Malaria incidence and invasive plants – is there a link?

Mosquito

25 April is World Malaria Day – a time to reflect on the steps we can take to tackle this terrible disease. Much progress has been made in the fight against malaria over the past 15 years, like the use of bed nets impregnated with pesticides, but 3.2 billion people are still at risk. If we are to achieve a 90% reduction in global malaria incidence and mortality by 2030 we must do more.

The path will not be easy. Mosquitoes are becoming increasingly resistant to pesticides – the front line of defence from malaria today. But there are other aspects we can consider, like the potential link between the incidence of malaria and invasive, non-native weeds.

It is widely known that mosquitoes need plant sugars, among other things, to survive and proliferate. Studies in Israel show that mosquitoes are much more likely (250 times more likely) to transmit malaria in areas rich in plant sugars. Could the improved management of invasive plants abundant on the African continent lead to a reduction in the incidence of malaria?

It is this question that brought together experts on malaria and plant invasions to a workshop in Kenya in December 2015, funded by the Bill & Melinda Gates Foundation. The broad objective of the workshop was to explore whether mosquitos benefit from invasive plants and whether these plants have a positive influence on the rate of malaria transmission. The workshop also looked at whether invasive plants could be managed on a large scale.

Experts agreed that access to particular plant sugars increases the ability of Anopheles mosquitoes to transmit malaria. Although it is not known if invasive plants produce more sugars, they are more widespread and abundant than native plant species. In fact, many have the ability to invade semi-arid and arid areas, possibly increasing the prevalence of malaria in regions where mosquitoes could not survive in the past. Invasive plants also actively grow and produce flowers and fruit for longer periods than native plants, thereby extending the availability of plant sugars over longer periods than in the past. This may allow mosquitoes to retain high population numbers for much longer periods in invaded areas than in areas where there are no suitable invasive plants.

If there is an obvious link between invasive or weedy plants andAnopheles mosquitoes, can we significantly reduce the incidence of malaria by managing invasive plants?

There is no doubt that problematic plants can be controlled. Landowners, especially farmers, do it all time. The Government of South Africa allocates approximately US$120 million a year to control invasive plants, especially in water catchments, biodiversity hotspots and protected areas. It also invests in biological or natural control of invasive species. This is considered one of the most cost-effective management options, ideal for use in developing countries that do not necessarily have the resources for chemical or conventional control.

So, we can control weeds but would it reduce the incidence of malaria?

Lowering the abundance and density of any plant species favoured by Anopheles mosquitoes should lower malaria incidence. Managing many of these non-native weeds will also result in a multitude of other benefits for poor rural communities – like protecting farmland, for example.

This possible malaria-invasives linkage must be explored further. We need to do more research to fill in knowledge gaps. This includes looking at what plant species the Anophelesmosquitoes use within a given environment. Methodologies are being developed to see if rapid assessments of mosquito gut contents can provide information on what plant species they have been feeding on. We also need to look at the impact of removing certain species of invasive plants on mosquitoes. If we can compare mosquito abundance, longevity and ability to transmit malaria in areas where the invasive plant is dominant and where it is less dominant, we can build a fuller picture of the potential problem and solution.

Malaria is a terrible disease that still affects too many people. We must do all we can to understand the possible link between the incidence of malaria and invasive, non-native weeds. If a link can be found, management of invasive weeds could offer hope to many living under the threat of malaria.

By Dr Arne Witt, Coordinator, Invasive Species, CABI

For more information about CABI’s work managing invasive species, click here.

New in July 2015 from the ISC

In July 2015 the following datasheets were published on CABI’s Invasive Species Compendium (ISC). You can explore the open-access ISC here: www.cabi.org/isc

July invasivesAkebia quinata (five-leaf akebia) – a highly invasive, aggressive vine native to East Asia, A. quinata has been introduced as an ornamental to Canada, Europe, Oceania and the USA. It can outcompete native understory plants and young trees, and its dense growth can block sunlight and prevent the germination of native plants.

Geophagus brasiliensis (pearl cichlid) – an ornamental freshwater fish native to southeast Brazil, G. brasiliensis has been introduced to Australia, Florida, the Philippines and Taiwan. Its fast growth, opportunistic diet and broad environmental tolerances have allowed it to colonize new waterways, particularly artificial and disturbed habitats.

Rudbeckia laciniata (thimbleweed)R. laciniata is an ornamental perennial plant that has been introduced to China, Japan, New Zealand and Europe. Native to eastern North America, thimbleweed grows best in bright, humid areas, such as wetlands, forest edges and roadsides. By producing lots of seeds and spreading from rhizome fragments, it can form dense monocultures which outcompete native plants.

Other invasive species datasheets recently published include:

Brugmansia suaveolens (white angel’s trumpet)
Chrysemys picta (painted turtle)
Macaranga tanarius (parasol leaf tree)
Paederia foetida (skunkvine)
Umbra pygmaea (eastern mudminnow)

Villagers front-line in the battle against Prosopis in Ethiopia

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Prosopis invading a flood channel near Badahamu

Paul Rogers, Business Development Manager at CABI, visits Badahamu to understand how invasive weeds affect their livelihoods

As we arrived in Badahamu, in eastern Ethiopia, negotiating the by-now familiar thickets of Hara Dergi, “Derg weed” as Prosopis is known in the local Afar language, the impact of last autumn’s floods were painfully apparent. We passed rows of half-destroyed and half-built houses and were warned to avoid local water due to the risk of cholera, which is now endemic.

