Why we can't seem to get rid of cockroaches
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.
"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