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 Te tranychus urticae Koch (Acarina: Tetranychidae). This was done by determini ng the intrinsic rate of population increase, r(m), of spider mites on leaf discs of plants from each of the cultivars. Large differences in r(m) valu es were found, ranging from 0.088/day on cultivar Bianca to 0.242/day on cu ltivar Sirtaki. This variation in r(m) 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 leave s 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 resist ant gerbera cultivar Bianca. We compared the r(m) of a strain that had been reared on this cultivar for approximately half a year with the r(m) of a s train that was reared on bean. The r(m) of the strain that was reared on cu ltivar Bianca increased to 0.208/day, which is however still substantially lower than the r(m) on the susceptible cultivar Sirtaki.