05
Sep
More Findings of Mosquito Resistance to Neurotoxic Pyrethroids Elevate Need for Eco-Compatible Strategies
(Beyond Pesticides, September 5, 2024) As insect-borne diseases like EEE (eastern equine encephalitis) become a focus of mosquito managers, there is a continuing pattern of mosquito resistance to synthetic pyrethroid insecticides, which are the primary tools in conventional mosquito control programs—a strategy more focused on attempting to kill adult mosquitoes than the management of breeding sites. A study published in PLOS One documents Aedes aegypti mosquito resistance to synthetic pyrethroids permethrin, lambda-cyhalothrin, and deltamethrin in study sites in Córdoba, Colombia. Aedes aegypti is a common mosquito species that can carry the EEE virus and others.
There is significant scientific literature highlighting the prevalence of pesticide resistance in mosquito, fly, and other insect populations. Pesticide resistance is an inherent problem with pesticide dependency generally, creating a complex fabric of threats from insect resistance to plant incorporated protectants (PIPs), weed resistance in genetically engineered crop production dependent on chemical-intensive weed control, to antibiotic resistance to medically important drugs, exacerbated by agricultural use and horizontal gene transfer. As the frequency of deadly mosquito vector diseases is expected to increase with rising temperatures and greater precipitation—which raises the number of breeding sites—advocates, physicians, beekeepers, and community leaders are calling on elected officials to prioritize ecological pest management strategies that protect biodiversity and public health.
Methodology and Results
The mosquito resistance study was conducted in the northwestern section of Colombia, an area that the researchers describe as having “eco-epidemiological conditions that facilitate the persistence of the transmission of [dengue virus]†as well as other arboviruses, such as Zika and chikungunya. Researchers at the Universities of Simon Bolivar and Córdoba collaborated with public health officials in the target municipalities in which they collected mosquito populations. They also teamed up with an entomologist at the Public Health Laboratory of Córdoba to analyze resistant mosquito populations using a Centers for Disease Control and Prevention (CDC) bioassay and World Health Organization (WHO) tube method for measuring mosquito mortality after exposure to the pesticide.
The goal of this study is to analyze the susceptibility and resistance rates of mosquito populations sprayed with the three insecticides to inform Córdoba’s public health strategy moving forward. The main mechanism of resistance for synthetic pyrethroids in mosquitoes is knockdown resistance (kdr) mutations, including F1534C, V1016I, and V410L mutations, which researchers identify in this study. Researchers partially attribute pyrethroid resistance to habitual household use of DDT, leading to cross-resistance since both types of insecticides impact mosquitoes’ voltage-gated sodium channel (VGSC), which can cause resistance mutations. While the organochlorine insecticide was prohibited in 1994, previous studies on DDT resistance in Colombia highlight the long-lasting impacts of decades of previous pesticide use (See here and here).
“The results varied between the two techniques used, with resistance to permethrin observed in thirteen of the fourteen populations, resistance to lambda-cyhalothrin in two populations, and susceptibility to deltamethrin in all the populations under study with the CDC method,†according to the lead researchers’ description of the impact of synthetic insecticide use on mosquito management. “In contrast, the WHO method showed resistance to the three insecticides evaluated in all populations.†The researchers successfully identify all three of the kdr mutations across all populations. Approximately 4,030 females of Ae. aegypti were assessed using the WHO method, with roughly one-third of each in this group showing resistance to permethrin, deltamethrin, lambda-cyhalothrin. Meanwhile, approximately 3,255 females of Ae. aegypti were evaluated for resistance in a similar breakdown across the three active ingredients.
“[T]he WHO tube test measures the mortality rate of mosquitoes exposed (usually for 1 h[our]) to a discriminating concentration of the insecticide for a specific time (regularly 24 h[ours]) [], while the CDC bottle test determines the time necessary to incapacitate a susceptible mosquito using a predetermined concentration of insecticide [].†The researchers discuss the implications of relying on one methodological approach versus the other: “It has been shown that the CDC method generates greater variability in mortality when compared to the WHO method, which may influence the actual interpretation of the susceptibility status of the populations evaluated []. The pyrethroids assessed by the CDC test may generate knockdowns in mosquitoes at the time of diagnosis, which does not necessarily generate mortality 24 h[our] post-exposure [].†Researchers also warn of less-than-ideal accuracy in the CDC bottle test because of a variety of potential outlets for human error, including washing the bottle, insecticide volatility, and the drying procedure. The use of both methods in this study corroborates the findings of previous research on insecticide resistance to insects carrying several arboviruses, which will only increase with the climate crisis and a lack of forward-thinking approaches to safer ecological pest management.
The mainstream approach to pest management that relies on chemical-intensive practices has led to deadly outcomes. In 2016, at least 2.3 million honey bees from 46 hives in Dorchester County, South Carolina perished from aerial spraying of organophosphate insecticide Naled. The state’s Department of Health and Environmental Control launched this spraying campaign in an attempt to kill adult mosquito populations in response to fears over the Zika virus. See the Daily News section on mosquitoes to learn more about the implications of “spray-as-usual†mentality, as well as the section on integrated and organic pest management to learn about success stories involving safer alternative approaches.
The status quo relies on responding to crises through least effective practices rather than taking a proactive approach that minimizes risks by focusing on root conditions for mosquito reproduction. This is true in state-level mosquito management programs in response to both West Nile Virus and Eastern equine encephalitis (EEE). For example, in Massachusetts last week, the Division of Crop and Pest Services in the MA Department of Agriculture announced that aerial spraying would begin in parts of Plymouth County and truck-based spraying in Worcester County on the evening of August 27, and several following days, after the presence of positive mosquito samples for the virus. It has been four years since the last EEE detection led to seven deaths and 17 cases in the same counties, leading to the use of Anvil 10+10—containing the neurotoxic, synthetic pyrethroid sumithrin (the active ingredient used to target the insect) and the synergist piperonyl butoxide/PBO, used to increase the potency of the active ingredient. (See Beyond Pesticides’ press release here.)
According to a Massachusetts Department of Agricultural Resources (MDAR) Crop and Pest Services’ 2019 summary report, spraying this product lasted 26 days, treated over 2,048,865 acres across the Commonwealth, and used 9,939 gallons of Anvil 10+10. The product was also applied by air in 2006, 2010, and 2012. It has been linked to a range of adverse health effects in humans, including a higher risk of liver disease—which increases the hazardous effect of exposure to the pesticide, given that the liver is the primary organ meant to filter out toxic residues including pesticides. In addition, the insecticide’s ingredients are also linked to cancer, kidney damage, threats to reproductive health, and endocrine disruption.Â
Aerial and truck spraying has also commenced in New Hampshire, Rhode Island, Connecticut, and New York for either EEE or  West Nile Virus since July.
See Gateway on Pesticide Hazards and Safe Pest Management to learn more about adverse health impacts of specific active ingredients or pesticide products to identify safer alternatives. See Safer Mosquito Management to learn more about the principles of ecologically based mosquito management plans from public health researchers and experts.
📣 To TAKE ACTION and reach out to your governor, click here to access last week’s special Action, Call for States to Adopt a Safer Strategy to Fight Eastern Equine Encephalitis (EEE) Transmission!
All unattributed positions and opinions in this piece are those of Beyond Pesticides.
Source: PLOS One
