Sustainable integrated management of whiteflies as pests and vectors of plant viruses in Asia

• Department of Agriculture, Thailand
• Regional Agricultural Research and Development Centre, Sri Lanka
• CSIRO Plant Industry, Australia
• Bogor Agricultural University, Indonesia
• Institute of Agricultural Sciences of South Vietnam, Vietnam
• CSIRO Entomology, Australia
• Asian Vegetable Research and Development Centre, Taiwan
• University of the Philippines at Los Banos, Philippines
• Malaysian Agricultural Research and Development Institute, Malaysia
• Bangladesh Agricultural Research Institute, Bangladesh
• Nepal Agricultural Research Council, Nepal
• Research Institute of Fruit and Vegetables, Vietnam
• Can Tho University, Vietnam
• University of Western Sydney, Australia

Whitefly (Bemisia tabaci) is a major insect pest of many vegetable plants. As well as causing direct damage by its feeding, the insect can also carry and spread disease-causing viruses to infested plants. Whitefly is a problem in many parts of the world, but its occurrence and habits as a vector in Asia have not been well studied, and neither has its impact on local agriculture.
The whitefly types and viruses in Asia may also pose a threat to Australian vegetables such as melon, tomato, capsicum, squash, pumpkin and brassica crops, as well as cotton production. Damage comes from whitefly feeding, but the viruses transmitted can make the situation even worse - in the Northern Territory, losses from 60 to 100 percent from the tomato geminivirus has rendered tomato-growing completely uneconomic,
Whiteflies carry geminiviruses, although not all viruses are present in all whitefly types in an area. There have been several reports of geminivirus outbreaks in Asia, but only a few of the viruses have been studied and there has been no systematic survey.

The project investigated the whitefly problem in Asia and Australia, and built collaborative research links between scientists in the region. Nepal, Bangladesh, Sri Lanka, Thailand, Malaysia, Indonesia, the Philippines, Vietnam and Australia took part in the research. Scientists assessed the extent of whitefly biotype diversity in Asia, the favoured host range for each biotype, the diversity of geminiviruses in the region, the plant species affected by those viruses, and the extent and diversity of potential biological control agents.

The scientists conducted detailed surveys of the occurrence of B. tabaci, and collected host plant leaves with mature and juvenile specimens of the insect. These collections were used to identify the key biotypes of the insect in Asia. Associations between insect biotypes and host plants were analysed. The team also collected parasitic or predatory insects from countries with absent or incomplete records (Malaysia, the Philippines and Indonesia). USDA had already made extensive collections elsewhere in Asia.
Samples of whitefly and of parasites and predators were sent to CSIRO Entomology in Australia, where potential control agents were identified and the information combined with existing data sets from Australia and the USA.
The project also determined the plant hosts affected by geminiviruses by collecting samples of infected cultivated crop plants from many locations for identification and characterization. Data from this process were then used to develop detection methods specific to each major virus.

Two genetically distinct tomato-infecting geminiviruses from north and southern Vietnam were isolated, cloned, and sequenced. The virus from north Vietnam is most similar to a geminivirus from Taiwan (88% homology) while the virus from southern Vietnam is a strain of a geminivirus from Thailand. The chances are high that AVRDC-resistant lines will hold up against the virus from northern Vietnam or similar viruses. Reaction of AVRDC resistance to the virus from the south is unknown and needs to be determined. The wide range of genetic diversity among the geminiviruses from Vietnam and other parts of Southeast Asia emphasises the importance of developing resistant lines carrying multiple resistance genes in order to have effective and durable resistance in the region.
After two years of on-station and on-farm testing in north Vietnam, two fresh market tomato hybrids, FMTT847 and TLCV15, were identified as highly promising for farmer adoption. Under moderate or high geminivirus pressure in farmers' fields, these two hybrids yielded 30-400% more than local geminivirus-susceptible cultivars. Parental lines of the hybrids have been provided to the Research Institute of Fruit and Vegetables, Vietnam (RIFAV) to enable seed production of the hybrid for farmers. AVRDC will make these two hybrids available to researchers in other countries for evaluation and possible release. However, both hybrids were susceptible to geminivirus when tested in an on-station trial at Cantho University in southern Vietnam.
The efficacy of petroleum spray oil (PSO) versus conventional pesticide and control (water) treatments in reducing/repelling whitefly populations and reducing geminivirus infection was evaluated in three on-station experiments at RIFAV in north Vietnam. Adult whiteflies but not nymphs were present on tomato plants, suggesting that local whitefly populations cannot complete their lifecycle on tomato. The Bemisia tabaci genetic group in north Vietnam may be Nauru instead of Asia 1 as previously thought. Weekly applications of PSO at rates of 1% or 2% reduced geminivirus incidence to 8-18% compared to 25% for the water control. For both non-resistant and resistant varieties, PSO application rates of 1% or 2% also increased marketable fruit yields by 50-92% compared to the water control. PSO technology combined with resistant varieties showed excellent potential as a component in an overall geminivirus IPM strategy and a safe substitute for insecticides. Furthermore, the large yield increases provide strong evidence that the PSO applications provide additional benefits to tomato crops beyond geminivirus control.