Dr David Lilly presents the findings of his studies into the effect of pyrethroids on common bed bugs.
It has been well established over the last decade that insecticide resistance in bed bugs, particularly to pyrethroids, has become a major problem around the world. In addition to contributing to the rapid resurgence in infestations, the resistance expressed by bed bugs has resulted in the efficacy of many formulated insecticides being compromised, and control in the field made more difficult as a result. In fact, a recent survey in the USA found that bed bugs are now regarded by professional pest managers as the hardest pest to control (Potter et al., 2015). Despite this, to date there has been no quick and cost-effective way for pest managers to determine if a field infestation is highly resistant and, similarly, no clear understanding of whether all bed bugs are equally resistant, or just a small percentage of them.
To examine this, we have been collecting bed bugs from across Australia for the past four years and evidence we have gathered suggests that, while resistance is certainly pervasive, not all bugs are the same. In fact, when 35 different field strains of the common bed bug, Cimex lectularius, were exposed to a high dose of deltamethrin (equivalent to nearly ten times the label rate), a full spectrum of responses was obtained (Figure 1). Several strains were completely controlled, with no survival, whereas in just over a quarter of the field strains more than 60% of the treated bed bugs survived. Moreover, across the whole study the average survival was close to 50%, suggesting quite a lot of variation is present within such field infestations.
A more concerning finding from the same study was that the trend of susceptibility of Australian bed bugs to neonicotinoid insecticides also appears to be changing (albeit only with C. lectularius). Up until very recently, exposing the same field-strain bed bugs with a dose of imidacloprid resulted in almost no survivors, however, several strains collected just within the last two or three years have started to show some reduced susceptibility (Figure 2), with some able to survive the lethal dose. It is not yet known if this is unequivocally the development of resistance, but a similar study in the USA found resistance to neonicotinoids was present at high levels (Romero & Anderson, 2016), thus suggesting that we will in all likelihood soon experience the same loss of efficacy.
The most likely reason for the differences we are observing in Australian bed bugs is because of variation in the mechanisms that confer resistance to the insecticides. We have also shown that Australian bed bugs can actively breakdown insecticides (a process called metabolic detoxification) and that one strain collected in Parramatta (in the western suburbs of Sydney) had evolved a thicker cuticle that enables a small proportion of the strain to survive continuous insecticide exposure (Lilly et al., 2016). When combined with kdr-type resistance (a process whereby the insecticide can’t bind to the target site anymore), which is also known to be present across Australia (Dang et al., 2015), this means that bed bugs are uniquely adapted to resist our control efforts.
This may all sound rather depressing, but there is actually a cheap and cost-effective way that professional pest managers can determine if they are dealing with resistant bed bugs or not. This method was developed by Dr Kai Dang (who also studied at Westmead Hospital) and simply involves confining ten bed bugs (of any life stage) onto a Mortein mosquito vaporiser mat (Figure 3). If the bed bugs survive longer than one hour – they’re resistant! Of course, even if some bed bugs die within the hour, our research would suggest that you should be particularly careful as it is highly likely that a small proportion of the infestation is still resistant anyway, thus making control very difficult if employing only traditional insecticidal means.
David Lilly*, University of Sydney, NSW
Further reading: Lilly, D. G., Dang, K., Webb, C. E., & Doggett, S. L. (2016). ‘Are Australian field-collected strains of Cimex lectularius and C. hemipterus (Hemiptera: Cimicidae) resistant to deltamethrin and imidacloprid as revealed by topical assay?’ Austral Entomology, (early view), doi: 10.1111/aen.12268.
*David Lilly completed his PhD at the University of Sydney (Department of Medical Entomology, Westmead Hospital) and is now working as principal entomologist at Ecolab Pest Elimination. David continues to be interested in the status of insecticide resistance in bed bugs in Australia, and can be contacted at firstname.lastname@example.org.