Invasive plant increases malaria transmission

In their article published recently in Malaria Journal, Gunter Muller and colleagues show that the invasive plant, Prosopis juliflora, promotes malaria parasite transmission, which could hamper vector control efforts. We talk to the team in more detail about their research and its implications.

Please could you briefly explain the results of your research?

In order to explain the results, it is important to put them in context of the study we carried out.  Basically, our study was a novel combination of natural comparisons and manipulative experiments. The effect on Anopheles mosquito populations was examined through a habitat manipulation experiment that removed the flowering branches of highly attractive Prosopis juliflora from selected villages in Mali, West Africa.

We knew from previous experience that malaria vectors of the Anopheles gambiae complex benefit from the presence of sugar rich plants, especially during the dry season. However, the change in mosquito species composition, following flower removal of this plant, was a particularly intriguing observation, as well as the drastic changes in population density, age structure, and sugar-fed status.

…our study provides the first field-based definitive evidence that malaria vector species of the An. gambiae complex during the dry season use the sugar present in nectar of an invasive alien shrub, Prosopis juliflora.

To put this in context, we found that villages where flowering branches of the invasive shrub P. juliflora were removed experienced a three-fold drop in the older more dangerous Anopheles females who are the predominant carriers of malaria. Total mosquito population density dropped by 69.4% and the species composition shifted from being a mix of three species of the Anopheles gambiae complex to one dominated by An. coluzzii. The proportion of sugar fed females dropped from 73% to 15% and males from 77% to 10%. The results are extremely important because, to our knowledge, our study provides the first field-based definitive evidence that malaria vector species of the An. gambiae complex during the dry season use the sugar present in nectar of an invasive alien shrub, Prosopis juliflora. We also show this source contributes significantly to the malaria transmission potential of vector species by contributing to the longevity and reproductive capacity of males and females. The ability of P. juliflora to survive and flower throughout the year, including during the dry season, may effectively extend the malaria transmission season in invaded areas, and therefore this new finding has implications for malaria control efforts.


Why are invasive plants beneficial to vectors such as the Anopheles mosquito?

Most invasive plants are abundant and widespread within a given habitat – more so than native or indigenous species. They not only grow rapidly but produce more flowers and seeds than their native congeners. In addition, invasive plants tend to have an extended growing period compared to native plants and in many cases also flower for significantly longer. Anopheles mosquitoes therefore benefit from the abundance of nectar produced by copious numbers of flowers and have an abundant supply of nectar for considerably longer periods than if the invasive species was not present. This will contribute to an increase in mosquito abundance and vectorial capacity – all other things being equal it may extend the period of malaria incidence in a community by a few months. However, it should be noted that not all invasive plants are attractive to Anopheles mosquitoes.

Why did you look at the invasive plant Prosopis juliflora specifically? Can your results be extrapolated to invasive plants in general, or are they more specific?

Prosopis juliflora and its hybrids are widespread and abundant in many parts of the world, including many semi-arid regions in Africa. In fact it is regarded as being invasive, among others, in Mali, Niger, Sudan, Somalia, Djibouti, Ethiopia, Kenya, Tanzania, Malawi, and Mozambique. The fact that it is extremely invasive, having invaded millions of hectares of land, forming vast monocultures make it an ideal candidate for such studies. In addition, previous studies have indicated that it is attractive to Anopheles mosquitoes but no studies were ever undertaken to validate this preliminary finding.

There are indications that other invasive plants such as famine weed (Parthenium hysterophorus), candle bush (Senna didymobotrya), castor-oil plant (Ricinus communis), and others are also attractive to mosquitoes but then again others are not. However, few have been tested – there are hundreds of invasive plant species in Africa. As such more studies need to be undertaken to determine which invasive plants are attractive to mosquitoes. What makes them all similar is, as I have explained above, that they are widespread and abundant, produce lots of flowers and seeds over extended periods, and actively grow for longer compared to native congeners.

How did local people react to your study? Do they find the invasive shrub a nuisance, or has it become useful to the local economy? 

Prosopis species were, in most cases, initially introduced to restore degraded lands. They have also been promoted as a source of fuelwood and their pods can be used to feed for livestock in limited quantities. However, they also have negative impacts. Dense stands reduce the amount of forage for livestock and displace other valuable plant species; invasions significantly reduce underground water resources; plants encroach on paths, villages, homes, crop- and pasturelands; invasions have contributed to the abandonment of agricultural land, and in some cases of homes and small villages; the pollen has been identified as a major allergen and the thorns can cause serious injuries. In fact P. juliflora has depleted the natural resources on which many thousands of people depend, spawning conflict between communities over the diminishing resources.

Although no impact studies have been undertaken in Mali we assume that impacts will be similar to those recorded elsewhere. The findings of this study may give impetus to communities in Mali to control prosopis.

That being said, the P. juliflora problem in Mali has not reached the widescale level that it has in other countries such as South Africa. Though some use P. juliflora for fuelwood, the locals in Mali did not see the presence of the shrub as particularly positive or negative. We did make sure to consult with communities about the nature and purposes of the study obtaining permission before removing the flowering branches, and when the villagers were made aware of the P. juliflora problem experienced in other countries, they became very concerned

What course of action would you recommend to the general public and local authorities on the management of Prosopis juliflora, specifically, and invasive plants in general?

In a nutshell – create awareness about the negative impacts of invasive Prosopis species and initiate control, with an emphasis on the introduction of host specific and damaging biological control agents.

The biggest barriers to invasive plant management, especially in developing countries, are a lack of policies or implementation thereof; little awareness as to the threats posed by invasive species; insufficient capacity to deal with the problem; and a lack of management activities. It is imperative that we develop and implement appropriate policies; create awareness as to which invasive plants are present, where they are problematic and what their impacts are; provide training as to how best they can be controlled; and develop and implement management strategies. The latter should include prevention (we need to prevent the introduction of invasive plants in the first place); surveillance to detect new invasions early so that they can be contained and possibly eradicated; and finally control which should follow an integrated approach (the use of cultural, manual, chemical and biological control, preferably in combination). Biological control, which is the use of host specific and damaging natural enemies, should be actively promoted, especially in the case of invasive Prosopis species – it is the most cost effective and sustainable management intervention.

In a nutshell – create awareness about the negative impacts of invasive Prosopis species and initiate control, with an emphasis on the introduction of host specific and damaging biological control agents.

Are there native plants that reduce malaria transmissions? And if so, are they being pushed out by successful invasive plants?

To our knowledge there are no native or ornamental plants that reduce Malaria transmission.

What will you be looking at next in your research?

We will be looking at ways to use a tool called Attractive Toxic Sugar Baits (ATSB), which uses highly attractive plant volatiles “spiked” with oral insecticide and combined with sugar (as a feeding stimulant) to attract and kill mosquitoes. ATSB’s were highly successful in reducing Anopheline and Culicine populations in Israel when sprayed on attractive blossoms or used as portable bait stations. We will be looking at ways to use ATSB’s in combination with highly attractive invasives such as P. juliflora to kill these dangerous mosquitoes.

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