An invasive whitefly has developed mutualistic relationships with the plant viruses it transmits and is able to increase its population much faster on virus-infected plants than on healthy plants, whereas its indigenous counterpart is unable to do so, according to the new research carried out at Zhejiang University and Chinese Academy of Agricultural Sciences, China.

Twenty years ago in 1986 in the USA, Florida experienced outbreaks of what is now known as whitefly (Bemisia tabaci) biotype "B", first in greenhouse poinsettia, then in a wide range of vegetable, ornamental and field crops. Soon similar outbreaks were seen in other States within the USA and many other countries around the world. The outbreaks of the B whitefly have often been followed by pandemics of a group of plant viruses called begomoviruses on crops such as tomato and tobacco. These viruses are transmitted by this whitefly. In many countries and regions, including China, the outbreaks of the B whitefly have also seen the gradual disappearance of some native whitefly biotypes.

Many scientists around the world have been investigating why the B whitefly is so invasive. It is now widely accepted that the B whitefly is most likely to have originated from the Mediterranean/North Africa region, and its recent widespread invasion has been assisted by the worldwide flower trade. The question remains how this pest can increase so rapidly and displace native biotypes of whitefly after it has been transported to new localities.

The research compared development, survival, fecundity and population increase of the invasive B whitefly and an indigenous whitefly (called ZHJ1) on both virus-infected and healthy tobacco plants. Compared to its performance on healthy plants, the invasive B whitefly had higher fecundity and longevity by 12 and 6 fold when feeding on plants infected by one virus, and by 18 and 7 fold when feeding on plants infected by another virus. Population density of the B whitefly on virus-infected plants reached 2-13 times that on healthy plants in 56 days. No doubt increase of infectious whiteflies will in turn speed up virus pandemics. In contrast, the indigenous whitefly performed similarly on healthy and virus-infected plants.

"This is the first study that shows an invasive insect has such a mutualistic relationship with the viruses it transmits, whereas its indigenous counterpart does not," said Professor Shu-Sheng Liu, corresponding author of the study, from the Institute of Insect Sciences, Zhejiang University. "We believe that the mutualism between the B whitefly and the viruses may contribute to the ability of the B whitefly to both invade and displace indigenous whiteflies, as well as causing disease pandemics of the viruses associated with this vector."

The study also shows that infection of the whiteflies per se has limited effects on the survival and fecundity of the vectors, and the B whitefly acquires the benefits through feeding on the virus-infected plants. Thus the mutualism is indirect. The researchers believe that this kind of mutualism may exist in many circumstances and should receive more attention in the research and management of biological invasions.