TOKYO—A new study offers a hint why American cockroaches are so hard to get rid of: Females can breed for years and multiply their numbers dozens of times alone, without ever mating with a male.

Scientists have long known that female cockroaches, like many other species, can give birth through parthenogenesis, a form of asexual reproduction in which the females produce eggs without male participation. In the case of roach parthenogenesis, the offspring are themselves all-female.

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