With dengue cases in Malaysia rising to an all-time high of nearly 80,000 cases and 113 deaths from January to 3 August this year, the search for an effective solution to eradicate dengue continues to intensify.
Controlling the spread of dengue is challenging because the Aedes aegypti and Aedes albopictus mosquitoes — the vectors for the dengue virus — are co-adaptive species that thrive in urban environments, says Dr Zoe Yek, an ecologist and senior lecturer at Monash University Malaysia’s School of Science.
Dengue – an urban problem
Of the 251 hotspots identified in nine Malaysian states (with the greatest number in Selangor), most were apartments.
Undertaking a study in the Kota Damansara Community Forest Reserve and Bukit Gasing forest area, Dr Yek and her students found 26 species of mosquitoes. However, Aedes aegypti and Aedes albopictus are the only two types found in urban areas in Bandar Sunway.
Dr Yek says that in the forest, there’s biological control for Aedes mosquitoes — natural predators such as fish, crustaceans, carnivorous plants and other mosquito species that prey on their larvae. The cooler temperatures in the forest also mean it takes two to four weeks for an Aedes mosquito to complete its life cycle.
“In warmer urban temperatures, it takes only a week. With no natural predators for the Aedes, there’s a population explosion,” says Dr Yek.
Garbage disposal is the more significant issue at hand when it comes to controlling dengue, as stagnated water collected in garbage creates the perfect environment for mosquitoes to swiftly breed. “The government should encourage the use of closed garbage bins, so there is no standing water,” Dr Yek says, adding that waste should be disposed of responsibly.
Fighting fire with fire
Whether rearing guppies in a home-kept pond or the government’s recent release of Wolbachia bacteria-infected Aedes mosquitoes, biological control measures are already in use in Malaysia for dengue control.
The Wolbachia method involves introducing Aedes mosquitoes that have been infected with the Wolbachia bacteria to breed with the existing Aedes population, with those infected having a reduced ability to transmit viruses to people. Female Wolbachia-infected Aedes mosquitoes are also sterile and unable to reproduce.
While the method has been successfully implemented in Queensland, Dr Yek says more research is needed in Malaysia to adapt it to the local Malaysian environment. “Even if the same strength of bacteria is used, the effectiveness is dependent on the local population of mosquitoes, and there are also different strains of Wolbachia. Tests need to be done to see if they can survive in Malaysia,” she says.
Going back to nature
She suggests that another form of biological control worth exploring is the introduction to the urban environment of more diverse flora and fauna.
“Most urban parks today are too manicured, with structured trees and trimmed plants that don’t support wildlife. We need more natural green parks to introduce ecological ways of controlling the mosquitoes, more natural predators of Aedes,” she says.
Planting different flowers in home gardens and employing “rewilding” in parks – conservation aimed at restoring and protecting natural processes and core wilderness areas – can reintroduce wildlife diversity in cities.
“When you have more flowers, you have more insects and birds. When you cut shoots, you kill the food source of caterpillars and beetles. You can encourage the rewilding process in lots of places by just leading a patch of greenery and allowing it to grow wild. Don’t use pesticides and don’t keep the structure,” Dr Yek says.
"We need more natural green parks to introduce ecological ways of controlling the mosquitoes, more natural predators of Aedes."
Horticulture is another option Dr Yek is exploring with her students in addressing Aedes mosquito populations. Hobbyists popularly cultivate carnivorous plants such as the rainbow plant, pitcher plant and sundews plant, which feed on insects.
“By the end of my students’ three-month research project, we’ll know whether the rainbow plant has potential or not. If effective, we’ll try to identify the enzyme the plant uses to attract the mosquito and extract it for application purposes,” she explains.
Fogging restraint urged
Citing research findings from her students’ research on the effectiveness of fogging in urban areas, Dr Yek says fogging should be used with restraint, as excessive usage can render it counter-productive.
“My student looked at the frequency of fogging on eight condominiums and compared it to mosquito numbers before and after fogging sessions. Regardless of whether fogging was done once, twice, thrice, or four times a month, he found no difference in the recovery rate of the mosquito population. As a research paper by the Journal of Pest Science puts it, dengue vector mosquitoes come back 24 hours post-fogging,” said Yek.
The use of fogging by private companies must be regulated to avoid overuse, and to avoid mosquitoes developing resistance towards the pesticides.
“Fogging should only be done when dengue outbreaks are reported, or during the rainy season. Due to many private companies offering fogging services, we don’t know what chemicals or dosage is being used, and it may result in mosquitoes developing resistance towards the pesticides instead. However, it’s challenging to regulate fogging services, because it’s market-driven,” she said.
“The downside to fogging is that it could potentially have destructive effects on pollinators such as bees and birds, adversely affecting the environment that depends on them to survive,” says Dr Yek, who’s conducting a study on this, with research data due to be released by the end of the year.
“Nature has a balance, but we disrupt that balance. People need to be made aware that our actions have serious consequences, and they impact the environment,” she adds.
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