THE global history of pest control through synthetic chemicals (pesticides/biocides) has shown that this approach is neither the only nor the best solution. Chemicalisation has caused enormous harm to humans, animals and plants, leading to biodiversity loss — particularly of beneficial organisms — and to the emergence of secondary pests as primary ones. New pests and diseases may also arise. Recurrent and improper use of pesticides and biocides promotes resistance in pests and disease vectors. Further drenching of ecosystems with chemicals then becomes futile, serving only to intoxicate the environment.
Dengue is a mosquito-borne viral infection transmitted primarily by Aedes aegypti. It causes flu-like symptoms such as headache, joint pain and rash, sometimes accompanied by nausea; severe forms may lead to haemorrhage or organ failure. Over the past few years, persistent reports of dengue-related deaths in Bangladesh — especially in Dhaka — have raised serious concerns about the effectiveness of existing control measures. Regular fogging with insecticides appears ineffective against Aedes, which has likely developed resistance. Moreover, there is little information on the harm caused by chemical spraying to non-target species, including humans. Continued spraying will have cumulative impacts on an already polluted Dhaka, a city under immense population pressure, where environmental awareness remains limited. Recurrent spraying poisons the environment, creating a lethal trap for residents and visitors alike. Those who persist with such practices raise questions that are not only scientific but also moral.
We often see spray operators — many of whom do not know how, where, when or in which direction to spray, nor the lethal consequences of the pesticides — using spray guns in the evening, which is probably the least appropriate time. Timing is crucial. In the evening, mosquitoes leave their hideouts and can easily avoid contact with insecticides; such spraying merely increases the chemical load in the environment. Around this time, insect-eating birds return to their nests to roost. Daytime spraying — especially in the morning, when birds are actively feeding — may irritate mosquitoes out of their hiding places and expose them to natural predators. In any case, population growth of any organism primarily depends on food availability and environmental suitability. Dhaka’s environment appears congenial for Aedes, but hostile for humans. Climate change may further fuel the proliferation of Aedes mosquitoes.
Dhaka is relentlessly inundated with hundreds — if not thousands — of chemical substances: toiletries, beauty products, dish, toilet and floor cleaners, detergents (whether biodegradable or not), pesticides and aerosols, many of unknown nature and toxicity. This continuous chemical influx further complicates the situation. Throughout the food supply chain — farmers, wholesalers and retailers alike — various toxic chemicals are used, eventually entering human diets and penalising unsuspecting consumers. Added to this are transport and industrial pollutants, municipal waste, medical waste, clinical and laboratory reagents, the excreta of millions, and much more, much of which is discharged directly into Dhaka’s ecosystem. Moreover, industries, businesses and advertising — by promoting excessive, non-essential consumption — contribute significantly to waste generation and environmental pollution. As a result of indiscriminate land-based activities, the four rivers surrounding Dhaka have already been declared Ecologically Critical Areas (ECAs), meaning they no longer function naturally. Environmental degradation weakens the ecosystem’s buffering capacity and natural control mechanisms, paving the way for disease outbreaks and pest infestations.
There are, of course, many approaches to pest control. What is needed is an effective, low-cost and minimally polluting solution. For sustainability, an integrated approach is essential, with biological control at its core:
Study the life cycle of Aedes in detail to identify its most vulnerable stage. There is always a weakest point. Generally, the immature stages — particularly larvae — are the most susceptible. Delicate larvae can be killed by very small quantities of insecticides; even sub-lethal exposure can retard growth and eventually cause death. Understanding Aedes behaviour — its hiding places, resting periods and breeding times — is crucial for designing effective control methods.
Identify and utilise natural enemies — such as birds, insects and pathogens — of Aedes. These can be reared and strategically deployed. Dragonflies, for instance, catch mosquitoes in flight, while their nymphs prey on aquatic mosquito stages. Increasing predator populations around breeding sites could significantly reduce mosquito numbers. The feasibility of sterilisation through radiation may also be explored.
Eliminate hosts or alternative hosts (if any) on which Aedes depends. Parasitism, starvation of larvae through destruction of food sources, and habitat modification may also serve as effective control tactics.
Ensure proper sanitation and municipal waste management. Improving Dhaka’s ecosystem is essential, as shallow stagnant waters provide ideal mosquito-breeding habitats.
The desire for quick and easy pest eradication has produced an ever-expanding array of chemicals, raising grave concerns about environmental poisoning. During long periods of environmental transit, chemicals almost certainly undergo transformations, yet we often do not know whether the altered compounds are more or less toxic than their originals. For example, heptachlor epoxide — derived from heptachlor in soil and in the tissues of plants and animals — is more toxic than heptachlor itself. Similarly, aldrin is transformed into dieldrin, an equally potent toxin. Another major concern is the interaction between chemicals, a problem that becomes especially acute once they enter the environment, where countless substances are mixed and transported together — a warning famously articulated in Silent Spring (1962). Before subjecting Dhaka to further chemical drenching, we must understand precisely what is happening to humans and to the ecosystem as a whole.
Dr Md Sohrab Ali, a former additional director general of the Department of Environment, is a writer and researcher.
