Finding bed bugs a little difficult to control? Got a few hardy survivors that just refuse to go away? Well, there may actually be a surprising reason behind it, according to David Lilly at the Department of Medical Entomology, University of Sydney.

In the last few years I have been studying bed bugs collected from across Australia and screening them for resistance to pyrethroids. And what we’ve found is that, essentially, all of the bed bugs we’ve collected are resistant, but that the magnitude of the resistance often varies from strain to strain. 

In an effort to understand what may be causing this variation I started to have a closer look at some of these bugs and, after a time, came to focus on one particular strain collected from Parramatta (a suburb of Sydney). This strain was found to be incredibly resistant to pyrethroids and, in fact, at the time it was collected we found it to be our most resistant strain ever. However, we also noticed that when we forcibly kept the bugs on a recently insecticide-treated surface, some of the bugs would succumb after a relatively short time, some would die after a few hours, and some simply wouldn’t die at all.  

What could be causing this? We knew based on previous research that this strain had a uniform genetic profile for another resistance mechanism that prevents insecticides working at their target site, so it shouldn’t have been that. And tests on enzymes that might detoxify insecticides similarly showed a fairly constant response. The only other option was that something was different in the cuticle of the bugs that died at different times. 

Knowing this, we carefully cultured some bugs so they were the same age, sex and had been fed and kept the same way, and then repeated the forced-exposure experiment. After separating them into their respective response times, we had a close look under scanning electron microscope at the thickness of the cuticle (based on a mid-point of the tibia of the middle leg). 

The result: thickness of the cuticle increased with how long it took for the bugs to be affected by the insecticide. When this finding is combined with what we already know about the presence of target-site and metabolic detoxification in this strain, it means that these ‘super-resistant’ bugs are perfectly evolved to resistant most pyrethroid insecticides thrown at them today. 

The important implication of this is that when attempting control of bed bugs in a field situation, you’re likely to be up against some bugs that are just naturally extra-resistant compared to all the other bugs in the infestation. Miss those bugs and they’ll either start the infestation anew, or potentially spread it to a new location.

Fortunately, there are two main ways to partially overcome this super-resistance. The first would be to use non-chemical means (no matter how resistant these bugs are they can’t withstand a vacuum!), and the second is to ensure you’re using an insecticide either with piperonyl butoxide (PBO) or one that is a combination product with pyrethroid and neonicotinoid (such a imidacloprid). Both methods significantly improve the effectiveness of control methods, and thus reduce the likelihood of call-backs. 

David Lilly is a PhD Student at the University of Sydney based at the Department of Medical Entomology, Westmead Hospital, Westmead. He is a recipient of an Australian Postgraduate Award that is generously supported by an Industry Top-Up Grant from Bayer CropScience, Australia.

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