Consultation on the Use of a New Biocontrol Agent to Tackle Mexican thorn
2 May 2023
Ascension Island Government, Press Release
The AIG Conservation Team is running a public consultation to get the public’s views on the introduction of a new biological control agent, a moth called Evippe, to slow the spread of Mexican thorn on Ascension
Mexican thorn is a highly invasive plant that is particularly well adapted to grow in arid to semi-arid environments like those found on Ascension. It has extensive lateral root systems to capture surface water after rain and deep tap roots (up to 30m), allowing it to survive prolonged drought by accessing the water table. It is fast-growing, nitrogen-fixing, and very salt and drought-tolerant. Having no natural enemies here on Ascension, it has thrived and became invasive.
The negative qualities of Mexican thorn include:
It monopolises groundwater, light, and nutrients and therefore easily outcompetes endemic and native vegetation such as the Ascension spurge.
The trees provide food and habitat for high densities of rats.
It poses a significant fire risk as Mexican thorn burns readily and considerable biomass now surrounds many human settlements, military installations, and commercial sites.
Roots can damage infrastructure such as roads, underground pipes, and wires.
The wide-spreading root system reduces the availability of suitable seabird and turtle nesting habitat.
Limits access, to infrastructure for maintenance or for recreational activities such as hiking.
Flowering branches attract and provide breeding grounds for mosquitoes.
Forms dense impenetrable thickets that can limit It monopolises available
Methods for controlling Mexican thorn
The Mexican Thorn Control Project is a Darwin Plus-funded project aimed at designing and conducting trials of chemical, mechanical and biological treatment methods for controlling Mexican thorn on Ascension. This process will allow AIG to produce an integrated Mexican thorn Control Plan, using several techniques, in an attempt to effectively manage and control the spread of this species. This plan will contain recommendations on the most effective treatments in different situations and will be made available to any interested parties that have to deal with controlling this invasive tree.
Mexican thorn is a tough plant and currently, multiple interventions are sometimes needed to kill a single tree. Mechanical and chemical control is usually physically demanding, labour intensive, slow, and therefore costly. Biocontrol is an entirely different way of controlling non-native species. It involves the release of a natural enemy (the biocontrol agent) from the problem species’ home range. If successful, the biocontrol agent will become established and continue to keep the problem species in check with no further effort.
What is being proposed?
AIG are proposing to release a new biological control agent, the Evippe moth. Research from Australia has shown that Evippe can cause over 90% defoliation within 12 months of its release and has since maintained similarly high rates of defoliation. As a result, Mexican thorn plants in Australia where Evippe has become established rarely produce much foliage. Repeated defoliation has contributed to the very low seed production and growth rates observed since 2000, and tree death is becoming apparent.
If this can be repeated on Ascension, long-term control of Mexican thorn may be possible meaning there will be much less need for costly and time-consuming cutting and spraying to protect infrastructure and Ascension’s native plants and animals.
How the risks have been considered
Once a biocontrol agent is released, it cannot be recaptured and so it is important to be certain it will have the desired effect and not damage other species. The Evippe moth has already been tested in Australia and South Africa. It only affects Mexican thorn and some very closely related species. To be certain it won’t affect Ascension’s plants, our partners in the UK have conducted further testing to see if Evippe would damage any of Ascension’s unique plants. Once again, these tests have shown that Evippe only targets Mexican thorn.
We can be confident that the Evippe moth will only affect Mexican thorn, but it is also important to consider if there would be any negative effects of stopping the spread and removing Mexican thorn.
The removal of Mexican thorn may facilitate the spread of other equally invasive weeds such as tree tobacco or yellow boy. It may also lead to dust and soil erosion. However, experiences from Australia indicate that Evippe will not necessarily result in the death of all plants, but the frequent defoliation is expected to make the canopy less dense and make the plants less able to regrow if they are cut and sprayed. The moth wouldn’t wipe out Mexican thorn on Ascension, but it would stop its spread and mean that efforts to remove it were effective for a much longer period. The continued presence of Mexican thorn, albeit with lower vigour, would mean it would, still function as a wind break to lower dust levels and soil erosion as well as deterring the invasion of other non-native plants. Although all non-native plants can be damaging, none of the other plants that could potentially take Mexican thorn’s place is anything as hard to remove.
