Using pesticides to prevent mosquito-borne diseases

Mosquitoes are an important part of the food chain in our watershed, but the few species that carry disease can be a human health risk. Reducing artificial mosquito breeding habitats is the best way to prevent mosquito-borne disease. Keeping waterways healthy, flowing freely, and full of fish that eat insect larvae, can reduce the need to use toxic chemicals. With proper surveillance and foresight, we can use only the pesticides that are absolutely necessary to control outbreaks of mosquito-borne diseases.

Pesticides can temporarily reduce local mosquito populations, but they can also harm other beneficial species like bees or fish. Carefully targeting the type and timing of pesticide applications can reduce the risks to the local ecology by:

  • limiting the total exposure to pesticides,
  • using chemicals that break down quickly so they do not persist in the environment,
  • avoiding times when beneficial insects are active, and by
  • targeting mosquito larvae in water.
  • Larvicides. Mosquito eggs hatch in still water and mosquito larvae spend up to two weeks growing until they pupate and fly away. Larvicides are pesticides that are added directly to water to kill the larvae. The most common larvicides are bacteria: Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus. These bacteria make a toxin that kills the larvae when they eat the bacteria. The toxin is specific to mosquitoes and a few closely related insects like black flies and midges, but is not toxic to bees or fish.

    Bti can knock down mosquito breeding at an application site within 24 hours, but it must be applied directly to the water that larvae live in. It is available in briquettes (like Mosquito Dunks©) and powders for use in rain barrels and catch basins. Spraying from helicopters in the early spring has been used to control mosquitoes in inaccessible wetlands, but the effectiveness for disease prevention is questionable. Bti can be sprayed from a truck, but it will only be effective if it actually lands in the water. Bti is only effective for a short time, which is why slow dissolving briquettes are a preferable form.

    Growth regulators like methoprene (Altosid©) can also be used to kill mosquitoes. Methoprene acts like a hormone and prevents mosquito larvae from developing into adults, but it has been shown to be toxic to amphibians and shellfish. Like Bti, methoprene is available in a briquette, so it is important to check the label of any mosquito control tablet to distinguish whether you are getting the bacteria-based larvicide or the more broadly toxic growth regulator. Methoprene lasts longer in the environment than Bti, which is why many communities use it in catch basins. Alternating bacteria and growth regulators can prevent the development of resistant larvae.

    Dr. Kiszewski collects a sample from one of 50 mosquito egg traps that he monitors in Concord and in the Boston area. Half-filled water cups with paper liners attract mosquitoes of many species to lay eggs. The papers are collected regularly and taken to the laboratory. The eggs are counted and hatched in the laboratory and then sorted by species. The eggs appear as little black spindle shaped specs that follow the water line on the paper. Adult mosquito traps, like the one on the right, allow scientists to look specifically for viruses and monitor for specific health concerns.

    Adulticides, like sumithrin (Anvil©), other pyrethroids, and malathion are very effective at killing flying insects, especially adult mosquitoes. But they also are toxic to bees, dragonflies, and fish. In high concentrations, they are also neurotoxic to human beings. Typically, adulticides are sprayed by handheld sprayers or trucks. When you see a truck spraying insecticide on a summer evening, it is most likely spraying an adulticide like sumithrin.

    Previous generations of adulticides, like DDT, persisted in the environment and killed fish and birds at the top of the food chain. So modern adulticides were selected because they break down fairly quickly in sunlight and do not become a persistent problem. But they are still neurotoxins, and can affect fish, and in higher concentrations, birds and mammals. They should not be sprayed directly on water. To prevent pollinators from being harmed by mosquito adulticides, they are often sprayed after dusk when mosquitoes, but not bees, are active. However, some beneficial insects are active at night. Bees and other pollinating insects will be out at dawn landing on flowers that may have residue before the sun has broken it down.

    Entomologist Dr. Anthony Kiszewski of Bentley University, questions whether spraying adulticides has a significant effect on the disease-carrying mosquito populations in Massachusetts. “These chemicals break down so quickly. They will only have an effect if a mosquito is actually touched by the chemical, and the trucks only have access to the spaces directly adjacent to the road.” The quick breakdown of pyrethroids means that they need to be sprayed when mosquitoes are active, usually between dusk and 11 pm. That might have a small impact on the Culiseta and Culex mosquitoes that carry EEE and West Nile virus. But the Aedes mosquitoes that carry Zika virus are active during the day, so there are only a few evening hours that sprays could be used to minimize effects on pollinators like bees. Even when the local adult population is reduced, new mosquitoes will hatch within days.

