Novaluron 8.8 Sc
Rosella Bowlan
According to Article 12 of Regulation (EC) No 396/2005, EFSA has reviewed the maximum residue levels (MRLs) currently established at European level for the pesticide active substance. Although this active substance is not authorised within the European Union, MRLs were established by the Codex Alimentarius Commission (codex maximum residue limits; CXLs) and import tolerances were reported by Member States and the UK (including the supporting residues data). Considering that no toxicological reference values are currently established in Europe for novaluron because the peer review for the approval was terminated before an EFSA conclusion was issued, the toxicological profile of novaluron was also assessed, in order to be able to perform the consumer risk assessment in the framework of the art 12 MRL review. Based on the assessment of the available data, toxicological reference values were derived, and a consumer risk assessment was carried out for the existing CXLs and import tolerances. All CXLs and import tolerances were found to be supported by inadequate data and a possible chronic risk to consumers was identified. Hence, further consideration by risk managers is needed.
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Dengue fever is a mosquito-borne tropical disease caused by the dengue virus. It has been estimated that 3.9 billion people in the world are at risk of the disease. In Sri Lanka, dengue fever (DF) has become a serious public health concern especially with the alarming increase of cases in the recent past (Epidemiology reports1. The disease is transmitted through the bites of the mosquitoes Aedes aegypti and Ae. albopictus2. Two transmission periods of the disease in Sri Lanka is parallel to the two monsoon seasons, showing the close association between dengue transmission and the rain fall. The most intensive transmission period June to August coincides with the southwest monsoon, whereas the less intense October to December period coincides with the northeast monsoon3,4.
Vector control plays the major role in reducing the disease burden especially in the absence of an effective vaccine against dengue. Suppression of vector densities is commonly attempted through various methodologies such as use of insecticides, source reduction, and implementation of new regulations for vector elimination5. Although the general public is urged to destroy or remove all possible indoor and outdoor Aedes breeding sites, it has not always been effective due to inadequacy of resources and poor community support6. Use of insecticides has emerged as a predominant and efficient approach in the control of Aedes vector populations, particularly during disease outbreaks7. Among the WHO recommended insecticides, pyrethroids with low mammalian toxicity and a higher efficacy are widely used in vector control programmes5,8. For more than two decades, Sri Lanka has been using pyrethroids as adulticides and temephos (organophosphate) as a larvicide for Aedes mosquito control. Constant application and indiscriminate use have led to emergence of insecticide resistance in Aedes populations. Several studies have reported high incidence of resistance to neuroinhibitory insecticides in Sri Lankan dengue vectors9,10,11,12,13,14,15. Also, the application of temephos as a larvicide is especially a concern due to its toxic effect on non-target aquatic life16,17. Furthermore, research has shown that diversifying control methods, such as incorporating biological control agents18 or insect growth regulators19 can provide sustainable and long-term solutions for managing dengue vector populations while reducing dependence on neuroinhibitory insecticides. Therefore dengue vector control programmes are looking for effective alternatives to minimize the use of neuroinhibitory insecticides.
Insect growth regulators (IGR) have been introduced as potential alternatives to control dengue vector populations20. Among IGRs, novaluron is considered as an active agent for insect larval control worldwide (WHO, 200721). This benzoylphenyl urea compound affects larval and pupal stages of the Orders Coleoptera, Diptera and Hemiptera22,23,24,25. Novaluron has been successfully used to control Aedes mosquito larvae26. Although the mechanism of action of novaluron has not been extensively investigated, the general mechanism of benzoylphenyl urea action has been well documented27,28,29. Benzoylphenyl urea changes the elasticity and rigidity of the endocuticle in insect immature stages by changing cuticular composition. This mainly affects molting stages of insects, causing death by abnormal endocuticular deposition and interrupted molting28. At sub lethal dosages, novaluron has a low risk to the environment, including its effect to mammals, birds and aquatic insect species other than mosquito larvae30,31.
During the period of January 2020 to April 2020, mosquito eggs were collected using ovitraps from Bandaranayakepura in Kurunegala Medical Officer of Health (MOH) area, where novaluron had not been used for vector control programs. Collected mosquito eggs were brought to the laboratory of Entomological Surveillance Unit, Office of the Regional Director of Kurunegala and transferred to hatching trays where they were allowed to hatch. Third instar larvae were used for all bioassay experiments with novaluron. Formulated emulsifiable novaluron (100 g/L, Rimon EC10) was a gift from Makhteshim Chemical Works Ltd, Israel.
