Can Bed Bugs Develop Thermal Tolerance?

The bed bug resurgence has prompted the pest management industry to utilize a multifaceted approach that includes chemical and non-chemical control tools. For controlling bed bugs with heat in urban environments, the three methods used by PMPs are steam, compartmentalized heaters and whole domicile heat treatments. All these methods can eliminate bed bugs when properly utilized.

The choice of what heating method to use depends on the scale of pest management that is needed. Steam is used to spot treat areas of heavy bed bug aggregations on furniture and other areas. A wide nozzle spout should be used to ensure that bed bugs are not propelled around the room. To ensure efficacy, the spout of the steamer should be placed directly on top of the bed bug harborage area; the steamer should move at a rate of 1 foot per second. Rutgers University researchers have shown that when steaming furniture, heat should be applied from the bottom up to prevent bed bugs from falling and resulting in sublethal exposure. They also have shown that steam causes low mortality in bed bugs harboring under leather fabric but does work well on bed bugs hiding under cotton and polyester fabric.

Compartmentalized heat chambers are a flexible option for heating items because these units are available in various sizes. Smaller heat chambers can be used for heat treating items of incoming patients at medical facilities or for treating personal items such as luggage. Research conducted at the University of California, Irvine has shown that when bed bugs residing on empty luggage are heated in a small container for six minutes at 158-167°F, complete mortality can be achieved. Infested items also can be removed from homes or apartments, etc., and heated in large transportable compartmentalized heat chambers.

When many items and areas within a home are infested by bed bugs, whole-home heat treatments can be used. Heat treating a home requires six to eight hours to achieve and maintain temperatures that are lethal to bed bugs within their harborages. Heat treatments can provide more immediate relief to individuals who are being impacted by a large infestation. Another advantage of heat treatment is that household items that cannot be treated by insecticides can likely be heated, unless they are temperature sensitive and therefore should be removed. The scale of the area to be treated should be considered because it can be difficult to reach lethal temperatures (greater than 122^°F) when the area to be treated is large.

TOLERANCE TO HEAT? In some cases, live bed bugs are found after heat treatments, which could be caused by uneven temperature distribution within the treated structure. Variable domicile temperatures, as well as items with different thermal conductivities, could help bed bugs escape to cooler areas or be exposed to sublethal heat. If bed bugs are repeatedly exposed to sublethal temperatures over many generations, they could be selected for greater thermal tolerance as has occurred with other pest insects. Therefore, the goal of our research was to 1) determine if bed bugs could develop greater heat tolerance in the laboratory and 2) determine if populations from urban settings had different heat tolerance.

To achieve the first goal, we tested if a laboratory population of bed bugs could develop greater thermal tolerance by exposing them to sublethal heat over several generations (see Figure 1 on next page). This was done by exposing large nymph bed bugs to heat (113°F) for a time that would kill 75 percent of the insects. The nymphs that had survived the heat exposure were then reared to adulthood. Once the offspring of the surviving adult bed bugs had reached sufficient numbers for more experiments, the heat exposure was then repeated. We did this heat selection experiment over multiple generations. The heat selection bioassays found that the survivorship of the heat-selected bed bugs first increased incrementally to 56 percent after three generations of selection, but subsequently decreased to initial levels (23-25 percent) in later generations. This data likely suggested that the ability of bed bugs to develop greater heat tolerance could be limited.

To help understand the reduction in survivorship that was observed during the selection experiments, we heat exposed a new group of bed bugs. We then offered the heat-exposed survivors of the experiment a blood meal and compared the number of insects that fed to unheated control bugs. We found that heat-exposed bed bugs fed at significantly reduced rates for up to two weeks after the experiment was conducted. This finding is similar to a conclusion reported by Rutgers University researchers that found that if bed bugs are exposed to sublethal steam, their ability to feed is reduced. When conducting a heat treatment in the field, you can expect that if some bed bugs are exposed to sublethal heat they may not take a blood meal for approximately two weeks afterwards. Therefore, when conducting follow-up inspections, a client may not mention being bitten, but this does not mean that bed bugs are not present.

During the heat selection experiments, we found that in some cases, exposure to sublethal heat impacted the ability of bed bugs to successfully molt to adulthood (see photos). Therefore, if bed bugs are exposed to sublethal heat in the field, delayed mortality may occur due to failing to successfully molt. Another impact of heat exposure has been identified by researchers in Norway, who have shown that female bed bugs exposed to sublethal heat will lay fewer eggs that hatch at lower rates. Finding that sublethal heat exposure negatively impacts the ability of bed bugs to feed, reduces their chances to successfully molt, and causes a reduction in egg laying suggests that heat also would increase bed bug development time if they survived heat exposure in the field.

In our final experiments, nine field- collected populations of bed bugs had their thermal tolerance determined by exposing them to heat using two techniques. For the first set of experiments, the field- collected populations were exposed to lethal heat (113°F) and then we determined the time required to reach 99 to 100 percent mortality in each population. We found that different field populations of bed bugs required similar heat exposure times to reach 99 to 100 percent mortality levels (see Table 1 on page 96). For the second set of experiments, we used the same field populations but they were first exposed to rising heat from 77°F to 113°F. When the lethal temperature of 113°F was reached, it was maintained for the estimated time required to kill 99 percent of the susceptible laboratory population. In the second set of experiments we observed complete mortality in all the tested populations, confirming that different bed bug populations likely have similar thermal tolerance.

CONCLUSIONS. Our findings that sublethal heat exposure reduces bed bug feeding and lengthens their development time, and that different populations have similar abilities to tolerate heat exposure, suggests that their ability to develop greater heat tolerance is restricted. If live bed bugs are found during a follow-up inspection after a heat treatment, the chance that these insects have developed heat resistance is probably low. One possible explanation for why bed bugs could remain after a heat treatment is that they were exposed to sublethal heat, escaped from high- temperature locations or were introduced to the domicile at a later time.

It is important to remember that heat is one of many tools that are available for bed bug elimination and when necessary, should be deployed with other IPM strategies and insecticides to enhance control outcomes.

Authors’ notes: This research was in part funded by the National Pest Management Foundation. To read the original peer-reviewed manuscript associated with this study, refer to the paper by Ashbrook et al. (2019) at https://buff.ly/3gL6zCH.

Aaron Ashbrook is a post doctoral researcher at North Carolina State University. Mike Scharf is a professor and O.W. Rollins/Orkin Chair at Purdue University. Gary Bennett is a recently retired professor and director of the Center for Urban and Industrial Pest Management at Purdue University. Ameya Gondhalekar is a research associate professor at Purdue University’s Center for Urban and Industrial Pest Management.

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