Why Don’t Mosquitoes in Collier County Show Resistance to Naled?

Keira J. Lucas, PhD

Deputy Executive Director

When it comes to mosquito control, one of the most effective tools in our toolkit is naled, the active ingredient in Dibrom Concentrate. Naled is part of a group of chemicals called organophosphates and has been reliably used across the United States for decades to reduce adult mosquito populations, including Collier County.

Naled continues to be an important tool for mosquito control because it has several advantages that set it apart from other products. Unlike pyrethroids, which are more commonly used, naled comes from a different chemical class (Sparks et al, 2020). Having more than one option is important in Integrated Mosquito Management because it helps prevent mosquitoes from developing resistance to any one product. Another benefit is that only a tiny amount is needed (less than half an ounce to cover an entire acre) making it very efficient. Just as important, naled breaks down quickly in the environment and does not linger.

And, the surprising thing: even after decades of use, mosquitoes in Collier County haven’t developed resistance to naled.

Why is that? The answer is a mix of nature and science. Southwest Florida’s unique environment plays a big role, and so does CMCD’s careful, science-driven approach to mosquito control. Together, they’ve helped us keep this tool effective year after year, allowing us to continue protecting both public health and our community’s quality of life.

Where We Do See Pesticide Resistance, and Why

While mosquitoes here remain susceptible to naled, resistance does occur with other products. In Collier County, it’s mainly seen in just two species:

• Aedes aegypti – an invasive mosquito that thrives around people and can spread diseases like dengue, chikungunya, and yellow fever (Estep et al, 2018; Parker et al, 2020; Schluep and Buckner, 2021; Lucas et al, 2022; Lucas et al, 2024; McDuffie et al, 2025).
• Culex quinquefasciatus – often found around homes and other artificial habitats, and known for spreading West Nile virus (Lucas et al, 2020; Watkins et al, 2021, Lucas et al, 2024).

Both species have developed resistance to pyrethroids, a chemical group widely used not only by mosquito control but also in repellents, household bug sprays and outdoor misting systems. So why is this happening? There are two big reasons:

1. Widespread use of pyrethroids at home. When residents rely heavily on sprays and misters for general pest control, mosquitoes that live in close association with people are indirectly and repeatedly exposed to the same chemical over and over again, giving them more chances to adapt.
2. The legacy of DDT. Heavy use of DDT decades ago created long-lasting “cross-resistance,” meaning today’s mosquitoes can still show resistance to related chemicals, such as pyrethroids.

Research shows that resistance in Culex quinquefasciatus mosquitoes is much stronger in urban areas compared to rural parts of Collier County, pointing directly to household product use of pyrethroids and other chemicals as a major driver (Lucas et al, 2020). And for the invasive Aedes aegypti, resistance is found throughout Florida, suggesting these mosquitoes may have arrived already carrying resistance traits, and further fueled by past DDT use (Estep et al, 2018; Parker et al, 2020).

Here’s a key point: CMCD’s operations are not driving this resistance. Prior to 2017, the District primarily relied on organophosphates such as naled, with minimal pyrethroid use. Yet resistance was already present in local mosquito populations before CMCD started using pyrethroids. This reinforces the evidence that residential use or historical relics, not CMCD treatments, are the main cause.

That’s why today, when we target species known to have pyrethroid resistance, we deliberately avoid using pyrethroids. By tailoring treatments based on science, CMCD ensures our control measures remain effective and that our community continues to be well protected.

Why Isn’t There Resistance to Naled?

If the District has used naled for decades, why hasn’t resistance developed to it? Here’s why:

• New Mosquitoes Are Always Moving In
  Collier County contains extensive state and federal conservation lands where mosquito treatments do not occur. Mosquitoes from these untreated areas regularly move into populated areas. These “naive” mosquitoes have never been exposed to pesticides, they mix with local populations and help prevent resistance from building up.

• Naled Isn’t Used by Homeowners
  Naled is a “restricted use pesticide”, meaning only trained and certified professionals can apply it. Since residents can’t buy or use it for everyday pest problems, mosquitoes aren’t constantly exposed to it in backyards or around homes. That makes it much harder for them to adapt and develop resistance.

• Smart Strategies and Early Detection
  Since 2017, the District has operated a rigorous pesticide resistance monitoring program. This allows us to detect early warning signs of resistance and adjust our control strategies before resistance becomes a problem. We also use a balanced approach called Integrated Mosquito Management, which combines different tools and methods instead of relying on just one product or control method.

• We Back Our Work with Science
  Our team doesn’t just apply treatments, we study them! CMCD has contributed research published in scientific journals and uses that data to guide decisions about which products to use, where to use them, and for which mosquito species. This commitment to science keeps our strategies effective and sustainable.