Sentinel cages were prepared by using 250 mL transparent plastic cups. Bottom of each cup was replaced with a plastic mosquito mesh. After placing larvae, the cups were covered with lids prepared by the same mesh. Each cup was emersed in water in the bucket or the barrel and its positions was stabilized using a wire linked to barrel/ bucket rim. After adding larvae to a sentinel cup, the water storage container was covered by a net to prevent mosquito egg laying. Control experiments were conducted without novaluron. Experimental set up included thirteen buckets (three test concentrations each with 4 replicates, and a control) and thirteen barrels, and were placed in an outdoor undisturbed area with a covered roof. Larval food was added once a week and water loss was replenished weekly.
A larval cohort (batch of 25) was introduced to the sentinel cage in the fresh novaluron solution filled bucket/barrel at the beginning of each experiment. After a 7-day exposure period, larvae of the first cohort were removed and a second cohort was introduced. Third cohort replaced the second cohort after another 7 days. Thereafter, each cohort was replaced by a new cohort once in a 14-day period. Before introducing a new cohort (batch of 25), all the dead/survive larvae from the previous cohort were removed. Assessment of larval survivorship was recorded. Adult emergence was assessed by counting pupal skins and emerged adults. After the count, remaining pupal skins were removed by a syringe or a fish net before the next cohort was introduced. Observations and recordings were continued until all the immature stages in the control emerged into adults.
Dose response curves were generated by using cumulative mortality in each concentration within the specific time period. Percentage of inhibition of emergence (IE%) was calculated using the following equation.
At the end of the 7-day period, larvae in control groups exhibited 100% emergence rate. The cumulative mortality of Ae. aegypti and Ae. albopictus 3rd instar larvae treated with eight different concentrations of novaluron under laboratory conditions are shown in Fig. 1. At the highest concentrations used (3 ppm and 4 ppm) Ae. aegypti larvae started to die within the first 24-h exposure whereas at the lowest concentration (0.0001 ppm) the mortality started only after 7 days. The highest concentration 4 ppm gave 100% mortality after 5 days of exposure and the concentrations 1.0 ppm and below did not result 100% mortality even after 14 days of exposure. Results showed that Ae. albopictus is slightly more resistant to novaluron. During a 24-h exposure period, Ae. albopictus larval mortalities were 3% and 10% lower than Ae. aegypti larval mortalities at the concentrations 3 ppm and 4 ppm, respectively. Concentrations 2 ppm and below could not achive 100% mortality at the end of the 14 days period (Figs. 1 and 2).
Lethal concentrations of novaluron which killed 50% (LC50) and 99% (LC99) of larvae/pupae at the end of 7-day period and at the end of 14-day periods and the calculated IE50 and IE99values for 14-day exposure periods are presented in the Table 1.
Although dengue infections have been documented from Sri Lanka since the 1960s, the first significant DHF outbreak with 206 clinically diagnosed cases and 20 deaths (CFR 9.8%) occurred in Colombo in 198940. Since then, dengue outbreaks have been gradually spreading to many regions of the island, with an increase in reported cases occurring every 3 to 5 years (Ministry of Health Sri41. Increasing tendency of dengue prevalence in Sri Lanka emphasizes the urgent need of effective vector control measures. Traditional vector control methods have several limitations including vector resistance15,42, 43. Biological insecticides based on Bacillus thuringiensis (Bt) have been employed in Sri Lanka as an alternative in integrated vector control strategies with varying success42. Storage challenges and cost-effectiveness, and resistance development are major challenges against Bt insecticides26,44, 45. Insect growth regulators (IGRs) are a group of chemicals that have been recently used as an effective alternative for controlling insect vectors worldwide7. They exhibit a good margin of safety to most non-target biota, thus offering some advantages in mosquito control programmes26 and are strictly arthropod-specific and environmentally safe and show very low toxicity to birds, mammals and honey bees24,46. Novaluron 10 EC formulation has been developed under WHOPES guidance and supervision, as an IGR against insects, including mosquitoes24,25.
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