A published study indicates that bed bugs have body clocks, just as we do, and that understanding these rhythms could potentially allow for more effective control techniques.
It is well documented that some insect species, including flies, mosquitoes and cockroaches, display varying levels of sensitivity to insecticides over the course of a 24-hour period. While research has been carried out on various pest insect species, to date, this phenomena has not been reported in bed bugs.
It is well documented that some insect species, including flies, mosquitoes and cockroaches, display varying levels of sensitivity to insecticides over the course of a 24-hour period. While research has been carried out on various pest insect species, to date, this phenomena has not been reported in bed bugs.
On June 17, 2019, an international team of researchers from the Universiti Sains Malaysia and the Department of Medical Entomology at Westmead Hospital, NSW (including Dr Stephen Doggett), published a study examining insecticide efficacy against the common bed bug, Cimex lectularius. The results showed that, like other species, a bed bug’s susceptibility to insecticides can vary dramatically over a 24-hour period
In the study, the researchers investigated the insecticide tolerance of C. lectularius, to two different insecticides widely used in bed bug control, belonging to two different classes of insecticides – deltamethrin (pyrethroids) and imidcloprid (neonicotinoids).
To assess the insecticide effectiveness, bed bugs belonging to an insecticide-susceptible laboratory strain were exposed to a residual treatment of the insecticides at different time points during the day. Two groups of bed bugs were tested; one group which was on a 12-hour light/12-hour dark cycle and one group which was kept in constant darkness, 24 hours a day. The efficacy of the insecticides was determined by the length of time taken to achieve knockdown of 50% of test insects (KT50).
The results showed that there were significant differences in the knockdown time at different times during the day, for both deltamethrin and imidacloprid treatments, with the time point at which bed bugs showed the least susceptibility to the insecticides being nine hours into the light cycle (4pm). Interestingly, researchers also found that C. lectularius kept in total darkness demonstrated the highest level of tolerance at the same time (after nine hours into the subjective day).
Further statistical analysis revealed that the daily fluctuations in deltamethrin susceptibility were rhythmic, although no obvious daily pattern was observed in the imidacloprid susceptibility. This indicated that the level of tolerance to deltamethrin is connected to a metabolic process that varies according to a circadian rhythm – a natural physiological or behaviour activity that is governed by an internal body clock. Indeed, biochemical analysis demonstrated that the enzymes involved in pyrethroid detoxification also peaked at nine hours into the day period.
“This peak production of metabolic enzymes during the day may be somewhat surprising as we tend to think of bed bugs being nocturnal creatures attacking sleeping victims. However, the strain used is one that it typically blood fed during the day, and so the strain has adapted to its local situation. The reason for greater production of metabolic enzymes when it is active, is that this is the time when the insects leaves its harbourage and is more likely to come into contact with lethal chemicals,” commented Dr Doggett.
Whilst this study is the first to demonstrate that a circadian rhythm exists in bed bugs influencing the production of enzymes involved in the detoxification of insecticides, the authors were keen to caveat the results. The study was carried out on an insecticide susceptible laboratory strain fed during the day. Apart from differences in susceptibility, it is hypothesised that field collected strains would show a circadian rhythm with maximum enzyme levels during the night, to coincide with their nocturnal periods of peak activity and feeding. As a consequence their peak insecticide tolerance would occur during the night, quite the opposite to the lab reared strain. However, this would need to be confirmed in future trials.
What does this mean for pest managers? This study provides clear evidence that bed bug susceptibility to insecticides varies depending on the time of day. However, the variations in susceptibility are not the same for all insecticides. Nevertheless, with more research, the researchers pointed out that knowing time-of-day susceptibility for the various insecticides could be useful in enhancing insecticide-based treatment regimes. For example, with bed bugs, topical treatments are known to be more efficacious. So the application of a pyrethroid treatment during the period of maximum susceptibility would deliver improved performance, particularly on resistant strains.
Of course for researchers and product manufacturers it has implications as well, as the results of insecticide testing and therefore the conclusions about performance can vary greatly depending on the time of day the testing is carried out.
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Further reading: Muhammad, F.K., Chow-Yang, L., Doggett, S. and, Veera, S. (2019). Circadian rhythms in insecticide susceptibility, metabolic enzyme activity, and gene expression in Cimex lectularius (Hemiptera: Cimicidae). PLOS ONE. 14. e0218343. 10.1371/journal.pone.0218343.
