Vector Disease


Mosquitoes Playing Hard to Get? Consider Resistance Monitoring

Posted by The VDCI Team on Apr 10, 2018 2:19:53 PM

Written By Kellie Nestrud, Biologist and Contract Manager in Louisiana 


There are several different components of a successful Integrated Mosquito Management (IMM) program. The consideration of chemical resistance in the local mosquito population is one of the components. Knowing, understanding, and monitoring for chemical resistance should begin as early as possible in an IMM program. It is recommended that all IMM programs monitor their mosquito populations for resistance at the beginning of a season and as often throughout the season as thought necessary. Resistance data is most valuable when collected over time to allow for comparison and monitoring of trends. There are many methods to monitor the effectiveness of an insecticide, and program managers may need to adjust their approach from season to season.

The Centers for Disease Control and Prevention (CDC) defines that a population of mosquitoes is considered to be resistant to an insecticide if a mortality rate is less than 90%. So how would one know if they are working with a population that has resistance?


CDC Bottle Bioassay

western-gulf-center-of-excellence-VBD-training_houston-TX-krisn (6)-716702-edited-825024-editedThe most widely recommended and accepted practice for monitoring resistance is the CDC Bottle Bioassay. This method involves using mosquito eggs or larvae from the population of concern and rearing them to the adult stage. The study can also be completed by collecting adults from the field population. Once the adult mosquito population is ready to be tested, the inside of the testing bottles are coated with a dilution of the pesticide that is being tested and allowed to dry. There should always be at least two bottles that are only exposed to the drying agent (usually acetone) and these bottles will be labeled the “Control” group. Having a control group allows for the demonstration of proper handling techniques of the mosquitoes. Once the chemicals in the bottles have dried completely, the adult female mosquitoes are added to each of the bottles. The control mosquitoes go through all of the stresses that each other set of mosquitoes must endure, except for actual exposure to the chemical being tested. Mortality counts are taken in 15-minute increments until all mosquitoes are dead or 2 hours, whichever comes first. Once the CDC Bottle Bioassay tests have been run, the data is then graphed to show the level of resistance, if any. The formulas for the chemical dilution as well as the guidelines for handling adult mosquitoes from the field, to help ensure that the population is healthy at the time of testing, can be found on the CDC website.

Is it Possible to Use a Chemical Routinely and NOT Observe Resistance?

20180228_135431-219546-editedThe great new is, it is! A research project on the topic was recently completed on one of our contracts in the Delta. After eight (8) years of using the suggested mid-label rate of application for the insecticide resmethrin, followed by nine (9) years of mid-label rate applications of the insecticide permethrin in Ruston, LA, local Culex quinquefasciatus eggs were collected and sent to the CDC lab in Fort Collins, CO. The CDC reared the eggs to the adult stage and ran CDC Bottle Bioassays using the chemicals resmethrin and permethrin. The results showed no signs of resistance with a mortality rate of 100% at 30 minutes. The Ruston example was a piece of a larger research project lead by the CDC

Overcoming a Resistant Population

Resistance has become more and more of a concern as years of chemical applications surmount. Evidence shows that the longer adulticide applications have been made in an area, the higher the likelihood of finding resistance in the local mosquito population, which makes perfect sense. Once resistance has been determined, there are a few management methods to address the problem that will allow for continued control of the resistant mosquito population.


Management by Moderation. This method is meant to maintain the susceptible genes in the overall mosquito population by using low insecticide rates, infrequent applications, and non-persistent compounds. 

Goal: Genetically speaking, you are trying to give the local population a chance to “recover.” 


Management by Saturation.  The approach is just as it sounds. The population is saturated with sufficient doses so that no survivors remain. This method can be especially useful during the early stages of selection of resistant genes.

Goal: To put a break in the resistance and not let it continue to the next generation. 


Management by Class Rotation. An additional option, for continuing to successfully manage a mosquito population with known chemical resistance, is to change to a different class of chemical that uses a different Mode of Action (MoA). There are many EPA approved pesticides available for the use of mosquito control. However, there are limited classes of chemicals - only three (3). This is important because each class of chemical has a unique MoA, which is the method of interrupting a mosquito’s vital processes that result in its death. But with only three options available, and typically a significant price increase between classes, it is important that this method is used responsibly.

