Biology

Why we can't seem to get rid of cockroaches

Why we can't seem to get rid o...
A new Japanese study reveals that the female American cockroach's penchant for co-operation could explain why there are so many of them
A new Japanese study reveals that the female American cockroach's penchant for co-operation could explain why there are so many of them
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The all-female cockroach colony that spawned from the 15 females that Japanese researchers placed together in a container
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The all-female cockroach colony that spawned from the 15 females that Japanese researchers placed together in a container
(A) Two virgin female cockroaches (B) A comparison of egg cases (C) A close-up of the interior of an egg case
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(A) Two virgin female cockroaches (B) A comparison of egg cases (C) A close-up of the interior of an egg case
A new Japanese study reveals that the female American cockroach's penchant for co-operation could explain why there are so many of them
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A new Japanese study reveals that the female American cockroach's penchant for co-operation could explain why there are so many of them

They've been around for the past 300 million years, outlasting the dinosaurs and teaming up with evolution to outsmart our attempts to get rid of them. Now, Japanese researchers at Hokkaido University have revealed yet another reason why we have been unable to put a dent in their populations: female solidarity.

Cockroaches, along with termites, snakes and sharks, have long been known to be capable of "virgin birth" or parthenogenesis, a form of asexual reproduction that occurs without fertilization. What is less known are the factors that trigger this process. Is the absence of male cockroaches the only condition necessary for asexual reproduction to take place or does the social environment play a part too? Given that cockroaches are social creatures that live in groups, the Hokkaido University researchers believed that there had to be factors other than male-absent conditions.

To test their hypothesis, the researchers conducted 11 sets of experiments with different groups of American cockroaches, a common pest. The control group comprised a male and female that were allowed to mate. Others comprised virgin females that were kept in isolation; in groups of up to five; and with castrated males. In addition, the researchers also added female sex pheromones – which are secreted in greater quantities by virgin females than those that have already mated – to containers housing single roaches to see if they would regard it as a male-absent signal and produce more eggs as a result.

What they found was that group-housed females, especially those with three or more insects, produced egg cases faster than any other group. In addition, the egg cases were produced in a synchronized manner. Bizarrely enough, this behavior was shared even by those kept in different containers. Furthermore, the group-housed females also produced their second batch of egg cases at shorter intervals than those kept alone (an average of 18 versus 27 days).

On the other hand, the presence of the castrated males and female sex pheromones did little to boost the production process. The researchers had included the former to find out what effect (if any) cohabitants of a different sex would have on the egg-laying process and discovered that it took the female cockroaches that were grouped with the castrated males almost the same amount of time to produce egg cases as the isolated specimens, thus suggesting that the promotion of asexual production depends on the females being able to discern the cohabitants' sex.

There was also a difference in the viability of the eggs. Only 30 percent of those laid asexually hatched, compared to around 47 percent of the ones produced by sexual reproduction. This could explain why the egg production process ramps up when virgin female cockroaches are grouped together, say the researchers. Synchronizing egg production in grouped females might result in their offspring hatching at around the same time. The nymphs would be able to increase their fitness by aggregation and the sharing of resources, which could counter the lower hatching rate of the asexually produced eggs.

According to the scientists, the female solidarity exhibited in this experiment is consistent with other observations of roach behavior. Rarely do fights ever break out among unmated females that are housed in the same container. Instead, they are often found huddling close together, whereas unmated males paired together will often fight until the antennae of both individuals are amputated.

Males? We don't need no stinking males

That said, while the hatchability rate of asexually produced eggs is generally lower than those laid via conventional means, the roaches that hatch from these eggs are nevertheless still able to form and maintain a colony for at least three generations without a male's input, as evidenced by the colony that formed when the researchers placed 15 random adult females in a container. Just over three years later, it had grown to comprise more than 300 females with nymphs and adults of different ages. Since they were kept in optimal conditions in the lab, the researchers estimate that some of the roaches may have even reached the fifth generation.

The all-female cockroach colony that spawned from the 15 females that Japanese researchers placed together in a container
The all-female cockroach colony that spawned from the 15 females that Japanese researchers placed together in a container

"Our study shows that female cockroaches promote asexual egg production when they are together, not alone," says researcher Hiroshi Nishino. "This is consistent with the fact that progenies produced by fifteen females in a larger container have maintained a colony for more than three years, whereas those produced by one female die out fairly quickly. In addition to the increased fecundity of group-housed females, the synchronized egg production could also assure higher survival rates via the aggregation of similar-aged larvae."

