposite and unequal, the first of the pair is 3–5.5 cm and the second is 1.5–2.5 cm long. Leaves and flowering stems often have a reddish tinge, seedlings are distinctly red on the underside. The white or pink flowers are small and occur in clusters. Distinctive club-shaped fruits are 3–4 mm long with five longitudinal ribs, and lack glands or hairs.

Figure 2. Erect spiderling: inflorescence including diagnostic smooth club-shaped fruits with longitudinal ribs (left); infestation to approximate height of 1 m (images courtesy Northern Australia Quarantine Strategy).

Similar species The taxonomy of Boerhavia is difficult as many species are plastic, variable and are sometimes treated as clusters (Chen & Wu 2017). In Australia there are several species of native Boerhavia which are superficially similar to erect spiderling. The two main differences between erect spiderling and the native Australian species are:

• Erect spiderling has a more upright habit with leaves growing off erect stems above the ground. In contrast, the leaves of native spiderlings generally grow on stems that are prostrate on the ground, sometimes forming mats.

• Erect spiderling can be reliably distinguished by their fruits which have five ribs and are smooth, without glands or hairs.

Erect spiderling seedlings have a reddish tinge around the veins of the upper side of the leaves and can be easily confused with seedlings of green amaranth Amaranthus viridis. Seedlings can be more confidently distinguished from other similar plants by the distinctive strong violet colour to the underside of the leaves produced by betalain pigments3 (Fig. 3).

3 Betalain compounds are also responsible for the deep red colour found in beetroot, bougainvillea and amaranth.



Figure 3. Seedlings of erect spiderling can be distinguished by a reddish tint around the veins of the upper surface of the leaf, and the strong violet colour on the underside leaf (Darwin, NT).

Habitat and ecology Generally reported as a weed of disturbed and open areas, waste ground and roadsides on sandy, gravelly or rocky soils, a common tropical weed of crops including maize, soybeans, bananas, coffee and cassava. Extremely well adapted to seasonally wet-dry environments due to its high water use efficiency, rapid development (setting seed within 3 to 4 weeks after germination), high seed production and tap root that can resprout after fire, drought or mechanical disturbance.

Other comments

• Erect spiderling is likely to have a much higher impact on irrigated cropping systems in part due to its ability to:

increase growth and reproduction in response to high nitrogen levels and other fertilisers; and

reproduce year-round when water is available.

• Boerhavia spp. are used locally as herb and medicine plants in many tropical countries in America, Africa and India. In the case of India, there is a long history (c. 2000 years) of using another species, Boerhavia diffusa, for medicinal purposes under the name “punarnava”. Boerhavia erecta has similar properties to B. diffusa, and the roots of both B. diffusa and B. erecta are harvested as punarnava in India for commercial processing and sale (Holm et al. 1997).

• Traditional medicinal applications of B. diffusa and B. erecta include: emetic and purgative (in high doses), diuretic, laxative, treatment for jaundice and kidney problems, anti-convulsant, cardiotonic and asthma (Holm et al. 1997; Schmelzer 2006). Research has been conducted to investigate the active pharmacological compounds present in these species (e.g. Stintzing et al. 2004).



3 Summary – Weed Risk Erect spiderling has been assessed as a low risk species in accordance with the NT Weed Risk Management System4. Weed risk is a quantified estimate of the relative risk of a weed having significant negative economic, environmental and/or social/cultural impacts in the NT (refer to the Appendix for method).

Table 1. Erect spiderling (Boerhavia erecta) weed risk score and band for the Northern Territory.

Section Score / 10 Weed Risk Score /1000 Weed Risk Band

A Invasiveness 6.7

29 Low B Impacts 0.5

C Potential distribution 8.4

Invasiveness Erect spiderling generally invades open and disturbed areas. A prolific seeder, it can set seed within 3–4 weeks of germination and a mature plant can produce up to 20,000 – 30,000 seeds per year (Schmelzer 2006). Approximately 60% of seed germinated after one wet season with germination continuing for more than 2 years (Holm et al. 1997).

