Ecological Mosquito Management

Boulder's mosquito program has been on the cutting-edge since its creation in 2002 by developing new tools and approaches to manage mosquitoes, while protecting fragile wetland ecosystems. The Xerces Society for Invertebrate Conservation recently highlighted Boulder's program as a  case study  for effective, ecologically sound mosquito management.

Wetlands are the most productive and  important  habitats on earth, providing critical services such as water purification, blunting the impacts of floods and storms, preventing erosion, and providing habitat for diverse plants and aquatic and terrestrial animals. Wetlands have been drained, polluted and damaged from human activities. As a result,  mayfly swarms  are becoming a thing of the past, and  dragonflies and damselflies are disappearing  and threatened with extinction.  Climate change  is also damaging wetland ecosystems. Protecting and restoring wetlands is  crucial for ecological resilience . Healthy wetlands can provide natural climate solutions as important carbon sinks, as well as protecting biodiversity and reducing disease risk. The practices used in conventional mosquito management can harm wetland health. And not only that, it can increase mosquito numbers by accelerating the decline of dragonflies and other important mosquito predators.

The underlying foundation of Boulder’s mosquito management program is based on an ecosystems approach and applying knowledge about the biology of mosquitoes and the wetland ecosystems where they live. Management options are carefully evaluated for environmental and human health impacts using the most current scientific literature and consulting with researchers and experts. The following sections provide basic information about mosquito biology and how the city is gathering biodiversity data to lower mosquito activity and reduce the risk of mosquito-borne diseases to the public.

In the past five decades, despite widespread mosquito control measures, mosquito populations have  increased  around the country by as much as 10-fold. This trend is occurring locally with combined mosquito trap counts throughout the county reaching the highest numbers recorded over the last few years. At the same time, the majority of insects are plummeting with  scientists issuing warnings  about the catastrophic consequences if immediate action isn’t taken to reverse this precipitous decline. Why is this happening?

Urbanization, habitat destruction and fragmentation, climate change, as well as chemically-intensive landscape management and agriculture have all led to disruptions in ecosystems. While mosquitoes can readily breed in poor-quality sites created by human disturbance, other organisms, including many of the predators that naturally control mosquitoes, require higher quality habitat. Pesticides that are used to try to control mosquitoes may actually be  making matters worse  as mosquitoes become resistant to the pesticides, while natural predators and other invertebrates are killed or harmed, which throws aquatic food webs out of balance. The city is studying how mosquitoes and other insects and animals live in 500 mapped mosquito breeding sites to address multiple concerns - public health, mosquito abundance and protection of the aquatic ecosystems and their surrounding terrestrial habitat. A review of basic mosquito biology and the different options that are used for mosquito management show why the city is taking this approach.

Of the 57 known mosquito species in Colorado, only the Culex species are known to transmit West Nile virus. There are two main categories of mosquitoes - those with the potential to transmit disease like West Nile virus - or vector mosquitoes. The other category are “non-vector” mosquitoes that pose risk little or no risk for transmitting disease to people. The majority of non-vector species are the “floodwater" mosquitoes that emerge in large numbers in the spring and early summer.

The diagrams below show the difference in lifecycles between Culex and floodwater mosquitoes. The two types of mosquitoes can occur at the same sites when areas with floodwater mosquito eggs are inundated with water from precipitation or irrigation and Culex females lay eggs at these sites. However, Culex also lay their eggs in containers, which is why it’s so important to carefully inspect yards to drain items that can hold water and make sure gutters and depressions are drained.

There are products available that target every stage of the mosquito lifecycle. Each of these products can have both direct impacts to non-target organisms and/or ecosystem-level effects.

Adulticides are insecticides used to kill adult mosquitoes that are sprayed from airplanes, fogged from trucks or applied in peoples’ yards as barrier treatments. The city doesn’t fog with adulticides due to lack of efficacy and human health and environmental impacts. Cornell University scientists  describe the poor efficacy  of adulticides due to the minute amount that actually reaches the mosquito, writing "target mosquitoes only receive about 0.0000001%” of insecticide droplets and that "1 million insecticide droplets must be produced to hit one target mosquito."

The family or chemical group of the insecticides most commonly used for mosquito sprays are pyrethroids that attack the nervous system of insects, which is the same as other animals. Even though the doses are fairly low from these mosquito fogs,  exposures do add up  from it and other sources and it’s common to find urinary metabolites of these chemicals in people that is  correlated with serious health issues  as well as  cognitive issues in children  and reproductive issues in both  men  and  women . These insecticides can cause both direct and ecosystem-wide  impacts on non-target insects and other wildlife . 

Surface oils and films can act as both pupacides and larvicides. They form a thin barrier over the surface of the water that prevents mosquito pupae and larvae from getting air. However, other aquatic insects are also suffocated from these products including important mosquito predators, harmless insects and surface-breathing small arthropods that are important components of aquatic ecosystems.

