It’s probably safe to say you don’t like cockroaches.

Even if you have one of those Madagascar hissing cockroaches as a pet, you probably don’t want to see its wild cousins getting their little cockroach feet all over your food. That’s a pretty popular sentiment, which is why stuff like roach motels and RAID got invented.

But it turns out, the human distaste for the humble cockroach has created some weird consequences. Like the fact that some female cockroaches now hate being wooed by the romantic advances of males. [♪ INTRO] The species of cockroach we’re talking about here is Blattella germanica, the German cockroach. Despite the name, they exist pretty much everywhere in the world, and when you do stumble upon them, it’s almost always inside human homes.

This has made them almost universally unloved, hence our frequent efforts to kill them dead. And several decades ago, our strategy for population control got pretty easy. Almost too easy.

See, German cockroaches have proverbial sweet tooths…Sweet teeth? Anyway, they’re biologically wired to like sugars. Especially simple sugars like fructose and glucose.

So in the 1980s, humans took advantage of that by creating sugary roach killer made from corn syrup. It’s like wrapping your dog’s medicine in a blob of peanut butter. Except instead of medicine, it’s poison.

And for a while, sweet roach baits worked pretty well. But as with all survival challenges, evolution found a way around that problem. By the 1990s, scientists discovered that at least some cockroach populations had become averse to the simple sugar D-glucose.

These cockroaches had a mutation on just one gene that caused them to absolutely abhor it. Because instead of stimulating the cockroach’s sugar-tasting neurons, like sugars are supposed to do, this mutation caused D-glucose to stimulate the roaches’ bitter-tasting neurons. So it’s not toxic to them, they just don’t like it.

It’s like chomping down on what you think is a piece of milk chocolate, but it’s actually 100% dark. Now, these glucose-averse cockroaches will eat glucose if there’s nothing else available to them, but only in small amounts. Which makes it a great evolutionary adaptation for outwitting roach poison, but not so good in other ways.

Glucose-averse roaches are less likely to check into a roach motel, but they’re also less fit than normal roaches, because they avoid foods that keep their not-glucose-averse counterparts healthy. In fact, some hate glucose so much that they might actually die rather than eat something sweet. Kind of like if your options were to starve or eat raw Brussels sprouts for the rest of your life.

And not the tasty Brussels sprouts you can buy today. The old school ones that gave Brussels sprouts their bad rep. But if you think that’s bad, it gets even worse!

This newly evolved glucose aversion is messing up their love lives. German cockroach romance normally goes something like this: Male cockroach meets female cockroach. Male cockroach gives female cockroach a box of chocolates.

Okay technically, it’s not a box of chocolates. It’s a sugary treat secreted from a gland on his back. Which is just as romantic, I’m sure.

This secretion contains the sugars maltose and maltotriose, which break down into glucose after interacting with the female’s saliva. But in order to taste this “nuptial gift,” the female cockroach just so happens to crawl into a position that’s exactly where she needs to be for the male to, well, uh… Cue Barry White. If the treat is tasty enough for her to hold that position, they’ll mate and that one mating provides the female with enough sperm to last the rest of her life, so she only needs romance just the one time.

However, if she’s glucose-averse, she’s not going to be so easily wooed. She might try tasting her would-be partner’s romantic gift, but she’ll likely run off before the relationship can be consummated. In a 2022 study, one team of researchers found that cockroach couples consisting of a glucose-averse female and a regular, or wild-type male had much lower mating success than two wild-type roaches.

They also had less success than when it was just the male that was glucose-averse. Which makes sense because he’s not the one who has to taste the nuptial gift. But interestingly, when a glucose-averse male was paired with a glucose-averse female, they were just as successful in mating as when neither hated glucose.

And there might be a clear reason for that. In a 2023 study, some of the same researchers found that some glucose-averse males may be learning how to better woo glucose-averse females. Their courtship treat contains more maltotriose, which doesn’t break down into glucose as quickly as maltose does in the female’s saliva.

So it takes a lot longer for her to start tasting that bitter flavor… as long as 300 seconds. Compare that to the 5 to 10 seconds it would take her to be put off by the typical nuptial gift. Some glucose-averse females also appear to be developing saliva that doesn’t break down maltose quite as efficiently, too.

Meanwhile, glucose-averse males have gotten quicker at initiating the actual “getting it on” part, so the female doesn’t have time to realize how disgusting her partner’s nuptial gift is. Now, it is worth noting these are lab-observed adaptations. The research team doesn’t know if they happen out in the wild.

But there’s certainly a chance they’ll pop up. It’s also possible that cockroaches in the wild…or in your house… will develop completely different adaptations so they can keep thriving. And that might sound like terrible news.

You might feel like it would be better for cockroaches to starve or stop mating. And I get that. No one likes finding a cockroach in the fruit bowl.

Or crawling across their face when they wake up in the morning. And while German cockroaches evolved to live alongside humans, cockroaches in general do play an important role in nature. They’re scavengers, food for other animals, and they help recycle the nutrients from food waste.

So they may not be welcome in our homes, but that doesn’t mean that, every once in a while, they don’t deserve a little romance. Thanks for watching this episode of SciShow. If you’d like to learn a bunch more weird science about creepy crawlies, you should check out our podcast SciShow Tangents.

Every other Tuesday, your hosts Hank Green, Ceri Riley, and Sam Schultz will try to one-up and amaze each other with weird and funny scientific research, taking some delightful detours along the way. October is Trick or Treat month, with each episode featuring a special guest and lots of special facts. Maybe cockroaches will get a shoutout.

You’ll just have to tune in and find out for yourself. You can head over to YouTube.com/SciShowTangents if you want to watch your hosts’ wonderful faces, and if you don’t, we’ve got a link in the description to take you to the right place, too. [♪ OUTRO]