We thought we'd beaten bedbugs. After World War II, they had been almost eliminated thanks to the powerful pesticide DDT. But in the past three decades, bedbugs have started to return, resistant to the chemicals that reliably killed almost all of them a half-century ago.
For the past nine years, Rose Zhu, an entomology professor at WSU, has been trying to figure out exactly which traits allow bedbugs to shrug off what once killed them.
Zhu has known the scourge of bedbug bites. A long time ago, in Washington, D.C., she remembers waking up covered in itchy bites. It wasn't until she started studying bedbugs that she knew what those bites were from.
Since then, she's been immersed in the bedbug's world. In 2014, Zhu went to a homeless shelter in Seattle, where she saw bedbugs that were so prevalent they were visible, brazen and bold, in the holes of the mattresses, in the cracks of the walls and crawling on the wooden bed frames.
For five years, exterminators had tried to rid the shelter of bedbugs. They had, but bedbugs kept returning when new residents stayed at the shelter.
"They still can come back," Zhu says.
Zhu dresses in white scrubs, making it easier to see bedbugs and reducing the risk of inadvertently smuggling out insect stowaways. Using insect forceps — essentially, soft tweezers — she snatches up the bedbugs, placing them in a vial that lets in just enough air for them to breathe, and takes them back to her lab. She keeps them alive, feeds them rabbit blood and studies them. And she's not the only one.
Zhu has joined with 80 other scientists over four years — essentially all the renowned bedbug experts in the world — to figure out why bedbug populations that once curled up and died when sprayed have become immune. That's meant diving deep into the DNA of the bedbug, mapping their genome, to identify what armors them against pesticides.
"We are just beginning to understand how the bedbug [resistance] was built, how the bedbug can adapt to the environment so well," Zhu says.
What this consortium discovered highlighted just how vast the problem was: It wasn't that bedbugs had evolved with a specific genotype that allowed them to resist toxic chemicals. It was that they had a bevy of different genotypes that allowed them to resist toxic chemicals.
"They have a whole battery of ways to be resistant to insecticide," says Laura Lavine, a WSU entomology professor who works with Zhu.
In other words, bedbugs don't have just a single weapon to defend against pesticides, but an arsenal.
"They don't just have a gun, they have a gun and an AK-47 and a bomb and a cannon," Lavine says.
So how did this happen? Lavine says the problem came down to how bedbugs have been battled for decades. When farmers spray their crops, they rotate through a whole range of different types of pesticides to insure that, even if some insects are immune to one or two of the chemicals, they'll be killed by the others.
"They use one chemical, then switch to a totally different chemical during the course of the season," says Lavine.
But spraying in homes is riskier. For years, only two types of chemicals have been used to try to kill bed bugs: pyrethroids — which include the toxic chemical DDT — and neonicotinoids.
"Imagine you're subjected to a chemical that's killing off 99 percent of the population, and you're the 1 percent of the population that survives," Lavine says. "You are now carrying the genetic material that is keeping you alive in the face of the chemical."
These insects reproduce very, very quickly. So throughout the country, isolated populations of bedbugs, with different pesticide-resistant genotypes, have separately developed different pesticide-resistant genotypes.
"The spray-and-kill solution is not really available anymore," Lavine says. There are a few other options — bedbugs are particularly vulnerable to heat — but for now, the best way to conquer the bedbug scourge is to avoid getting it in the first place.
"You just have to be vigilant and check when you travel," Lavine says. "That's really the only solution. Be vigilant when you travel or [when] you have guests."
It's possible that the discoveries from Zhu's consortium still have more to teach us. There are still threads to pull on: For example, the researchers have discovered that there are 805 genes in bedbugs that have been transferred from bacteria. It appears to be some sort of symbiotic relationship, but Zhu says exactly what impact that discovery has on bedbug resistance will take more research.
There's another challenge. Funding for studying bedbugs is very limited, Lavine says. Bedbugs are considered a "nuisance" but not a vector for disease. But the work researchers like Zhu are doing remains crucial. Bedbugs often hit the most vulnerable populations the hardest, impacting homeless shelters, nursing homes and low-income apartments.
"It's a social justice issue a little bit," Lavine says. ♦