Spotlight: Resistance Management Program at Harris County Mosquito and Vector Control Division

You may feel like the topic insecticide resistance is dynamic, complex, and ever evolving. In many ways, you’re right. As a vector control professional, you are challenged operationally every day to overcome resistance and successfully control mosquitoes. While this may be a new topic to some of us in vector control, it is a common theme across several entomological fields (such as agriculture and apiculture). To support you in your efforts to control mosquitoes, ADAPCO works to gather information and resources to better support your program’s needs for resistance monitoring and management.

So, what do you do if you think there is insecticide resistance in your area? How do you monitor for it and respond to it when it’s detected? With such an important topic, we wanted to take the opportunity to highlight programs in various regions throughout the US that have developed a resistance monitoring and management program to help them better control mosquitoes.

resistance management and monitoring

In the south-central region, we interviewed Chris Fredregill, Director for Harris County Public Health, Mosquito and Vector Control Division (HCPHMVC). Chris found his way to mosquito control while a student at Texas A&M. Dr. Jim Olson, a major proponent and supporter of the mosquito control industry, encouraged Chris to explore a career in vector control. Now, Chris manages a district that covers 1,778 square miles and protects 4.7 million residents from the public health threat of mosquitoes.

Since 1988, HCPHMVC has been monitoring resistance in some capacity. These efforts meant they were able to document resistance in their Aedes albopictus populations only a few years after their arrival to Texas. In 2000, they also found malathion resistance in some of their Culex populations, but after “resting” their organophosphate use for a few years, saw susceptibility recovered in those populations. Through collaborations, some external support, and prioritization of internal resources and expertise, HCPHMVC was able to establish a formalized and robust insecticide resistance management plan in 2005. Today, it continues to grow and evolve to address the important issue of insecticide resistance in mosquito vectors.

Harris County Public Health’s Approach to Monitoring and Managing Resistance

The insecticide resistance management plan at HCPHMVC is primarily focused on three species:

  • Culex quinquefasciatus, the southern house mosquito
  • Aedes aegypti, the yellow fever mosquito
  • Aedes albopictus, the Asian tiger mosquito

These species are important from both a nuisance and public health perspective. To monitor the resistance in these populations, there are several sentinel sites and survey areas throughout the county where mosquitoes are collected and used for both field and laboratory testing. Resistance testing is managed by HCPHMVC’s entomologist who supervises two full-time technicians and a seasonal technician as part of the testing and evaluation team.

field testing

Cages of Culex mosquitoes placed in spray plot awaiting treatment.

Field testing is considered their gold standard method for assessing product efficacy, but these field tests are run in parallel with the CDC bottle bioassay, which is a laboratory assay that also assesses phenotypic resistance. For Culex quinquefasciatus, there are 7 sentinel areas and 5 additional survey areas where truck-mounted field cage tests with permethrin and malathion are run annually. In tandem with this, CDC bottle bioassays are conducted with permethrin and malathion. For Aedes aegypti and Aedes albopictus, the approach is similar, but field tests are done with only permethrin and are conducted with a handheld ULV sprayer and the CDC bottle bioassay is done with both permethrin and malathion.

handheld ulv sprayer field test

Handheld ULV treatment of Field Cage Test plot, testing Aedes mosquitoes vs. permethrin.

looking for resistance

Preparation of samples for melt curve analysis of Knock Down Resistance Aedes aegypti.

A recently incorporated piece to HCPHMVC’s resistance monitoring capabilities is their ability to conduct melt curve analysis on their mosquito populations. This analysis allows them to gain an understanding of resistance at the genotypic level. This assay allows the identification of mutations implicated in knockdown resistance (kdr). Taking it one step further, Chris and his team also have plans to determine the feasibility of doing genomic sequencing on their mosquitoes to gain an even more granular look at resistance.

In summary, HCPHMVC utilizes laboratory and field assays to assess both the genotypic and phenotypic resistance of their mosquito populations. This combination of assays allows HCPHMVC to have a detailed and dynamic understanding of their local mosquitoes and how to best control them while also being good stewards of the limited chemical toolbox available.

