Survival of whiteflies during long-distance transportation of agricultural products.

Besides being pests of tropical and subtropical crops and of protected crops in temperate regions, some whitefly species are also vectors of many plant viruses. Worries about their possible movements to new areas are therefore justified. Although whitefly species are known to have invaded new areas through natural spread, in general whiteflies are poor fliers and their long-distance movements are likely assisted by humans. The high risk of moving pests from place to place by human activity is one of the few agreed reasons for imposing quarantine restrictions. Hence, it is very important to know, besides other things, the conditions that permit survival of the pest. Among the abiotic factors influencing survival of whiteflies, temperature and relative humidity are important, particularly at extreme values. Temperature has two limits for the survival of whiteflies. High transport temperatures are not compatible with transportation of fresh plant products, but may be compatible with other types of goods that do not require cooling or controlled conditions. At 30 to 40°C, survival of adult B. tabaci is very poor after 6 hours, independently of RH. At the lower limit, adult whiteflies are the least resistant stage, but they can survive for long times at temperatures often recommended for the transportation of potted ornamental plants and florist green materials. Other stages, like eggs and nymphs, are much more resistant to cold. Within the ranges of temperature and time that allow survival of the non-adult stages, products can be moved substantial distances. There is also a possibility that plant pathogenic viruses can be moved together with whitefly immature stages. Among the biotic factors that could influence survival, only the osmotic pressure of the host leaves seems to have a direct bearing. Variations in host leaf water content, and its associated soluble sugar concentration, strongly influence egg survival of T. vaporariorum. On the other hand, 4th instar nymphs can survive desiccation of the leaves they are attached to and emerge as adults after a few days. Given that the risk of transporting undesirable species is high, the imposition of quarantine regulations is not a decision for scientists. Biologists, though, should provide the rationale behind decisions of this type: not only the basic knowledge of pests (biology, taxonomy), but also risk analysis, in order to establish the priorities for political decisions and for future research, and instruments for making regulations enforceable. Methods are available for identifying the undesirable pests both at the species (official diagnostic protocols, identification guides, ELISA tests, multiplex real-time PCR assays) or biotype level (analysis of esterase patterns, sequencing of whitefly genes, RAPD-PCR, AFLP, PCR-RFLP). The analysis of biotypes/races can be even more detailed by microsatellite markers. Other more general instruments for reducing the risk of exporting pests are available or being developed, with the aim of excluding pests. If exclusion fails, we can still use our knowledge to confine, and hopefully eradicate, the undesired "guest". Agriculture operators and international travelers should be made aware of the existence of destructive, exotic (plant) pests threatening to enter a country in which they are not known to occur, because it is clear that the success of plant quarantine programs greatly depends upon public cooperation with quarantine legislation.