The story of Badahamu is common across southern Afar. One best told in the words of its inhabitants, it is a tale of the usual challenges of drought and flood being complicated by an invasive species, which removes land from productive cultivation and pasture, and challenges the wealth and identity of households and communities.

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Food Aid being delivered to Badahamu

The real human impact of this stressed and vulnerable habitat was apparent when we found the group of community members who we had arranged to interview. They had gathered near the village storage barns, where the latest consignment of food aid was being unloaded. A hundred or more villagers were waiting around the trucks, keen to see what was being delivered and what would temporarily alleviate their chronic hunger.

When we started to discuss Hara Dergi the community listed a number of critical indigenous species that are disappearing due to the invasion of three-quarters of their land. These included trees like cassalto (Acacia nilotica), which is used to feed camels, build houses and as a fuelwood, angalita (Cadaba rotundifolia), which goats and camels browse on, and hedayito (Commiphora habessinica) which provides good grazing. When it invades the banks of the Awash river, the region’s lifeblood, it causes silting which obstructs and deviates the flow, increasing the frequency and severity of floods and reducing the community’s ability to manage dry season droughts.

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Focus group respondents at Badahamu

There were bitter complaints over the cost of clearance, which places a significant financial burden on households and the community. The fierce thorns of Hara Dergi are causing both lameness and jaw deformity in livestock, and is likened locally to “AIDS for cattle”. It is also reducing income that could be used to fund local measures to control the spread of the weed.

It is not just the animals that suffer from thorn injuries – the local administrator estimated that the thorns had caused over 30 disabilities within the community, a human tragedy worsened by the effect Prosopis has on obstructing access to the local health centre.

Hunger and despair are driving the community to increasingly drastic and destructive measures. The encouragement, and then discouragement, of charcoal production by the government has caused anger and conflict amongst pastoralists who suspect that the few remaining local trees, rather than the intended Hara Dergi, are being used. Then there is the sugar cane plantation.  Positioned adjacent to the community’s lands as a tauntingly green, year-round oasis, over 15 of their members have been imprisoned for illegally grazing their herd around its edges. Despite warnings and sanctions, the scant rewards outweigh these evident risks.

This is the real tragedy. Such is the devastating impact on their livelihoods, and the desperation that this causes, that members of the community are engaging in illegal activities that risk ostracization by officials whose support is crucial in improving their circumstances. Once proud, the community now recognizes that it cannot confront Hara Dergi alone but is coincidentally conscious of the fact that it was introduced not by them, but by foreigners – tempering shame with indignation. Whoever’s ultimate responsibility it is, a solution will only be found through collective action, action which above all must be quick.

New edition of weed biocontrol catalogue gives information on more than 2000 releases

The Plantwise Blog

Himalayan balsam infected with Puccinia rust Himalayan balsam infected with Puccinia rust – a method of biocontrol being used in the UK. Photo credit: Rob Tanner © CABI

The fifth edition of Biological Control of Weeds: A World Catalogue of Agents and Their Target Weeds has been released after years of literature searches and the involvement of 125 weed biocontrol specialists.

The publication of this catalogue, available as a searchable online database and as a PDF book, was led by Mark Schwarzländer, University of Idaho CALS professor of entomology and biological control of weeds (and a former CABI researcher), and current CABI biological weed researcher, Hariet Hinz. Several prominent invasive species researchers co-edited the catalogue, including CABI’s Chief Scientist, Matthew Cock.

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Tuta absoluta, a new invasive invading India

Additional information about Tuta absoluta can be found on the CABI Invasive Species Compendium http://www.cabi.org/isc/datasheet/49260

The Plantwise Blog

Tuta absoluta (commonly known as tomato leaf miner) is a devastating pest of tomato which originated from South America. It can breed between 10-12 generations a year and each female can lay upto 250-300 eggs in her life time.  This pest has been very quickly crossing borders and devastating tomato production in both protected and open fields. The infestation of this pest is also reported on other solanaceous crops like potato, aubergine and common beans. The pest has spread from South America to several parts of Europe, entire Africa and has now spread to India. This pest is observed for the first time infesting tomato crop in Maharashtra, India reported by Indian Council of Agricultural Research. It has a potential to cause up to 90% loss of yield and fruit quality under greenhouse and field conditions. Plants are damaged by larval stages by direct feeding on leaves, stems, buds, calyces, young fruit, or…

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The Plantwise Blog

A species of whitefly that transmits cassava mosaic virus has been detected in South Africa for the first time. The whitefly, Bemisia tabaci is a cryptic species complex containing some important agricultural pests and virus vectors. The term ‘cryptic species complex’ means that Bemisia tabaci is considered to be a complex of at least 24 different species that look almost identical but are in fact genetically different.  Researchers from a range of organisations including the University of Johannesburg, the University of Witwatersrand and ARC-Vegetable and Ornamental Plant Institute conducted surveys to investigate the diversity and distribution of Bemisia tabaci species in 8 provinces in South Africa. The study aimed to update the information regarding the different Bemisia tabaci types present in the country.

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The Plantwise Blog

Researchers are working towards developing a cost effective solution to controlling  Ash Dieback fungal disease, a major threat to 80 million ash trees in the UK. As part of the plan to tackle Ash Dieback and other invasive pests and diseases, the government has formulated a team of ten internationally recognised experts in plant health, forestry and wider related disciplines as part of the Tree Health and Plant Biosecurity Taskforce. The taskforce includes three entomologists, Professor Charles Godfray from Oxford University, Professor Simon Leather from Harper Adams University College and Professor John Mumford from Imperial College as well as a number of social and environmental scientists.

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