By enhancing the energy reserves of mosquitoes, an invasive American weed could be setting back malaria control measures in Africa, according to new work published in PLoS ONE this month.
Climate change is already negatively impacting human health by sustaining the spread of vector-borne diseases. And now invasive plant species might be making it worse. Parthenium hysterophorus, also known as the Santa Maria feverfew, is a highly aggressive invasive weed. Native to the subtropics and tropics of North and South America, this notorious invader has now made its way to Africa, Asia, and Australia. It grows especially well in parts of East Africa where malaria is a major concern. And while some of the plant’s compounds are toxic to humans and livestock, previous work found that the important malaria vector, the mosquito Anopheles gambiae, is highly attracted to the flowering weed, feeding on its nectar frequently. But until now there’s been no evidence that the weed improves mosquito survival.
A team led by Baldwyn Torto from the International Centre of Insect Physiology and Ecology wanted to investigate the impact of the neotropical weed and its toxins on the survival and energy reserves of Anopheles gambiae, compared with two other plant species that are abundant in malaria endemic parts of western Kenya: the castor bean plant (Ricinus communis), which is toxic to humans, and the hairy beggartick (Bidens pilosa), which is consumed as a vegetable by people. The researchers fed batches of 200 one-day-old female mosquitoes one of five diets: water, glucose solution, or nectar from the feverfew, castor bean, or beggartick.
The Santa Maria feverfew, they found, does improve mosquito survival. The bloodsuckers survived the longest on the glucose solution, followed by castor bean, feverfew, beggartick, and water – in that order. The mosquitoes accumulated more energy reserves when they were fed feverfew and castor bean plants compared to beggarticks, which have the lowest sugar content.
When the researchers examined the gut contents of the mosquitoes, they found that toxic plant compounds had indeed been ingested. That means the mosquitoes can tolerate – or perhaps clear from their system – much of the plant substances that are toxic to us. They couldn’t, however, tolerate the toxins produced by beggarticks.
If invasive plants are replacing less competitive species that are also less suitable host-plants for disease vectors, that means the spread of plant invaders could lead to higher disease transmission. According to New Scientist, the team is now studying the impact of the feverfew’s toxins on mosquitoes infected with the malaria parasite.