Goal: To prevent further growth of resistance in a population by introducing a chemical that has an alternative process to attack the mosquito’s body functions and result in death.

Potential Resistance Phenomena

western-gulf-center-of-excellence-VBD-training-cdc-bottle-assay-250x152-wb_houston-TX-krisnSometimes IMM programs will run into additional resistance problems that can further convolute treatment plans.


Cross Resistance means that the population is not only resistant to one insecticide of a particular class, but also to other insecticides in the same class, even when that population has never been treated with that insecticide. 


Multiple Resistance refers to separate detoxification mechanisms for unrelated insecticides are present resulting in a mosquito population that is resistant to different classes of insecticides (with different MoA) which makes chemical control of that population extremely difficult. 


Behavioral Resistance. There is also documentation of a phenomenon called Behavioral Resistance where mosquitoes have been found to no longer rest on surfaces that have been treated with a residual adulticide chemical in the past.

Ignoring the Problem

What happens if you continue to apply a pesticide that the local mosquito population is known to be resistant to? You end up killing only the susceptible mosquitoes leaving behind a population that is completely resistant to that chemical. This population will pass on their resistant genes to their offspring. Then that particular chemical becomes completely useless in managing the local population. 

Advice: Monitor, Monitor, Monitor

western-gulf-center-of-excellence-VBD-training-cdc-bottle-assay-sheet-250x152-wb_houston-TX-krisnResistance is not a new concern in the mosquito management industry. However, it is possible to minimize the occurrence of resistance if the chemicals are applied responsibly and the local mosquito population is tested regularly in an attempt to detect resistance in the earliest stage possible. As mosquito control professionals, the top priority of mosquito management is to be effective at protecting the public from the many vector-borne diseases that are transmitted by mosquitoes. In order to maintain our ability to do such, we must be familiar with chemical resistance levels and be proactive in addressing resistance once found. Monitoring for chemical resistance is just one of the many components of a successful Integrated Mosquito Management Program, but it is an important one that should not be overlooked.


VDCI is committed to public education and spreading awareness throughout the U.S. about the dangers of mosquito-borne diseases and their preventability, with the overarching goal of reducing illness and fatality statistics in 2018. Our dedicated and experienced team works tirelessly to prevent the spread of mosquito-borne diseases in all of the contracts we service. If you would like more information about any aspect of an Integrated Mosquito Management (IMM) Plan, including mosquito surveillance, disease testingadult control, or aerial applications, please contact Vector Disease Control International (VDCI), and we will help you get started immediately.

Mosquitoes in Your Community? Let's Talk.

bio-pic-kellie-nestrud-lowery_100x150Kellie Nestrud is a Contract Manager for Vector Disease Control International (VDCI) in our Delta Region and has been a member of the VDCI family since 2002. Kellie holds a degree in Biology with a minor in Chemistry from Louisiana Tech University. Her experience includes research trials for new chemicals, bottle bioassays for tracking chemical resistance in mosquito populations, and participating in Emergency Response missions following hurricanes or major flooding. She is one of the few privileged to study mosquito identification under the late Dr. Richard Darsie Jr. (Co-author of Identification and Geographical Distribution of the Mosquitoes of North America, North of Mexico) at Florida Medical Entomology Laboratory (FMEL). She can be reached through the VDCI website or by calling 800.413.4445.


Since 1992, Vector Disease Control International (VDCI) has taken pride in providing municipalities, mosquito abatement districts, military bases, industrial sites, planned communities, homeowners associations, and golf courses with the tools they need to run effective integrated tick and mosquito management. We are determined to protect the public health of the communities in which we operate. Our tick and mosquito management professionals have over 100 years of combined experience in the field of public health, specifically vector disease control. We strive to provide the most effective and scientifically sound mosquito surveillance and control programs possible based on an Integrated Mosquito Management approach recommended by the American Mosquito Control Association (AMCA) and Centers for Disease Control and Prevention (CDC). VDCI is the only company in the country that can manage all aspects of an integrated tick and mosquito management program, from surveillance to disease testing to mosquito aerial application in emergency response  situations.


Contact the professionals at 800.413.4445 for all of your integrated tick and mosquito management needs.

Topics: Integrated Mosquito Management (IMM), Surveillance and Disease Monitoring