How Pesticides Pushed Cockroaches Into Rapid Evolution

In the 1980s, manufactures began making cockroach baits that combined sweet glucose with deadly insecticides. By 1993, many cockroach populations somehow developed an aversion to the bait. Now, 20 years later, scientists finally understand how the roaches beat these traps.

It turns out that this glucose actually sets off the bitter receptors in the insects' "taste buds." And given the genetic basis for this glucose aversion, this trait quickly spread throughout cockroach populations, rendering the traps useless.

Top image: Neil T/Flickr.

There's no denying that cockroaches are survivors. Despite our best efforts, we just can't seem to beat them. "We have had this ongoing arms race with roaches for as long as we have been around," says Coby Schal, an urban entomologist at North Carolina State University. "When we come up with something, they come up with an adaptation to survive."

Over 4000 species of cockroaches exist today. They live in a range of habitats, from deserts to forests, and have diverse behaviors. Interestingly, only one percent of cockroaches actually live among humans, Schal tells io9. And of that one percent, only a single species lives exclusively among human populations and in human-built structures: the German cockroach (Blattella germanica).

Cockroaches are born to like sweet food and hate bitter food, just like humans. And given that our kitchens and garbage are often loaded with glucose, a simple sugar, it's not surprising that German cockroaches love to hang around us. So when Schal's colleague, Jules Silverman, first discovered the pests' aversion to glucose-laden bait two decades ago, it was quite the mystery. Through experiments he found that the cockroaches were specifically avoiding glucose (pdf), which made the behavior all the more strange. At the time, however, bait manufacturers — one of which Silverman worked for — had little incentive to invest in costly research to pinpoint the whys and hows of it all. They were more interested in developing better baits.

Now at North Carolina State University, Silverman finally got a chance to figure it all out — he teamed up with Schal and post-doc Ayako Wada-Katsumata. One of the hypotheses the researchers had was that these glucose-averse (GA) cockroaches had experienced some kind of change in their chemosensory system, as compared with normal cockroaches. They decided to take a look at the insects' gustatory (taste) system.

Both humans and cockroaches have specialized structures to detect and discriminate different chemicals, called tastants — humans have taste buds, while cockroaches and other insects have sensilla, or taste hairs. Inside of these sensilla are gustatory receptor neurons (GRNs), which encode for different taste classes. The team took two groups of cockroaches — normal and GA — and exposed them to six different tastes, including glucose, fructose (the sugar found in honey and fruits) and coffee.

The normal cockroaches behaved as expected. Glucose and fructose stimulated the insects' sugar receptor neurons and coffee stimulated their bitter receptor neurons. The GA cockroaches showed similar responses, except when it came to glucose — the sweet chemical stimulated both sugar and bitter GRNs. Additionally, the bitter GRNs suppressed the sugar GRNs.

"It's sort of an amplification effect," Schal says. "In order to respond maximally to something bitter — which denotes toxicity, as plants couple bitterness with nastiness — it's a good idea for the bitter system to suppress the sweet system."

So when the GA cockroaches sense glucose in something, their "bitter" GRNs kick into high gear and tell them to stop eating. Past research has shown that this glucose aversion comes with a cost, however. Under normal circumstances, where no baits are involved, GA cockroaches are at a huge disadvantage to their glucose-eating cousins — without the nutritious substance, they grow slower than normal cockroaches. But when glucose-laden baits are present, the GA cockroaches are more likely to survive than normal cockroaches.

Just how this mechanism evolved so quickly remains unknown, though the researchers have a few ideas. One possibility is that glucose-aversion is actually an ancient trait that plant-eating cockroaches developed to avoid ingesting nasty plant compounds. When they moved in with humans in caves and later buildings, the trait became useless and was selected against. And then when we introduced baits that coupled glucose with toxic insecticides, the ancient trait, which likely still occurred at very low frequencies, was rapidly selected for.

So does this mean that our cockroach baits are all but useless? Well, no — manufactures have developed many other baits that use other attractants, including sweet fructose. "But we have had several populations that are now averse to fructose," Schal says. Our arms race with cockroaches rages on.

If you really hate cockroaches, one comforting thought is that German cockroaches will likely die out when we do, considering their dependence on us for food. But that says nothing for the other 4000 species of cockroaches.

The work was published in the journal Science.

Top image via Lmbuga/Wikimedia Commons. Videos courtesy of Ayako Wada-Katsumata.