The resurgence of bed bugs that began in the early 2000s has been linked to insecticide resistance. However, bed bugs’ ability to develop resistance is not a new story.
Historically, bed bugs have developed resistance to insecticides from the chlorinated hydrocarbon class, such as DDT. Recently, researchers have found that certain bed bug populations are resistant to chemicals within the pyrethroid, neonicotinoid and pyrolle classes.
How a population becomes resistant to an insecticide is simple: before a product is applied, some individuals in a population have rare resistance genes that are beneficial in the presence of an insecticide. Once a chemical is applied, a few of these insects survive exposure because of their resistance genes. These insects then reproduce and pass on resistance genes to their offspring.
It is important to consider that just because bed bugs are found alive after a treatment does not automatically mean they are resistant to a particular insecticide. They may have avoided exposure or could have been reintroduced. Still, there are steps that can be taken to identify insecticide resistance and prevent its development while also eliminating an infestation.
Additionally, there are a variety of new products on the market to which bed bugs have not developed resistance to yet. Incorporating the strategies described in this article into a control program can prevent resistance development and help maintain the efficacy of products that have recently been introduced to the market.
PHYSICAL CONTROL. One effective way to prevent insecticide resistance development is to use physical or non-chemical control methods that cause mortality through a non-specific mode of action in combination with insecticides. Physical control methods reduce the amount of liquid insecticide needed and can be used in certain areas where insecticides cannot be used. There are a variety of non-chemical techniques that can be deployed.
For example, bed bugs can be removed from furniture by thoroughly vacuuming or by using a steamer that expels hot steam higher than 200°F.
One natural product that can be used is silicate dust, which abrades an insect’s cuticle, causing them to desiccate. Furthermore, because of how desiccants work, bed bugs cannot develop resistance to them. Desiccant dusts should be applied in cracks and crevices because of inhalation hazards to people.
Preventive measures, such as placing pitfall interceptors on the legs of furniture or using mattress encasements, can trap insects and may cause them to starve or desiccate. Deployment of interceptor traps also serves as a way to monitor the efficacy of a treatment and detect future introductions.
In heavy infestations where bed bug numbers are very high, consider using thermal remediation. Lethal heat can kill all bed bug life stages if infested items reach 122°F for a sufficient length of time. After a heat treatment, insecticides can be applied to bed bug harboring areas or to spots that did not reach lethal temperatures. By integrating a variety of non-chemical strategies into a control program, the number of insecticide resistance genes in a population can be reduced and better control can be achieved.
|Bed Bug Control Tools||Options|
|Prevention and Detection||Education of residents, active or passive interceptor traps, mattress encasements, canine inspections|
|Non-chemical Methods||Vacuuming, steaming, desiccant dusts, freezing infested items, thermal remediation (above 50°C)|
|Insecticide Classes||Pyrethrins, pyrethroids, neonicotinoids, pyrolles, synergists (added to an insecticide), insect growth regulators, fumigants (DDVP and sulfuryl fluoride), essential oils (biochemicals)|
|Table 1. Available control options for bed bug prevention, detection and elimination.|
USING JAR TESTS. Methods also have been developed to test the presence of insecticide resistance in a population. If time permits, conducting a “jar test” is a way to test how effective an insecticide will be on a bed bug population, whether they are resistant or not and how a substrate will affect insecticide performance.
To prepare a test, treat a container with a small amount of insecticide. A good time to prepare a testing container is right after mixing an insecticide and allowing it to completely dry. Next, collect insects and place them in the treated container either before applying an insecticide in the infested account or upon a follow-up visit. If any of the insects survive exposure to chemicals from the pyrethroid or neonicotinoid classes after 24 hours, then there are likely some resistant individuals present in the population. Insecticides from the pyrolle class are slower acting, therefore it may take longer for susceptible bed bugs to die.
If needed, there are also a variety of pre-made kits available for testing resistance to an insecticide (see authors’ note on page 83). Kits may come treated with an insecticide or have different substrates inside to test how a surface influences product performance. Testing different substrates is valuable because studies have shown that porous substrates can reduce the amount of insecticide available on the surface. If insects survive exposure to the chemical inside the jar test, mixing a synergist with insecticides such as pyrethroids, pyrethrins or neonicotinoids, while also using non-chemical control methods, can prevent further development of resistance.
ADDING SYNERGISTS. By adding an appropriate synergist to a product, the efficacy of the insecticide will increase against some resistant individuals. Synergists, such as PBO and MGK-264, have low toxicity by themselves but interfere with a specific mechanism of insecticide resistance, where individuals are resistant due to the presence of higher quantities of detoxification enzymes.
When the liquid insecticide/synergist mixture enters a bed bug, the synergist interferes with the insect’s enzymes that detoxify the active ingredient, thus enhancing efficacy. A caveat to using a synergist is that in order for them to make an insecticide more efficacious, the insects must be contacted by the liquid mixture, since dried synergists lose their efficacy.
Before combining a synergist with an insecticide, be sure to read and follow all label directions because there are some insecticides that cannot be used with synergists. For example, pyrolle class insecticides may not be used with synergists. Besides having higher amounts of detoxifying enzymes, other mechanisms of insecticide resistance have been found in bed bugs. Therefore, another chemical-based strategy also should be used to prevent resistance development.
PRODUCT ROTATION. This other chemical-based strategy is called product rotation theory and is implemented when conducting a follow-up treatment. For this strategy, if an insecticide treatment is needed during a follow-up visit, the product applied is from another chemical class with a different mode of action than the insecticide applied on the previous visit.
Product rotation theory works on the idea that if any individuals in a population are resistant to an insecticide from one class, they are likely not going to be resistant to an insecticide from another. Therefore, if they are exposed to an insecticide from a different class each time, then they will likely not develop resistance to any single product. The time for rotating to an alternative product is based on the life cycle of the pest.
For bed bugs, it takes a little longer than a month for an egg to hatch and reach adulthood. Therefore, when rotating to a different product for bed bug control, the change should occur at least once every month when visiting an account. For more information on the groupings of insecticides and resistance management, visit the Insecticide Resistance Action Committee’s (IRAC) website at www.irac-online.org.
FINAL THOUGHTS. In conclusion, history has taught us that we should not ignore the potential for bed bugs to develop resistance to products. However, there are a variety of strategies that we can use to not only prevent the development of insecticide resistance, but to also detect resistance and to deal with it if it occurs.
Combining non-chemical control techniques that have a non-specific mode of action with an insecticide, and possibly a synergist, will reduce the selection pressure that leads to resistance build-up in a population. When conducting a follow-up treatment, applying an insecticide with an alternative mode of action will further reduce the potential for resistance development. By utilizing a variety of control tools, the efficacy of products can be maintained while also providing the much-needed relief for those impacted by bed bug infestations.
Aaron Ashbrook is a Ph.D. student, Ameya Gondhalekar is a research associate professor and Gary Bennett is a professor of entomology at Purdue University.
Authors’ note: Resistance test kits can be obtained from Protect-A-Bed (http://ow.ly/LjbW30jTF9N) and the World Health Organization (http://ow.ly/eD6G30jTFfA).