One Exception

So far, scientists at CMCD have only found one unusual case of low-grade resistance to naled in Aedes aegypti mosquitoes (Lucas et al, 2024). This happened at a golf course, where the regular use of pesticides, fertilizers and other chemicals may have created unusual pressures that made mosquitoes more broadly resistant to multiple products, not just naled.

But this was an isolated situation, not something seen across the county. The golf course has since closed and been replaced by a neighborhood, and those resistant mosquitoes are no longer around. In fact, when scientists tried to raise those mosquitoes in the lab to study them, the resistance quickly disappeared once the mosquitoes were no longer exposed to the same chemical pressures.

The bottom line: outside of that one short-lived case, no resistance to naled has been found in Collier County mosquitoes (Parker et al, 2020; Lucas et al, 2020; Watkins et al, 2022; Lucas et al, 2022; Lucas et al, 2023; Lucas et al, 2024; McDuffie et al, 2025).

Not All Resistance Is the Same

A common misconception is that if mosquitoes are resistant to a pesticide in one place, they must be resistant everywhere. But that’s not how it works. Resistance can look very different depending on the mosquito species, the chemical being used, and even the neighborhood. Science has shown this time and time again.

That’s why CMCD runs a strong local resistance monitoring program. Our scientists regularly test mosquitoes from across the county and build resistance “maps” showing which species are resistant to which products and where.

Armed with this information, we can match the right treatment to the right place for the right mosquitoes. It’s a careful, science-based approach that helps us protect the effectiveness of the limited tools available. By staying ahead of resistance, we make sure these tools continue working when they’re needed most.

Missed our first pesticide resistance blog post, read it here: https://cmcd.org/managing-pesticide-resistance-in-mosquito-control/

 References

Estep AS, Sanscrainte ND, Waits CM, Bernard SJ, Lloyd A, Lucas KJ, Buckner E, Vaidyanathan R, Morreale R, Conti L, Becnel JJ. (2018) Quantification of permethrin resistance and kdr alleles in Florida populations of Aedes aegypti (L.) and Aedes albopictus (Skuse). PLoS Neg Trop Dis. 12(10): e0006544.

Lucas KJ, Bales RB, McCoy KD, Weldon C. (2020) Oxidase, esterase and kdr-associated pyrethroid resistance identified in Culex quinquefasciatus field collections from Collier County, Florida. J Am Mosq Control Assoc. 36(1):22–32.

Lucas KJ, Bales RB. (2022) Insecticide resistance evaluation of Aedes aegypti mosquitoes from Collier County, Florida. Arthropod Management Tests. 47(1).

Lucas KJ, Babcock E, Bales RB. (2023). Baseline susceptibility and effectiveness of adulticides to local Aedes taeniorhynchus from Collier County, Florida. J Am Mosq Control Assoc. 39: 212-15.

Lucas KJ, Heinig R, Lake L, Williams K, Parker-Crockett C, Bales RB, Decyo McDuffie. (2024) Evaluation of a novel triple-action adulticide containing a pyrethroid, macrocyclic lactone, and fatty acid against pyrethroid-resistant Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae). J Med Entomol. 61: 701-709.

McDuffie D, Kacinskas S, Li S, Parker-Crockett, Lucas KJ*. (2025) Evaluation of ground and aerial ultra-low volume applications using ReMoa Tri against deltamethrin-resistant Aedes aegypti from Collier County, Florida. Trop Med Infect Dis 10: 119.

Parker C, Ramirez D, Thomas C, Connelly CR. (2020) Baseline Susceptibility Status of Florida Populations of Aedes aegypti (Diptera: Culicidae) and Aedes albopictus. J Med Entomol. 57: 1550-1559.

Schluep SM, Buckner EA. (2021) Metabolic Resistance in Permethrin-Resistant Florida Aedes aegypti (Diptera: Culicidae). Insects. 12: 866.

Sparks TC, Crossthwaite AJ, Nauen R, Banba S, Cordova D, Earley F, Ebbinghaus-Kintscher U, Fujioka S, Hirao A, Karmon D, et al. (2020)  Insecticides, Biologics and Nematicides: Updates to IRAC’s Mode of Action Classification—A Tool For Resistance Management. Pestic Bioche. Phys. 167: 104587.

Watkins A, Babcock E, Lucas KJ. (2021) Ornamental bromeliads of local Botanical Gardens serve as larval production sites for pyrethroid-resistant Culex quinquefasciatus in Collier County, Florida. J Florida Mosq Control Assoc. 68: 14-23.

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Dr. Keira Lucas serves as the Deputy Executive Director of the Collier Mosquito Control District, bringing a background in vector biology. She specializes in mosquito reproduction and biology, integrated pest management and pesticide resistance management, with experience in applying science-based strategies for effective and sustainable mosquito control.