Intrinsic rate of population increase of the spider mite Tetranychus urticae on the ornamental crop gerbera: intraspecific variation in host plant and herbivore.

Eight cultivars of the ornamental crop Gerbera jamesonii Bolus (Asteraceae) were compared in host plant suitability for the two spotted spider mite Tetranychus urticae Koch (Acarina: Tetranychidae). This was done by determining the intrinsic rate of population increase, rm, of spider mites on leaf discs of plants from each of the cultivars. Large differences in rm values were found, ranging from 0.088/day on cultivar Bianca to 0.242/day on cultivar Sirtaki. This variation in rm was mainly caused by differences in developmental time of the spider mites. We assessed the performance of spider mites on young and old leaves of the two gerbera cultivars Bianca and Sirtaki. On Sirtaki the spider mites had a shorter developmental time and higher peak oviposition rate on young leaves than on old leaves. However, on Bianca such an effect was not found. We also determined the performance of two spider mite strains on the resistant gerbera cultivar Bianca. We compared the rm of a strain that had been reared on this cultivar for approximately half a year with the rm of a strain that was reared on bean. The rm of the strain that was reared on cultivar Bianca increased to 0.208/day, which is however still substantially lower than the rm on the susceptible cultivar Sirtaki.