Devastating weed that favours mosquitoes

In a study published on 20^th July 2021 (https://rdcu.be/cpfvh), the Centre demonstrates that the weed, which has aptly earned the alias of ‘famine weed’ due to its phenomenal adverse impact on people’s health, agriculture, livestock and the environment, has contrastingly favourable effects on Anopheles mosquitoes, which transmit the malaria parasite. Also, the researchers note the possibility of exploiting the Parthenium-mosquito relationship to control the insects. 

“In general, mosquitoes lay their eggs in standing water. However, we have established that Parthenium releases from  its roots, chemicals known as terpenes that have a distinct blend of mosquito-attractive fragrances. When these chemicals leak into stagnant water, they enhance its attractiveness as an egg laying site for mosquitoes, in comparison to plain water,” explains Prof. Baldwyn Torto, Head, icipe Behavioural and Chemical Ecology Unit (BCEU). 

He adds: “As our research further demonstrates, this preference has major implications on the ability of mosquitoes to survive and thrive. The Parthenium root chemicals enable mosquito larvae to emerge two to three days earlier, and they also extend the lifespan of the adult mosquitoes arising from the contaminated breeding sites to a week longer than normal, thus boosting their chances to bite people and transmit the malaria parasite.”
 

Seminal knowledge

These findings are especially significant considering that Parthenium – a native of North and South America and one of the world’s most devastating invasive plants – is widely spread across East Africa including in flooding-prone malaria endemic zones. Parthenium aggressively colonises its invaded regions, killing other plants and reducing crop yields. It also produces a highly toxic compound called parthenin that causes dermatitis, hay fever and asthma in people, poisons animals and contaminates meat and dairy products in livestock that has fed on it.
 
In 2015, icipe published a seminal study that made the first global connection between Parthenium and mosquitoes. The research demonstrated that the weed is a preferred nectar source for Anopheles mosquitoes and it can sustain these insects by extending their lifespan even in the absence of a blood meal from people. Moreover, female Anopheles mosquitoes that feed on Parthenium survive longer, accumulate substantial energy reserves and they are capable of laying more eggs. Importantly, the researchers found that parthenin does not have the same toxic effect on adult female mosquitoes as it does on people and animals, indicating that the insects can tolerate and possibly detoxify themselves of the compound.

Prospective solutions

“Our recent findings present a silver lining in that the chemical fragrances found in the roots of Parthenium could be used as a bait in combination with traps, to selectively capture pregnant female mosquitoes seeking egg laying sites,” explains Trizah Milugo, a Kenyan student who conducted the study as part of her PhD research based within the icipe BCEU. “We also noted that only half of the eggs deposited in water containing these chemicals hatched. We singled out parthenin as being responsible for the low egg hatch rate, meaning that  female mosquitoes can compensate the cost of exposing their juveniles to plant toxins for improved survival as adults.”   
 
“Globally, invasive species are considered one of the most important perils to nature due to their severe  impact on many socio-economic aspects,” notes Dr Segenet Kelemu, icipe Director General & CEO, “Africa is one of the most susceptible regions, with a long and diverse list of such menace. Therefore, icipe has prioritised the management of invasive species as a key area of focus.”
 
In addition to the Centre’s invasive species initiatives, icipe and partners have developed a Strategy for Managing Invasive Species in Africa (2021 – 2030), which focuses on a three-stage approach of prevention, early detection and control, and a ‘One Health’ focus to tackle the cross-cutting impact on human, animal, plant and environmental health.
 

Notes for Editors

Research details: Milugo TK, Tchouassi DP, Kavishe RA, Dinglasan RR and Torto B (2021) Root exudate chemical cues of an invasive plant modulate oviposition behavior and survivorship of a malaria mosquito vector. Scientific Reports 11:14785 https://doi.org/10.1038/s41598-021-94043-5
 
Collaborating institutions: International Centre of Insect Physiology and Ecology (icipe); Kilimanjaro Christian Medical University College (KCMUCo), Tanzania; Department of Infectious Diseases & Immunology, College of Veterinary Medicine, Emerging Pathogens Institute, University of Florida (UF), Gainesville, FL, USA.
 
Corresponding author: btorto@icipe.org
 
Further reference: Nyasembe VO, Cheseto X, Kaplan F, Foster WA, Teal PEA, Tumlinson JH, Borgemeister C and Torto B (2015) The Invasive American Weed Parthenium hysterophorus Can Negatively Impact Malaria Control in Africa. PLoS One 10 (9): e0137836. https://doi.org/10.1371/journal.pone.0137836
 
Funding: This research was supported by DELTAS Africa Initiative (grant # DEL-15-011 to THRiVE-2) and International Foundation for Science (IFS) grant number: I-1-F-6127-1. The DELTAS Africa Initiative is an independent funding scheme of the African Academy of Sciences (AAS)’s Alliance for Accelerating Excellence in Science in Africa (AESA), supported by the African Union Development Agency-NEPAD
(AUDA-NEPAD) with funding from the Wellcome Trust grant # 107742/Z/15/Z and the UK government. We acknowledge icipe’s core donors: Swiss Agency for Development and Cooperation (SDC), Switzerland; Swedish International Development Cooperation Agency (Sida), Sweden; UK’s Foreign, Commonwealth & Development Office (FCDO); Ministry of Education, State Department of University Education and Research, Kenya; and Government of the Federal Democratic Republic of Ethiopia.
 
The International Centre of Insect Physiology and Ecology (www.icipe.org): Our mission is to help alleviate poverty, ensure food security, and improve the overall health status of peoples of the tropics, by developing and disseminating management tools and strategies for harmful and useful arthropods, while preserving the natural resource base through research and capacity building.