Controlling mosquito larvae is one of the most effective methods to reduce adult mosquito populations. Unlike adult mosquitoes that can move and travel, larvae are confined to the water where they live and treating thousands of larvae in one small area can greatly reduce overall mosquito populations. There are a number of products that target mosquito larvae, but the most common that are used in our region are insect growth regulators and naturally-derived bacterial larvicides. Growth regulators are compounds that prevent mosquito larvae from emerging as adults and keep them in an immature state. The hormones these products target occur in all insects and crustaceans and can result in non-target impacts to individual species and overall ecosystem changes.

One of the major tools used in mosquito management is the larvicide, Bacillus thuringiensis israelensis (Bti). In most situations, Bti is effective at killing mosquito larvae. However, there are downsides due to its direct toxicity to non-target organisms, such as tadpoles and harmless and/or beneficial insects, as well as indirect effects, which can disrupt ecosystem function ranging from water quality to bird reproductive success.

Studies have shown cascading impacts from Bti that disrupt overall wetland health. See Attachment A in this  memo  for a review of the scientific literature regarding the ecological impacts from Bti.

Mosquito breeding sites cover a wide range of types from muddy depressions in soil, stagnant water in containers or storm drains to high quality wetlands. If breeding sites can be eliminated by inspecting and draining artificial sites, cleaning clogged trash guards in ditches or managing flood irrigation, this is the quickest and most effective approach.

Bti is used where appropriate, but alternative treatments are the first option for high quality wetland sites. Over 500 mosquito breeding sites are mapped on city-owned property. These sites are regularly monitored for mosquito larvae and are treated when mosquito larvae reach a threshold. Due to the potential for ecosystem-wide impacts, Bti is cautiously and judiciously applied. The main focus is lowering Culex larvae to reduce the risk of West Nile virus. Areas in lower ecologically functioning sites that breed high numbers of nuisance or floodwater larvae are also treated when larval densities are high - particularly when few or no mosquito predators are present. Sites with low mosquito breeding history and high ecological value are not routinely treated with Bti, but are still monitored for mosquito larval activity and treated on a case-by-case basis.

Monitoring adult mosquitoes is an important part of the city’s program. Adult mosquito populations are monitored through 19 city traps. The city also follows data from the University of Colorado and Boulder County mosquito programs to track local mosquito activity.

Wetlands with high ecological function can possess built-in mosquito controlling organisms, such as fish, predatory insects, birds and spiders that can keep mosquito populations naturally low.

When people see a wetland they often assume that it's breeding mosquitoes. But a healthy wetland is filled with mosquito predators and competitors that naturally keep mosquito numbers low, as this factsheet from the Indiana Department of Natural Resources explains. Besides the ecological benefits from protecting wetland habitats, increasing biological diversity  decreases the risk  of mosquito-borne disease.

A team of field ecologists have conducted biodiversity surveys on all 500 sites for three years and are collecting focused data on a subset of sites during 2022. An aquatic net is used to sample invertebrates in the water. The contents of the net are placed in an enamel tray where expert field technicians identify and record the type and abundance of invertebrates. This information is analyzed at the end of each season to learn more about the insect predators that feed on mosquitoes, as well as the characteristics of wetlands and how it correlates to mosquito populations.

Adult insects are sampled with sweep nets on the vegetation around the wetland site. Both adult and immature (nymph) dragonflies and damselflies are important predators for adult and larval mosquitoes.

The data from wetland ecological surveys provides information about biodiversity which is analyzed to determine the relationship between mosquito larvae, mosquito predators and other life at wetland sites. In addition, this data can used by city wetland and wildlife ecologists to improve the stewardship of our most fragile ecosystems.

Both adult and immature aquatic beetles are important mosquito predators. During a routine mosquito larval monitoring dip—a cup on a long stick is used to "dip" for mosquitoes—a larval beetle was also captured when refusing to let go of the Culex larva it was devouring.

Biodiversity surveys were conducted from 2019-2021. Over 4000 indvididual biodiversity records have been collected from mosquito breeding sites. Collaborating scientists are using advanced statistical methods to better understand how the presence of different species and groups of species of insects, birds, amphibians and fish impact mosquito larvae. Initial data analysis shows that increasing aquatic biodiversity richness correlates with lower mosquito larval numbers and that both dragonfly nymphs and adults are particularly important.

A subset of sites are being more intensely studied to gather more detailed data about insects, plant communities, water depth, temperature and dissolved oxygen and how it relates to overall biodiversity and mosquito populations.

The city is currently collaborating with mosquito disease researchers to design protocols and collect data to understand how biodiversity is related to mosquito-borne disease risk to better understand the characteristics of sites with high biodiversity and low larval mosquito density.