The insect arrived in Australia and China in the 1990s through the international flower trade and has since displaced native populations of the same type in both countries.

The marauding aphids have blighted tomato and vegetable crops and in some cases spread plant viruses, forcing Australian producers to shell out millions of dollars to pay for additional supplies of insecticide.

But the invasion also intrigued entomologists who wondered how this member of the silverleaf whitefly Bemisia tabaci species -- a genetic variant known as Biotype B -- could so quickly establish its dominance in a new territory.

In Australia, for example, it took the invaders just three years to supplant the native population in Queensland and five years to do the same in the southwest of the country, according to the study published in Science.

"We were trying to find out what made B. tabaci biotype B such a successful invader and the answer appears to be sex," said Paul De Barro, a research scientist at CSIRO Entomology, Australia's national science agency in Brisbane.

De Barro and colleagues at the Institute of Insect Sciences at Zhejiang University in China conducted regular field sampling of whitefly populations in Zhejiang, China, from 2004 to 2006 and in Queensland, Australia from 1995 to 2005 to monitor the insect's behaviour as it spread and displaced native whitefly populations.

They found what they called an "asymmetry" in mating interactions between the invader and native whiteflies which over time translated into a population advantage for the incoming aphids, who probably originated in the Middle East and Mediterranean.

Immediately after the invasion, the two populations try to interbreed because they cannot tell each other apart, but they cannot successfully reproduce.

This results in a greater number of male offspring because this species of aphid is a haplodiploid which means that males are produced from unfertilized eggs and females from fertilized eggs.

The invasive females respond to the ensuing abundance of males in their environment by becoming more promiscuous and having more frequent sex with those males they can mate with, which leads to an increase in female offspring.

In contrast, the indigenous females do not become more sexually active due to the increased availability of male partners.

What's more the invading male aphids tend to court indigenous females as well as females from their own "clan", reducing opportunities for copulation between native aphids.

The upshot is that the proportion of females among the invading population of Biotype B insects keeps climbing, while the number of indigenous females goes down, eventually leading to extinction.

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