Using carnivorous plants to control malaria-transmitting mosquitoes

Malaria remains one of the major vector-borne diseases in the world. It is responsible for approximately one million deaths per year, mainly of children below the age of five, particularly in Sub-Saharan Africa [1]. Mosquitoes of the genus Anopheles are the sole vectors of malaria in subSaharan Africa [2]. Current vector control methods are based on larvicide application in breeding sites, indoor residual spraying (IRS) and/or insecticide-treated bednets (ITNs). Larvivorous fish have also been used to control immature stages in breeding sites. However, insecticide resistance poses a serious threat to sustainable insecticidebased vector control in many African countries. Insecticides may also pose serious environmental and human health threats. These insecticide-based control methods are not sufficient on their own to reduce or even eliminate the malaria burden. There is therefore an increasing need to develop novel vector control methods that can complement or replace existing intervention tools. Carnivorous plants trap and kill their prey, including mosquitoes and their larvae, by ingenious pitfalls and traps, and then use their prey as a source of nitrogen for protein synthesis. They have modified leaves that trap insects by pitfalls (Waterwheel), flypaper traps with sticky mucous and snap traps (Sundews) or bladder trays that suck the insects (Bladderworts). Waterwheels and Sundews also have brightly colored flowers that attract insects including mosquitoes. The shiny droplets of sticky fluid attract mosquitoes [3,4]. Twenty three species of carnivorous plants have been described in Uganda. They are classified into those that grow in water (aquatic) and those growing on land (terrestrial)[5]. The Drosera spp. (Sundews) grow on land whereas Utricularia (Bladderworts) and Aldrovanda L. (Waterwheel or waterbug plants) are found floating on water. There are three species of the genus Drosera distributed throughout Uganda. Drosera madagascariensis and D. burkeana are found in southwest Uganda while D. indica is found in the Northern District of Gulu. Nineteen species of Utricularia and one species of Aldrovanda (vesiculosa) have been found in various places in Uganda. Drosera madagascariensis, with its leafy stems, offers high chances of trapping mosquitoes that may not necessarily be attracted to the shiny sticky leaves but that would be flying around. At the same time, the leaves of D. burkeana, occurring in basal rosettes, would enhance the visibility of the sticky leaves hence increasing the chances of attracting mosquitoes. The aquatic Aldrovanda and Utricularia plants are suitable for mosquito larvae capture because they can be viewed as motile if the water is a little agitated, hence increasing their chances of finding prey. Those growing in water trap and eat small insects and may be able to trap Anopheles larvae while those on land may trap and eat adult mosquitoes [3]. These properties imply potential for them to be used as biological control agents against malaria-transmitting vectors. We hypothesise that if propagated and deployed appropriately in and around houses and in mosquito breeding sites (like stagnant water), these plants may reduce the population of adult mosquitoes, thereby reducing transmission of malaria.