Mosquitoes are notoriously non-discriminate; they will take a blood meal wherever they can get one. They prefer areas that are marshy or full of puddles because they have ready-made places to lay eggs. A house that isn’t protected with pesticides means that entry and exit is easier.
In the Ngara district in northwest Tanzania is a small village with some of the ideal conditions for mosquitoes. Villagers lack the income to pay for the typical mosquito protection. Many are wary of chemical sprays used on their property or inside the house. Because the village was nearly ideal for mosquitoes, it was also ideal for a group of British researchers who wanted to test the effectiveness of an exotic invasive plant on repelling mosquitoes.
The idea of using one pest to control another is not new; however, using an exotic invasive plant to control a pest insect presented a new set of problems, as the research team discovered. The experiment also raises the recent debate over whether exotic invasive plants have any redeeming value in an ecosystem.
In Tanzania, the typical technology for mosquito prevention is a process called house screening. House screening involves modifications to a house including physical screens and insecticide treated curtains. Although the practice is common in urban areas, many people in the poorer rural areas cannot afford the $25 to implement the screening. In addition, many of those families live in mud brick houses where screens are not practical.
So in 2008, scientists from a large Tanzanian nonprofit, Concern Worldwide Tanzania, and from universities in London, England, decided to test the efficacy of plants known to have chemicals that repel mosquitoes. Their findings were published in a peer-reviewed article in PLoS ONE in October 2011. The scientists experimented with planting Lantana camara, an aggressive, non-native weed that grows rampant in eastern Tanzania.
The Ngara district is popular to migrating refugees from Burundi and subject to migrating Tanzanians seeking more land. The district is located in the Kagera region of Tanzania, one of the most remote regions in Tanzania, where 33% of the households fall below the poverty line. According to demographics collected during the study, most families had lived in the village for fewer than 5 years. The majority were also subsistence farmers, had low levels of education and had little disposable income. Many villagers were familiar with mosquito control and prevention, but several were not. Malaria-carrying mosquitoes are endemic to the region; in fact, a mosquito collection in 2008-09 revealed a plethora of Anophales gambiae mosquitoes, the vector for African malaria.
The study team was seeking a method of mosquito control that would be simple for villagers to use and affordable enough for them to be practical. Lantana contains caryophylene and alpha-pinene that are known to repel mosquitoes. Plants can be purchased for $1.50, and because the species is invasive, it is easy to grow but must be regularly pruned.
The team collected data for one year and had to discontinue the study after the government began their own mosquito spray program in the district. The data showed some modest reductions in mosquito populations, but they team had not collected population data before and after planting, so they could not attribute the reductions specifically to the Lantana.
Concern Worldwide planted Lantana only at 231 houses whose residents had requested the plants, and they collected data from 90 additional residents with no repellent plants. Reactions to the plants were mixed; some residents thought Lantana had reduced the number of mosquitoes around their home, while others complained about the amount of labor it took to keep the plants from becoming overgrown. One owner of a banana plantation removed the plants after he said that they invaded his fields and reduced his banana yield.
As has been found in other studies, knowledge about mosquitoes and prevention strategies varied. The baseline survey included a question about how malaria could be prevented, to which 41% of participants mentioned bednets and 14% mentioned filling in puddles. However, 12% thought that eating cleaner food could prevent malaria, and 21% did not know how to prevent it. In fact, only 81% of participants knew that mosquitoes caused malaria, and 60% knew that mosquitoes bred in standing water.
In terms of preventing mosquito bites, most residents said that they covered themselves in long clothes (79%), and 52% used bednets. However, 53% of residents did not have a bednet at all—one percent more than those who claimed to use one.
Probably the most interesting—and troubling—finding involves the attitudes of the majority of residents toward mosquito and malaria prevention. According to the data, residents perceived malaria as an inevitable part of life, while having to do additional labor to control an aggressive plant was considered a serious impediment to their quality of life. Researchers noted, however, that the increase in labor would need to be studied, especially since many children are involved in household labor. In addition, the study noted that the plants should be used in concert with other mosquito control technologies, not relied on as the sole pest management strategy.
In the United States, our war against exotic invasive plants involves issues much less opaque. Weeds typically reduce our quality of life, whether they are choking native vegetation in a public forest or reducing crop yield on a farm. Most families in the U.S. can afford some kind of spray for mosquitoes, and those who prefer not to spray deal with a pest that is, for the most part, just an incredible annoyance. It’s not often that we have to decide between preserving the health of native vegetation or saving people’s lives.
This post is not here to point out the benefits of invasive plants. Clearly, for the most part, their costs far outweigh any advantages. I pose the debate here purely for the sake of discussion and to tender a reminder that in some situations, the solutions are not always clear cut.