Old mosquitoes usually spread disease, so Australian researchers figured out a way to make the pests die younger -- naturally, not poisoned.

Scientists have been racing to genetically engineer mosquitoes to become resistant to diseases such as malaria and dengue fever that plague millions worldwide, as an alternative to mass spraying of insecticides.

A new report today suggested a potentially less complicated approach: Breeding mosquitoes to carry an insect parasite that causes earlier death.

Once a mosquito encounters dengue or malaria, it takes about two weeks of incubation before the insect can spread that pathogen by biting someone -- meaning older mosquitoes are the more dangerous ones.

The Australian scientists knew that one type of fruit fly often is infected with a strain of bacterial parasite that cuts its life span in half.

So they infected the mosquito species that spreads dengue fever -- called Aedes aegypti -- with that fruit-fly parasite, breeding several generations in a tightly controlled laboratory.

Voila: Mosquitoes born with the parasite lived only 21 days -- even in cozy lab conditions -- compared with 50 days for regular mosquitoes, University of Queensland biologist Scott O'Neill reported in the journal Science.

Mosquitoes tend to die sooner in the wild than in a lab. So if the parasite could spread widely enough among these mosquitoes, it "may provide an inexpensive approach to dengue control," O'Neill concluded.

Next month, O'Neill's team begins longer studies in special North Queensland mosquito facilities that better mimic natural conditions. Still, "determining whether it can remove enough infectious mosquitoes to be useful will be a challenge," the team cautioned.