Erect spiderling seeds can be moved by water. Fruit are sticky only when wet, unlike the fruits of some other Boerhavia spp. which gives them limited ability to stick to passing animals.

Due to the high number and tiny size of the seed (1.5 mm), a likely means of long-distance dispersal is accidentally as a contaminant in soil, on boots, attached to fur or machinery or similar means. This appears to have been a likely cause of recent incursions in Taiwan and Darwin, Australia. Accidental movement of seeds in soil remains a potential mechanism of long-distance dispersal from any existing infestation.

Impact Erect spiderling was considered to be a low impact weed in native ecosystems. There is no evidence to suggest that it is capable of transforming habitats, its leaves are able to be consumed without ill-effect and it does not have an impact on movement of people or animals.

There is widespread evidence of high impact on irrigated cropping systems overseas. It is on this basis that erect spiderling was listed as a target species to increase likelihood of detection if it arrived in Australia (DAFF 2001), with subsequent declaration as a Class C weed in the Northern Territory.

Potential distribution Erect spiderling is well adapted to seasonally dry conditions and high temperatures. The Climatch model predicts that nearly all of the Australian continent (except for cool temperate regions) is climatically suitable for growth (Figure 4).

4 Weed risk management systems in some Australian jurisdictions evaluate specific land uses separately − e.g. agricultural versus pastoral land. However, while there are varied land uses in the Territory, the majority of the landscape is largely intact and is managed in a relatively unmodified state. Also, weed risk is assessed at a Territory-wide scale. For these reasons, the NTWRMS evaluates the impact of weeds on intact native vegetation as the default context (NTG 2017).



Figure 4. Potential distribution model of erect spiderling (Boerhavia erecta) in Australia (NT Weed Management Branch 2018) using Climatch (BRS 2008) and worldwide distribution records (GBIF 2018). Areas of suitable climate are indicated by a climatic suitability score of 7 or above out of 10.

In addition, erect spiderling:

• Would be capable of invading parts of other open, well-drained habitats particularly along roads, tracks and in gravelly, or sandy substrates, along animal tracks and adjacent to riparian areas.

• Does not tolerate extended periods of waterlogging and does not thrive in shaded areas. • Prefers higher levels of nitrogen than other spiderlings including the native Boerhavia spp.

4 Summary – Feasibility of Control The Feasibility of Control assessment (FoC) relates to the comparative ease or feasibility of controlling a weed species in a given weed management region. Feasibility of control is only calculated for regions where the weed is present.

The feasibility of control of Boerhavia erecta in the Darwin region was assessed as very high.

Table 2. Feasibility of control scoring and determination for erect spiderling (Boerhavia erecta).

Region Section Score / 10 FoC Score /1000 FoC Band

Darwin

A Control costs 6.4

342 very high B Current distribution 10.0

C Persistence 5.4



Control Costs Erect spiderling is easily controlled in highly restricted areas with the regular application of glyphosate. The highest cost is labour as it requires repeat visits every three weeks during the growing season to ensure that no plants are able to set seed. If erect spiderling were to become more widespread it would become impractical to control due to the rapid rate at which it is able to mature (3-4 weeks), its high seed production, and the inconspicuous nature of the plants which require targeted survey to detect.

Current Distribution The only known population in Australia is at a single property in the Darwin rural area.

Persistence Most of the seed bank emerges by the end of a single wet season, but some seed remains viable for at least 2 years (Holm et al. 1997).

5 Management Matrix The NTWRMS determines a recommended management responses for an assessed plant species by inputting the weed risk and feasibility of control outcomes into the Weed Risk Management Matrix (Table 3).

Table 3. Weed Risk Management Matrix indicating recommended responses for erect spiderling (Boerhavia erecta).

Feasibility of control

High-Very high Low – Medium

Wee

d ris

k

Very high Prevent movement; regional eradication; contain regional spread; protect priority sites

Targeted control (including biocontrol); protect priority sites

High Prevent movement; contain regional spread; protect priority sites Targeted control

Medium Targeted control; improve general weed management; monitor; protect priority sites Improve general weed management

Low Monitor; assist interested parties Assist interested parties

Darwin region

The recommended responses for erect spiderling in the NT from the management matrix are:

• monitor • assist interested parties.