Operational Application of Resistance Management Program

With such a wealth of resistance information on their local mosquito populations, it’s important to understand how that information is translated into operational control. In the early days of their resistance management program, the detection of resistance led the HCPHMVC team to reduce use of an entire chemical class for several years.

The result was a relative return to susceptibility, and they regained use of both chemical classes. Today, HCPHMVC minimizes adulticide use by reserving it for mosquito-borne disease cases or in the event of a declared emergency. When they do adulticide, products from different chemical classes are rotated to ensure that the mosquito populations aren’t being hit with the same materials repeatedly. This rotation is done on a weekly basis between a synergized permethrin product (pyrethroid) and a malathion product (organophosphate).

adulticiding truck

Additionally, HCPHMVC’s efforts are not focused solely on adulticides. Instead, they implement a full Integrated Vector Management strategy that emphasizes surveillance information, community education, source reduction, and larviciding. Additionally, they continuously consider and evaluate novel control methodologies such as the sterile insect technique (SIT), traps like the In2Care® trap, or Wolbachia-infected mosquito releases. By having a comprehensive IVM program and keeping their pulse on emerging technologies and control strategies, HCPHMVC can reduce insecticide exposure and delay/ prevent the development of resistance.

When asked what changes had been observed as a result of the resistance management program and what the biggest benefit was, Chris responded:

“One of the biggest changes I have observed was the re-introduction of malathion after several years resting. Malathion use was discontinued in the early 2000s due to resistance issues, but after reevaluating it in field cage tests, we discovered that Culex quinquefasciatus populations had regained susceptibility. Malathion was then reincorporated into a rotational treatment strategy. The biggest benefit of the program is the ability to ensure that mosquito populations maintain susceptibility so that our efficacious applications are not only saving lives, but money.”

Challenges to Resistance Monitoring and Advice to Get Started

In Chris’s opinion, the biggest challenges are:

  1. Labor requirements associated with collecting, rearing, and testing mosquitoes for insecticide resistance.
  2. The multitude of tests required to characterize resistance mechanisms.
  3. Determining the best way to correlate the various tests for a complete picture of insecticide resistance across an area as large as Harris County each season.

Despite these challenges, Chris still believes that it is beneficial to have a resistance management plan.

CDC bottle bioassay

“My advice is to start small with CDC bottle bioassays. Kits are available for free from CDC. This is the easiest way to get in the insecticide resistance management game. First, get a snapshot of what your local mosquito populations resistance profile looks like, then begin to refine your efforts to get a more complete picture. Once you have that baseline data, you can begin to make decisions and guide activities to use products more effectively.

Don’t think you have to build a complete program overnight; some information is better than none. Don’t let perfection be the enemy of better. Also, don’t feel you have to go it alone, there are a lot of resources: colleges & universities, the Centers of Excellence, state agencies, and industry all have a vested interest in IRM and are willing to collaborate and help. Insecticide resistance is an enormously complicated, important issue. It won’t be fixed in a day as it wasn’t created in a day. Try not to get overwhelmed by attempting to boil the ocean and approach it methodically, one step at a time. It is going to require the efforts of all of us (vector control, pest control, homeowners, regulators, industry, academia, etc.) to get the issue under control so buckle up, do your part and keep protecting folks from mosquitoes and the diseases they transmit.”

As you can see, there is not just one right answer when it comes to tackling and responding to insecticide resistance. Let the capabilities of your vector control program, your local mosquitoes, and the needs of your constituents drive what your insecticide resistance management plan looks like. Identifying the problem and taking even a small step to combat it is better than no attempt at all.

~Written by Casey Crockett, PhD
Technical Development Specialist, ADAPCO

ADAPCO would like to note that the CDC is currently offering free bottle assay kits: Programs in the continental United States and its territories can order free Insecticide Resistance Kits by sending an email to and requesting an order form. Kits include bottles, insecticide, and manual.

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