    Mosquito Control Projects. Pesticides that break down quickly--which are healthier for the environment--must be applied when mosquitoes are active. But most towns do not have the resources to assess when there is a public health need to spray. The Mass. Dept. of Agricultural Resources oversees mosquito surveillance programs in districts across the state. Individual towns join their local Mosquito Control Project, which monitors mosquitoes, maintains ditches, and educates the public. The Control Projects also apply pesticides: adding larvicide to catch basins, doing aerial spraying with larvicides over waterways that have been identified as mosquito breeding grounds, and sending out trucks to spray adulticides.

    Surveillance programs collect mosquitoes in member towns with CO2-baited traps, and identify the number and species collected. This is particularly important because most of the 51 mosquito species found in Massachusetts do not cause disease. The program also identifies mosquito breeding grounds, and interfaces with the DPH to identify disease presence in mosquito populations. Surveillance can help distinguish between mosquitoes that harbor arbovirus (mosquito-borne virus) and innocuous species that are just a nuisance.

    Mass Audubon Senior Policy Analyst Heidi Ricci points out that, “One of the biggest flaws in the state mosquito control system is that communities can’t get mosquito testing services unless they are part of a district, but then they often are forced to sign onto a program that allows homeowners to request neighborhood spraying based on an individual’s perception of mosquito nuisance.” Members of the Central Mosquito Control Project do not get to pick which services they get. The Project applies larvicides in catch basins early in the season, and decides for itself when adulticides will be applied. A detailed adulticide spraying schedule is published on the Project’s website: http://www.cmmcp.org/pesticide.htm

    Surveillance programs collect mosquitoes in member towns with CO2-baited traps, and identify the number and species collected. This is particularly important because most of the 51 mosquito species found in Massachusetts do not cause disease. The program also identifies mosquito breeding grounds, and interfaces with the DPH to identify disease presence in mosquito populations. Surveillance can help distinguish between mosquitoes that harbor arbovirus (mosquito-borne virus) and innocuous species that are just a nuisance.

    Mass Audubon Senior Policy Analyst Heidi Ricci points out that, “One of the biggest flaws in the state mosquito control system is that communities can’t get mosquito testing services unless they are part of a district, but then they often are forced to sign onto a program that allows homeowners to request neighborhood spraying based on an individual’s perception of mosquito nuisance.” Members of the Central Mosquito Control Project do not get to pick which services they get. The Project applies larvicides in catch basins early in the season, and decides for itself when adulticides will be applied. A detailed adulticide spraying schedule is published on the Project’s website: http://www.cmmcp.org/pesticide.htm

    But not all projects operate by the same rules. East Middlesex Mosquito Control Project superintendent Dr. David Henley points out that his Project provides more flexible services, allowing members to select whether they want only larvicides or, at a greater cost, comprehensive adulticide applications. Dr. Kiszewski, a former Concord EMMCP representative, points out that Concord has limited its pesticide use to larvicides in catch basins and mosquito breeding sites. But Concord’s restraint is just one end of the pest management spectrum. According to the East Middlesex website, Sudbury and Bedford were sprayed with sumithrin this summer: https://sudbury.ma.us/emmcp/

    Environmentalists like Ms. Ricci would like to see public health be the focus of the mosquito control projects. This could be facilitated by adding representatives from the Departments of Public Health and Fisheries and Wildlife to the Control Project management, and adding local public health official representation to the regional districts. Surveillance could be expanded to look for impacts of pesticide use on wildlife, especially pollinators, fish, and birds. Management techniques could be continually updated to incorporate the best scientifically-validated methods.

    In the meantime, it is up to individuals to support the most environmentally-friendly practices in their own towns by making their opinions known to their own town representative. Home owners who wish to opt out of pesticide applications need to file a request with their local town clerk by March 1st and mark their property with orange tape. If you want to opt out after the season has started, you should contact your Control Project superintendent directly--most towns will respect a late request and a posted sign like a paper plate that says “no spray”. During a public health emergency it is unlikely that an individual homeowner will be able to opt out of spraying.

    Homeowners can contract with pest control companies to spray pesticides on their property, but buyer beware: you pay these companies to apply chemicals, not prevent disease. They may plan to apply pesticides when it is convenient, not when mosquitoes are active. And their treatment may not distinguish the disease-carrying mosquitoes from the nuisance of the many disease-free mosquito species. Pesticides that are over-used can also lead to pesticide resistance.


    Articles:

    Mosquito-borne diseases

    How to reduce mosquito habitats

    How to protect yourself from mosquito-borne diseases

    What will global warming bring?

    References