While this may be an impressive feat of female solidarity in the insect world, it does not bode well for human societies. Given that female American cockroaches already have several advantages over males that allow them to adapt to new habitats – for a start, they have longer lifespans and their larger body size protects them from environmental changes – their ability to reproduce asexually and maintain colonies for several generations makes them a health threat to be reckoned with, given the way they transfer disease. Hence the importance of understanding how they reproduce so that more effective cockroach traps can be built, say the researchers.

"The traps utilizing sex pheromones to attract only male cockroaches are not sufficient," says Nishino. "Understanding the physiological mechanism behind the reproductive strategies should help us find more effective ways to exterminate pest cockroaches in the future."

The study was published in the Zoological Letters.

Source: Hokkaido University

8 comments
MK23666
So basically ... from now on concentrate on killing off the female cockroaches and the males will eventually kill off each other.
Jose Gros
When I moved to the house I'm now, I past many months with no food purchase, I always had meals out, or consumed packed food leaving no residua edible for insects. Shortly after entering, I found a big cockroach dead, probably from starvation, and no one more show, even if later I changed my cooking habits. Did the granny cockroach leave some kind o 'pheromone', pointing place was deadly, and keeping away others from coming? btw, when during a German language course in 1986, Cologne, I was asked if I had 'House Tiere'= 'Pets', I answered, 'Yeah, cockroaches'
ljaques
Too bad they didn't include a base group of 15 males/15 females to show the differences in colony levels between them. When I moved into an old rental house in Phoenix in 1971, I set my dinner plate down next to me while watching dinner. 20 minutes later, I reached down to pick up my plate and I couldn't see it for the cockroaches! After some research, I found that nicer restaurants used Indian Head pesticide, so I got a gallon ($25 in 70s dollars) and sprayed the place inside and out, giving the rest to the 2 other renters (three 4-br apartments in the big old house), who also sprayed. That killed them all, at least for the year I was there. As for the sewer cockroaches (4+ inches long), when you walk across the street at night barefooted and step near them, they would grab you by your big toe and throw you onto the sidewalk. ;)
bwana4swahili
So, cockroaches have been around for 300+/- million years, survived several global extinction events, don't require sex to reproduce and thrive in the companionship with us recently evolved homo sapiens, AND we think we're going to eliminate them!? Good luck with that!! But it was an interesting article...
Tanstar
They should have tried an equal mix of males and females and then soaked them all in male pheromones. Convince them they are all males. The females wait for the males to do their thing and the males try to kill everyone. Maybe. It's worth a try!
William Juno Roehling
They are within every inter-dimensional realm and reality throughout the infinite living universe...they are where live ends, and when life begins... Cockroaches are anywhere and everywhere...some are huge monstrous beasts, and others smaller than a pinhead...!!!
rude.dawg
Because of their ease of rearing and resilience, cockroaches have been used as insect models in the laboratory, particularly in the fields of neurobiology, reproductive physiology and social behaviour. The cockroach is a convenient insect to study as it is large and simple to raise in a laboratory environment. This makes it suitable both for research and for school and undergraduate biology studies. It can be used in experiments on topics such as learning, sexual pheromones, spatial orientation, aggression, activity rhythms and the biological clock, and behavioural ecology. Whereas household pest cockroaches may carry bacteria and viruses, cockroaches bred under laboratory conditions can be used to prepare nutritious food. In Mexico and Thailand, the heads and legs are removed, and the remainder may be boiled, sautéed, grilled, dried or diced. In China, cockroaches have become popular as medicine and cockroach farming is rising with over 100 farms. The cockroaches are fried twice in a wok of hot oil, which makes them crispy with soft innards that are like cottage cheese. Fried cockroaches are ground and sold as pills for stomach, heart and liver diseases. A cockroach recipe from Formosa (Taiwan) specifies salting and frying cockroaches after removing the head and entrails. In China, cockroaches are raised in large quantities for medicinal purposes. In other words, we need them more than they need us. Mwahahahaha!
b@man
Just more proof that evolution is not a viable theory... never was:)