Repelling the Invader: Turning the Tide on Ascension’s Mexican Thorn

Repelling the Invader: Turning the Tide on Ascension’s Mexican Thorn

 

23 May 2022

 

 

Update on Darwin Plus funded project

 

The Conservation and Fisheries Directorate (AIGCFD) hosted a public presentation at Two Boats Club on Wednesday 20 April 2022. It was aimed at introducing members of the public to the Mexican Thorn Control Project “Repelling the Invader: Turning the Tide on Ascension’s Mexican thorn”. This externally funded Darwin Plus project seeks to establish viable control methods to tackle the invasive Mexican thorn population in Ascension.

 

 

Type: Non-native Species | Status: Least Concern | Nativeness: Introduced

 

 

The presentation was well attended and was followed by a question and answer session with the project lead, Chrisna Visser.

 

For those that missed the presentation, below is some of the information covered:

 

An introduction to Ascension Islands Mexican Thorn

 

Where does it come from and why was it introduced to Ascension?

 

Mexican thorn is native to Central and South America and was brought to Ascension during the 1960s to help consolidate the soil around the Two Boats settlement after its construction.

 

Why does it do so well in Ascension?

 

It’s an arid specialist.

 

Mexican thorn grows multiple tap and lateral roots which can grow up to 30m in length, allowing groundwater to be monopolised. Seeds stay viable for long periods of time, even during drought. These trees also have the ability to regrow from the stump or roots after being cut or even chemically treated.

 

It’s a nitrogen fixer.

 

This tree is able to take nitrogen from the air and change it into nitrogen compounds that fertilise the soil. This feature helps the plant grow and compete for space, water and nutrients.

 

Allelopatic properties

 

The tree produces biochemicals that prevent other plants from growing around it.

 

No natural enemies here

 

Mexican thorn has the ability to spread and establish itself quickly across the island as it has no natural enemies to keep it in check.

 

Mexican thorn has expanded so that it is now the dominant plant species over much of the island. It is continuing to spread into new areas as well as becoming denser where it is already present.

 

What damage is this tree doing to our Island?

 

Out-competes our native vegetation, some of which is at risk of becoming extinct in the wild.

Supports pest species such as rats, mosquitoes and Indian myna birds by providing food and shelter.

 

Roots damage infrastructure such as roads, buildings and pipelines.

 

Extensive root systems reduce suitable seabird and green turtle nesting habitats.

 

Poses a serious fire risk, especially around our settlements.

Impenetrable thickets limit access to infrastructure for maintenance, recreational activities or sites of historical importance.

 

Control Methods being trialled

 

Through this project the Mexican thorn control methods being trialled both on island and off island can be split into three different categories:

Mechanical – The use of physical force to remove plants or damage plants to such an extent that it dies.

 

Chemical – Killing plants by using a registered herbicide.

 

Biological – Using the plants’ natural enemies to control host plant populations.

It is hoped that by trialling the various methods listed below, a suitably effective one can be identified to help begin tackling and managing the Mexican thorn across the island.

 

Control method	Types being trialled
Mechanical	1)     Bark stripping –        Bark is removed from the trunk between ground level and up to 1m above ground. 2)     Ring barking –        Used on trees with stem diameters greater than 150 mm. –        The bark and cambium (the layer below the bark) is removed in a continuous band around the trunk at least 25 cm wide, starting as low as possible. 3)     Uprooting –        The entire plant and root system is removed. 4)     Bulldozing road verges –        Heavy machinery is used to uproot or break trees down.
Mechanical and Chemical	1)    Cut stump –        Cut stump down as low as possible to the ground and herbicide is applied. –        Need a smooth surface to ensure optimal herbicide absorption. 2)    Foliar application –        Herbicide is applied directly to the leaves with a knapsack sprayer. –        Best results are expected in the growing season of the plant. 3)    Chemical frilling –        Downward cuts to the bark and cambium layer. Herbicide applied. –        The entire circumference of the trees needs to be treated.
Biological	AIGCFD and our partners the UK Centre for Agriculture and Bioscience International (CABI) are currently putting together a risk assessment for the possible introduction of the Evippe moth (Lepidoptera: Gelechiidae). This risk assessment will evaluate the effects this biocontrol agent will have on the island if introduced. The Moth has already been introduced to parts of South Africa and Australia where Mexican thorn has also invaded. In Australia the moth has greatly reduced the growth of Mexican thorn in areas where it has been released.

 

 

Will the Evippe moth be released on Ascension?

 

The use of biocontrol agents such as the Evippe moth can be an extremely effective way of controlling invasive species, as it creates a natural way to hold back the invader and restore balance. However, it is very important to consider all the possible consequences before any decision to introduce the moth is made to ensure it doesn’t have any unintended negative impacts.

 

This will be done by collecting data, conducting a risk assessment and then holding a public consultation to determine if the Evippe moth is the best way forward for controlling Mexican thorn on Ascension.

 

What will be considered in the Evippe moth biocontrol risk assessment?

 

We will assess the possible impact the moth may have on plants and animals already present in Ascension Island.

 

We will all consider what would happen if the moth is released and works well so that Mexican thorn is removed from large parts of the island. This will include whether other plants would fill its place, if dust and soil erosion would increase and whether there would be a loss of food and shade for donkeys and sheep.

 

The project team would like to say a huge thank you to all community members that attended and have shown such interest in the project. If members of the public would like further information or have questions about the project, please get in touch with the Mexican Thorn Project Control Officer by email through chrisna.visser@ascension.gov.ac.