6 General Management Recommendations

In addition, the Weed Risk Technical Committee made the following general management recommendations:

• Noting the highly restricted nature of the species in the NT, the significant impacts of this species on crop yields overseas, and the fact that this is the first reported incursion in Australia, maintain the existing Boerhavia erecta eradication program, so long as it remains feasible to eradicate.

• In collaboration with the Northern Australia Quarantine Strategy, conduct follow up surveys in the vicinity of the eradication site to determine:

o Whether the species has spread to adjacent or nearby properties; o Raise awareness among local landholders in relation to the species; and o Provide some confidence that extent of the incursion has been delineated.

• Ensure relevant NT Government officers are made aware of the species and are equipped to

identify and report new infestations.

• Reassess the viability of the eradication program in the event that Boerhavia erecta be detected at another location in the NT. At this time, if a decision is made to continue the eradication program, then it is recommended that consideration be given to raising a public alert in the immediate region.



7 Determinations – Weed Risk



8 Determinations – Feasibility of Control



9 Evidence Used – Weed Risk











10 Evidence Used – Feasibility of Control







11 References



12 Appendix – Summary of Method The weed risk and feasibility of control of a species is determined using published evidence, expert advice and observations that are presented to or provided by the NT Weed Risk Technical Committee. Members of this committee represent three broad interest groups—primary industry, environmental protection and weed management. Assessment is made against a standardised set of 39 questions:

• 25 relate to Weed Risk with sections on invasiveness, impacts and potential distribution; and

• 14 relate to Feasibility of Control with sections on control costs, current distribution and persistence.

The evidence is prepared and presented to the Technical Committee who then determine the outcome for each question by consensus. The Weed Risk and Feasibility of Control scores are calculated and then converted to a band (low, medium, high and very high). The Weed Risk Management Matrix uses the bands to provide broad management recommendations.

Weed Risk Weed risk is the quantified estimate of the relative risk of a weed having significant negative economic, environmental and/or social/cultural impacts. The model used calculates Weed Risk for the whole of the Northern Territory; however, it can be used to assess weed risk in a specific region or area when required.

Feasibility of Control Feasibility of control is the comparative ease or feasibility of controlling a weed species in a given weed management region. There are four Weed Management Regions in the Northern Territory - Darwin, Katherine, Tennant Creek and Alice Springs.

Scoring Weed Risk and Feasibility of Control are each determined with reference to three specific components, that being:

• Weed Risk — invasiveness, impacts and potential distribution. • Feasibility of Control — control costs, current distribution and persistence.

For each component, the scores are totalled and adjusted to be expressed out of 10. The total score is calculated by multiplying the three component scores, resulting in a score out of 1000. Multiplying scores acknowledges the interactions between the attributes. To get a high score, a species needs to score at least moderately well in each of the three sections. Conversely, if a species scores poorly in any section, it will result in a low score.

Scores are converted to bands (‘low’, ‘medium’, ‘high’, ‘very high’) for both Weed Risk and Feasibility of Control. The bands are based on all possible combinations of scores and indicate how high the score is compared to all possible combinations.

Table 1. Score comparison relating to band determination

Band Combinations Quartile Weed Risk Score

FoC Score

Very High Top 25% 76–100% > 198 > 294

High Next highest 25% 51–75% 133–197 168–293

Medium Next highest 25% 26–50% 84–132 44–167

Low Bottom 25% 1–25% < 84 < 44

For further information refer to the NT Weed Risk Management System User Guide, available at www.nt.gov.au/weeds/risk.

http://www.nt.gov.au/weeds/risk

Documents

Northern Territory Weed Risk AssessmentReport: Erect spiderling … · 2019-08-14 · Annual to short-lived perennial erect herb with a strong tap root, up to 1 m in ideal conditions