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Report: Cutting Food Source Leads to Dramatic Drop in Number of Mosquitoes

  • Ben Thompson

A photo of the shrub Prosopis juliflora. (Gunter Muller/Malaria Journal, 2017)

Insecticides, mosquito nets, and disrupting breeding grounds all reduce mosquito populations and slow the spread of malaria. Now, researchers want to take away the insect's food to fight the disease that kills a child every two minutes.

Mosquitoes mostly feed on plant sugars that can be hard to find during the dry season in Africa, where 90 percent of malaria cases develop. Researchers thought one potential source of food might be from the flowers on a small type of mesquite tree. The tree, imported from Mexico 40 years ago to provide firewood and shore up irrigation dykes, quickly became invasive and grew out of control.

To test their idea, researchers monitored mosquito populations in six villages in the Bandiagra District of Mali. After a week, they removed the flowers from the mesquite trees in half of the villages.

Anopheles gambiae mosquito, which spreads the malaria parasite
Anopheles gambiae mosquito, which spreads the malaria parasite

The report, published in Malaria Journal, found that with less food around, the mosquitoes didn't live as long and populations dropped 69 percent. This didn't just mean fewer mosquitoes, it meant fewer old mosquitoes. That's important because it takes 12 days for the malaria virus to get to the salivary glands of a mosquito where it could infect a human. So if mosquitoes die even a couple of days earlier, that could greatly reduce the number of mosquitoes that pose a threat.

"This suggests that removal of the flowers could be a new way to shift inherently high malaria transmission areas to low transmission areas," said Gunter Muller, lead author of the study from Hebrew University Hadassah Medical School.

Devil tree

But getting rid of mesquite is easier said than done. It's not as if people haven't tried to control the tree before. It encroaches on crop lands, makes areas inaccessible, and can use up what little water there is. It has been known to grow up though the floors of huts. Even getting to the flowers is a challenge, due to the 10-centimeter-long thorns that grow along the branches.

Getting to the flowers is a challenge, due to the 10 centimeter long thorns that grow along the branches.
Getting to the flowers is a challenge, due to the 10 centimeter long thorns that grow along the branches.

Many refer to it as the devil tree, but Medusa tree may be just as apt a name, since it can grow back from just its roots after it is cut down.

Biologist Dawn Wesson from the Tulane University School of Public Health and Tropical Medicine said this was one of the first attempts she has seen to control mosquito populations by restricting their food source.

Wesson, who was not involved in the research, highlighted that not only were populations depressed, but that the degree of impact varied greatly depending on the species of mosquito. In this case all of the species can carry malaria, but Wesson hopes that in other contexts this could be used to help a benign species of mosquito displace a dangerous species of mosquito. That impact could extend beyond the end of any food control measures.

Approach could backfire

But Wesson also cautioned that removing mesquite might backfire. Without flowers to feed on, these mosquitoes might turn to blood meals. This could lead to more frequent bitings and increased transmission of malaria. "It's probably unlikely," she told VOA. "They did show a nice decrease ... in the older female mosquitoes. But remember their study only took place over a period of about eight days."

The next step, she suggests, should be to measure the impact of removing mesquite, not just on mosquito populations, but also on the incidence of malaria.

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