Replacement of CTV-susceptible sour orange rootstock by CTV-tolerant ones may have triggered outbreaks of Tetranychus urticae in Spanish citrus

The two spotted spider mite Tetranychus urticae Koch (Acari: Tetranychidae) is a major pest of citrus in Spain. Commercial citrus plants are always propagated asexually by bud-grafting onto a seedling rootstock. During the last 70 years, millions of citrus trees grafted on sour orange have been destroyed because of Citrus tristeza virus (CTV). Therefore other rootstocks are nowadays predominant. The aim of this study was to asses the influence of the rootstock on selected life history parameters of T. urticae. Mites were reared on leaves of Citrus unshiu trees grafted on six different rootstocks: sour orange, Troyer citrange, trifoliate orange, Volkamer lemon, Alemow and Cleopatra mandarin. Some parameters (sex ratio, egg hatching and survival) were not affected by the rootstock, whereas many other important ones were (fecundity, oviposition rate, lifespan and development time). These differences may explain the diversity found in the selected life history parameters (net fecundity, generation time and intrinsic rate of increase, rm). Sour orange appeared as the worst rootstock for T. urticae (rm = 0.140). It was closely followed by Troyer citrange and trifoliate orange (6% and 9% increase in rm, respectively), then by Alemow (35.3% increase in rm), Volkamer lemon (77.6% increase in rm) and finally Cleopatra mandarin which yielded an rm 89.1% higher than sour orange. These results can explain, at least partially, the increasing prevalence of T. urticae as a pest in Spanish citrus orchards. Furthermore, problems could become much more severe on salt-tolerant rootstocks because of global warming, which is expected to exacerbate the threat of salinity, especially in areas with a semi-arid climate.