The effects of releasing Evippe and the subsequent reduction in Mexican thorn cover on Ascension’s native wildlife were also considered. For the most part, Mexican thorn is damaging to our wildlife because of the loss of natural habitats and the harbouring of pests such as rats and ants. Very few endemic or native species occur in areas where Mexican thorn is present and a reduction in its cover will have many benefits and few negative consequences for wildlife. The only small loss might be to land crabs that sometimes use Mexican thorn for shade. However, land crabs evolved in the absence of Mexican thorn and are able to use other plants or rocks for shade. Evippe itself feeds only on Mexican thorn and so will not compete with any native species of insects.
The donkeys may also be affected by the loss of shade, but it should not necessarily affect their feeding habits. Their tough digestive system allows roughage, grass, leaves and stems to be efficiently broken down.
What is the aim of the consultation?
AIG Conservation would like to hear people’s views, thoughts, and ideas. Do you think Mexican thorn should be controlled? Do you think there is a strong case that the moth will contribute to this control? Do you have any concerns about the release of the moth?
We encourage everyone to come along to the public meeting at Saints Club at 19:00 on Wednesday, 10 May 2023. You can also email your thoughts to conservationenquiries@ascension.gov.ac or chrisna.visser@ascension.gov.ac. The closing date for responses is the 31 of May 2023.
Frequently Asked Questions
What is biological control?
It is the use of the plant’s natural enemies to control a problem plant population, in this case, the Mexican thorn on Ascension Island. Biological control offers a long-term, sustainable, and environmentally friendly option for managing this plant.
What is so bad about Mexican thorn?
It out-competes our native vegetation, some of which is at risk of extinction.
It provides shelter for pests such as rats, mosquitoes, and Indian myna birds.
Its roots can damage infrastructure such as roads, buildings, and pipelines.
Its presence on beaches and coasts could reduce suitable seabird and green turtle nesting habitats.
Poses a serious fire risk, especially around our settlements.
It forms impenetrable thickets which limit access to infrastructure for maintenance, recreational activities, or sites of historical importance.
Why can’t it just be controlled by cutting it back?
Mexican thorn is a hardy plant and multiple interventions are sometimes needed to kill a single tree which is labour-intensive and costly and usually limited to protecting the most sensitive sites and important infrastructure.
The addition of a biological control agent will mean that the natural enemy will feed on and damage the Mexican thorn, making them easier to manage through mechanical and chemical control methods and reducing the rate of spread across Ascension.
Why do you think this moth will work?
Research from Australia and South Africa has shown that Evippe causes extensive and repeated defoliation of Mexican thorn plants. This will cause very low seed production and growth rates and trees will stop flowering which is good for Mexican thorn control and management. In Australia, it happened in the Pilbara Region which has a similar climate to Ascension.
What will the island look like if the moth is released?
The defoliation caused by the moth will mean Mexican thorn trees will end up leafless for long periods.
In Australia, it caused over 90% defoliation within 12 months of its release and has since maintained similarly high rates of defoliation. As a result, trees in Australia where Evippe has been established rarely produce much foliage.
If we don’t like the effect the moth has, can we stop it?
No, once the moth is released we will not be able to recapture it. This is why it is so important to make sure it won’t have unintended harmful effects before it is released.
Won’t other plants just move in and take the place of Mexican thorn?
Yes, there is a possibility that other invasive species such as Yellow boy and tree tobacco may fill any gaps opening up in the vegetation. In this case, it is expected to still result in an improvement as these species grow less vigorously than Mexican thorn and are more easily controlled by other control methods.
What else can we expect to happen once the Mexican thorn is removed?
It is expected that there will be an increase in dust and soil erosion. However, we anticipate that Mexican thorn will remain present to a certain degree, albeit with a lower canopy density, where it can still function as a wind break to lower dust levels and in part reduce soil erosion compared to the situation before its introduction.
Will the moth attack any other plants on Ascension?
No, extensive testing has shown the moth only attacks Mexican thorn and very closely related species. It will not affect Ascension’s endemic plants or those growing in people’s gardens.
What will the donkeys eat if Mexican thorn dies?
Donkeys have other sources of food and it is usually only the seed pods of Mexican thorn that they eat during the periods when these are available. The trees do provide shade for donkeys, but there are other areas of shade available to them.
If the moth is successful will we never need to cut Mexican thorn again?
No, we will still need to control the spread of Mexican thorn. Experiences from Australia indicate that Evippe will not necessarily result in the death of all plants, but the frequent defoliation will weaken the plants and slow down the Mexican thorn’s population growth. Integrating chemical, mechanical, and biological control methods will deliver the best results for controlling Mexican thorn in the long term.
What will you do if the release of the moth doesn’t work?
The project will still continue to develop improved chemical and mechanical control methods, which will be implemented on the island to control Mexican thorn.
