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The resistant bed bugs had multiple genetic defenses against the toxins. There are three specific genes (cytochrome P450 monooxygenases, carboxylesterases, and glutathione S-transferases) that can bind to, deactivate, and break down insecticides, and all three were cranked way up in the wild bed bugs. The pyrethroid also targets the insects sodium channel, which in response had mutated, making them partially resistant to the effects.
Ladies and gentlemen, the very definition of evolution. A constant pressure on the population in the form of a limited number of insecticides means that bed bugs with mutations that offer them a resistance are substantially more likely to live long enough to reproduce.
This means that there's a definite biomarker that separates the resistant insects from the rest. And hopefully, we can now engineer a smart pesticide which will actually succeed in killing the horrible things.