California red scale (CRS), Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), one of the most important pests of citrus worldwide, began to cause damages in eastern Spain in 1986. The main biological control agents of A. aurantii in this zone are the native parasitoid A. chrysomphali (Mercet) and the introduced A. melinus DeBach (both Hymenoptera: Aphelinidae). Nevertheless, the control they exert is insufficient. In order to improve the biological control of A. aurantii we studied several biotic factors that may affect the efficiency of A. chrysomphali and A. melinus in the field. More concretely, we studied the spatial and temporal variation in the scale size, the host sizes used by A. chrysomphali and A. melinus as well as the influence of host size on various parasitoid traits. Moreover, we studied the foraging ecology and the effect of the ants native to the Mediterranean on the populations of the scale and, finally, the nutritional state and food sources used by adult A. melinus in the field. In the field, the body size of A. aurantii varied with plant substrate, locality, time of the year, and probably, with the nutritional state of the host plant. Plant substrate was found to substantially influence the body size of A. aurantii with scales being significantly larger on fruits that on leaves or twigs. Another important source of variation for A. aurantii size was geographic location since significant differences were found among orchards. Moreover, significant seasonal variation in the body size of A. aurantii was observed; body sizes were smaller during summer and autumn, apparently due to the effect of temperature. Finally, a positive relationship between the content of potassium in leaves and scale size was observed. From all the above factors, temperature related seasonal variation had the most profound effect on A. aurantii size. Aphytis chrysomphali and A. melinus used different sizes of A. aurantii in the field. / Pekas, A. (2011). Factors affecting the biological control of California red scale Aonidiella aurantii (Hemiptera: Diaspididae) by Aphytis (Hymenoptera: Aphelinidae) in eastern Spain citrus: host size, ant activity, and adult parasitoid food sources [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/10293 / Palancia