Study Shows Removal of Invasive Shrub Can Help Reduce Malaria Transmission
Removing the flowers of the invasive shrub Prosopis juliflora from mosquito-prone areas might be a simple way to help reduce malaria transmission, according to a new study co-authored by John Beier, Sc.D., a world-renowned University of Miami Miller School of Medicine entomologist, professor of public health sciences and director of the Division of Environmental and Public Health.
The collaborative study, published in the open access Malaria Journal, revealed that removing the flowers from villages in Mali decreased the local mosquito vector population by nearly 60 percent. The study, carried out in the Bandiagra District in Mali, is the first of its kind to try a direct environmental manipulation as a way to control mosquito vector populations in areas at risk of malaria transmission.
“The presence or absence of Prosopis juliflora in villages has a significant influence on the size of the mosquito population in general, on their species composition and on their feeding source,” said Beier. “As well as offering a potentially environmentally reasonable and sustainable strategy in reducing the incidence of malaria, there are also other benefits to be gained from removal of these plants. For example, these plants are known to encroach on crop and pasture lands.”
The study was led by Gunter Muller, M.D. Ph.D., of Hebrew University Hadassah Medical School. “Mosquitoes obtain most of their energy needs from plant sugars taken from the nectar of flowers, so we wanted to test the effect removing the flowers of the shrub Prosopis juliflora would have on local mosquito vector populations,” he said. “Our results show that removal of this particular shrub reduces total population levels of mosquitoes and reduces the number of older female mosquitoes in the population, which are known to more often transmit malaria parasites to humans. This suggests that removal of the flowers could be a new way to shift inherently high malaria transmission areas to low transmission areas, making elimination more feasible.”
The investigation focused on the removal of the flowers of the invasive shrub Prosopis juliflora, which is native to Central and South America but was introduced to new areas in the late 1970’s and early 1980’s as an attempt to reverse deforestation. Prosopis juliflora is a robust plant that grows rapidly and has become one of the worst invasive plants in many parts of the world. The shrub now occupies millions of hectares on the African continent, including countries such as Mali, Chad, Niger, Ethiopia, Sudan and Kenya.
Light traps to catch mosquitoes were set up across nine villages in the Bandiagra District, six of which were home to flowering Prosopis juliflora and three that had no presence of the shrub. After a first round of analysis was conducted to assess mosquito populations, the researchers cut all the flowering branches from Prosopis juliflora in three of the six infested villages, before setting up light traps to determine the effect removal of the shrub had on mosquito vector populations.
The researchers found that villages where they removed the flowers saw mosquito numbers collected in the traps fall from an average of 11 to 4.5 for females, and 6 to 0.7 for male mosquitoes. The total number of mosquitoes across these villages decreased by nearly 60 percent after removal of the flowers. The number of older, more dangerous vector females in the population dropped to levels similar to those recorded in the villages that had no presence of the shrub. Villages infested with Prosopis juliflora also had a higher proportion of mosquitoes with a sugar meal in their gut, which enhances their survival. This proportion was reduced five-fold following removal of the flowers.
According to the researchers, it may be worthwhile to abstain from the introduction of exotic plants that have the potential to become invasive, not only because of their potential negative impacts on the environment and livelihoods, but because some of them may have significant negative consequences for public health and specifically for malaria.