By Cameron Webb
The wellness industry is booming, but it’s not just people looking for the latest “detox” diet. Some of our closest parasitic foes are well ahead of us in mastering the art of detoxification.
Bed bugs are one of the most loathsome of all pests that attack and bite humans. Their resurgence in recent decades has caused heartache to many a suffering victim. Eliminating an infestation can be time consuming and, more significantly, extremely expensive. This is of increasing concern as bed bug infestations are often hitting the least well-off members of our community.
There are many reasons why bed bugs have made a comeback, but their resistance to commonly used insecticides is the most widely accepted explanation.
It is one thing to know insecticides aren’t working, but understanding the biological reason why they’re not working is critical. In a new paper, “Evidence for metabolic pyrethroid resistance in the common bed bug (Hemiptera: Cimicidae),” published in the Journal of Economic Entomology, scientists from the University of Sydney and NSW Health Pathology describe research that provides evidence that metabolic detoxification is a major contributing factor to pyrethroid resistance in the common bed bug, Cimex lectularius.
Metabolic detoxification is simply the process by which bed bugs break down insecticides. In this particular research, two types of detoxification enzymes — broadly called “esterases” and “oxidases” — were the focus. These two types of enzymes change the chemical composition of insecticides so that they’re less harmful to the insect.
To understand which enzyme type might be responsible for resistance to commonly used insecticides, chemicals called “synergists” can be used. Synergists can inhibit or lower the levels of detoxifying enzymes, thereby increasing the toxicity of the insecticide. By using different types of synergists, it is possible to determine which enzymes may be present.
One of the most widely used synergists is a chemical called piperonyl butoxide (or PBO), which is commonly found in most fly sprays. PBO is a very useful chemical as it can inhibit both esterases and oxidases, but that ability in turn makes it hard to determine which enzyme type is contributing to the resistance.
However, a new synergist known as EN16/5-1 only inhibits oxidases, and not esterases, so it provides an opportunity to investigate the role of metabolic resistance and to determine which enzyme type may be responsible for resistance.
“What we set out to do was to determine the biological processes by which bed bugs were becoming resistant to insecticides,” said David Lilly, a PhD candidate at the University of Sydney and lead author on the study. “We knew our bed bugs were highly resistant and that metabolic detoxification was almost certainly involved, but in using PBO we didn’t know which type of enzymes might be involved. The development of EN16/5-1 was the solution we were looking for.”
A range of bed bug strains originating from cities across Australia were selected for testing. This included two strains from Sydney (New South Wales), two from Melbourne (Victoria), and one from Alice Springs (Northern Territory).
The bed bugs were exposed to six different treatment categories. These categories included a combination of insecticide and synergist, as well as controls without insecticides. The mortality of more than 200 individual bed bugs that were exposed to these treatment categories was observed and recorded.
So how do you apply insecticides to bed bugs?
“We’ve found that the easiest way to apply precise doses of insecticides on bed bugs is by first ensuring the bugs can’t move,” Lilly said. “We do this by attaching them to small strips of sticky tape! Once secured, it is then very easy to apply a minute drop of insecticide using a small piece of equipment called a microapplicator, and the bugs are then released into a recovery container for monitoring. Interestingly, the bugs don’t seem to mind their short time being adhered to sticky tape.”
Analysis demonstrated that the presence of both PBO and EN16/5-1 greatly improved the effectiveness of the insecticide for many bed bug strains. However, for some strains, the addition of EN16/5-1 resulted in little improved mortality compared with the PBO, demonstrating for the first time that different strains have different metabolic enzymes — some have oxidases, some esterases, and some both.
The results of this study may have important implications for bed bug control in the future. One of the authors, Stephen Doggett of NSW Health Pathology, explained just how significant this research is.
“The findings of this research are particularly important, as metabolic resistance is often known to confer ‘cross resistance,’ whereby resistance to one chemical group can result in resistance to a whole range of different insecticides, which limits what we can use now and even in the future for controlling bed bugs,” he said. “This emphasizes the need for an integrated approach to bed bug control using all of the available tools, both chemical and non-chemical.”
Watch this space. It doesn’t look like we’ve got bed bugs beat just yet.
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Dr. Cameron Webb is principal hospital scientist at NSW Health Pathology and clinical lecturer at the University of Sydney. He was a contributing author to the study mentioned in this article. He primarily studies mosquitoes, their associations with constructed and rehabilitated wetlands, and links to public health concerns. He provides advice on medical-entomology-related health threats to local, state, and federal authorities in Australia. Cameron maintains a blog on mosquito research and management at https://cameronwebb.wordpress.com and can be followed on Twitter at @mozziebites.