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Migration and local dynamics of Trialeurodes vaporariorum in bean crops
The population dynamics of Trialeurodes vaporariorum (Westwood) in fields grown with French bean (Phaseolus vulgaris L.) were followed at two different locations in Kenya with the purpose of studying the interplay between local and regional dynamics. The fields were surrounded by two types of window traps which provided data on the relative rates of whiteflies arriving and leaving the fields. These data were supplemented with density estimates of adult whiteflies within the fields obtained by means of pot traps. Whiteflies arrived five days after the crop germinated, causing a quick increase in the proportion of infested plants, which reached 98% of all plants one month after germination, corresponding to ca 6 adult whiteflies per plant. Flight activity had two daily peaks, one around 12 hours and one about 16 hours. Activity level was found to increase with temperature (up to 27.5oC) and solar radiation (up to 0.73 kW/m2) and to decrease with wind speed. Immigration rate at first increased with crop age, but reached a maximum of approximately 1.5 whiteflies arriving per plant per day once the crop had reached maturity. A simple statistical model was fitted to sampling data of the number of whiteflies per plant (Nt). The model has three predictor variables: t which is the number of days since germination of the crop, Co(t) and Ci(t) which are the cumulated catches of whiteflies obtained from the outer and inner side, respectively, of the window traps from day 0 and until day t. The model explained 66% of variation in Nt, but surprisingly the model showed that Nt was positively associated with Ci, but negatively associated with Co. A possible explanation for this counterintuitive result is that Ci reflects trivial (non dispersal) flights, which is likely to be correlated with the population size, whereas Co reflects the emigration rate, which tends to increase when the whitefly population declines as a result of reduced food quality due to aging or overexploitation. If there are no other suitable fields in the vicinity, a fraction of the emigrating whiteflies will return to the original field to be caught at the outer side of the window traps. Such captures thus represent “false” immigrants.
