Ecological impacts of larvicidal oil on the marine ecosystem : implications on its management

Abstract

Mosquitoes are some of the most influential insects to mankind in the world due to their ability to transmit diseases to humans, which results in millions of deaths every year. Prevention of mosquito-borne diseases and elimination of mosquitoes are important to protect human health. Among different methods of eliminating mosquitoes, the use of larvicidal oil is the most common way being employed in Hong Kong. It functions by forming a thin film over the water surface, killing mosquito larvae by suffocation. However, larvicidal oil will be eventually released into the marine environment due to rainfall and surface runoff, and thus it may affect marine organisms. However, its toxicity and risk to marine organisms remain largely unknown. Therefore, this study aims at investigating the toxicities of larvicidal oil to marine organisms at different trophic levels along the food chain. The composition of larvicidal oil was characterized by gas chromatograph-mass spectrometer (GC-MS). It was found to consist mainly of aliphatic petroleum hydrocarbons (n-alkanes) ranging from n-octane (C8) to n-pentacosane (C25). Being a complex mixture of organic compounds, it has highly varied physical properties and solubility in water. Standard acute toxicity tests were conducted by preparing water-accommodated fractions (WAFs) of larvicidal oil to investigate their toxic effects to the marine microalgae Isochrysis galbana (a primary producer), the intertidal copepod Tigriopus japonicas (a primary consumer) and embryos of the marine medaka fish Oryzias melastigma. My experimental results, for the first time, showed that larvicidal oil was toxic to these three marine species, and among them, the microalgae was the most sensitive to this oil with a median effect concentration (EC50) of 0.47 mg/L while the copepod was the least sensitive to this oil with a median lethal concentration (LC50) of 2.14 mg/L. The concentrations of larvicidal oil in the marine environment were also determined with results obtained ranging from 4.3 mg/L to 20.4 mg/L, by analyzing water samples collected near Tolo Harbour, Hong Kong, where there were some local fish farms operating and they were suspected to be contaminated by larvicidal oil. To safeguard marine biodiversity and ecosystem services, the current results were used to derive the predicted no-effect concentration (PNEC) of larvicidal oil with 0.00047 mg/L which might be used as a water quality criterion for management purposes. Risk quotients (RQs) of larvicidal oil were determined to be extraordinarily high that were greater than 1 by four to five magnitudes, indicating that the current risk was extremely high, hence, the monitoring and control on the use of larvicidal oil as mosquito control pesticide would be urgently needed to mitigate its environmental and ecological risks.