Summary
   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. Aphytis chrysomphali was recovered mostly from second instars (0.152-0.300 mm2 in A. aurantii body area), but parasitized more heavily third instars, reaching an average of ~10% parasitism on scales sized between 0.80-0.85 mm2. Aphytis melinus developed mostly, and parasitized more heavily, third instars reaching an average of ~30% parasitism on scales sized between 0.70-0.75 mm2. Gregariousness and parasitoid size were positively influenced by host size. Moreover, host size affected A. melinus sex ratio; male eggs were laid on small hosts and female eggs on large hosts. The host size at which the sex ratio of A. melinus turned female biased was found to be around 0.40 mm2 and this threshold remained constant whether relatively small or large hosts were available. Given the seasonal variation in the size of A. aurantii, between May and October most scales were not suitable for production of female A. melinus. Thus, a decrease of parasitoid populations is likely to be expected in this period of the year, which in turn may result in insufficient control of the scale. Augmentative releases of A. melinus should be carried during the period when hosts suitable for the production of females are scarce.
   The most abundant ant species (Hymenoptera: Formicidae) in eastern Spain citrus, the dominants Pheidole pallidula (Nylander) and Lasius grandis Forel foraged in mutually exclusive territories within the same orchard but they both share their territory with the subordinate Plagiolepis schmitzii Forel forming a distribution pattern known as “ant mosaic”. Ants were ascending to the canopies from April until November. Temperature and colony nutritional requirements shaped their seasonal foraging patterns. The daily activity pattern of P. schmitzii was strictly diurnal whereas L. grandis and P. pallidula were active during the 24 hours of the day. Citrus nectar and predation/scavenging did not represent important food sources. On the contrary, hemipteran honeydew was the principal food source for the ants on the canopies. More than 60% of the total honeydew sources, and 100% of the citrus mealybug Planococcus citri (Risso) (Hemiptera: Pseudococcidae) colonies, were tended by ants during spring and summer.
   Ant-exclusion from the citrus canopies resulted in significantly lower scale densities on fruits at harvest when compared with the control treatment where ants had access to the canopies. Scale density on the fruits was positively correlated with the number of ants that climbed to the citrus canopy. This result suggests that the increase of A. aurantii densities induced by Mediterranean ants depends on the intensity of the ant-activity.
   The comparison of the total sugar content and the glucose-fructose ratio between field-collected A. melinus and individuals that received a reference feeding treatment in the laboratory revealed that nine out of eleven field-collected A. melinus had recently consumed carbohydrates. The laboratory reared parasitoids did not synthesize oligosaccharides after sugar feeding. On the contrary, all the field-collected parasitoids characterized as “fed” contained oligosaccharides like melezitose, raffinose, erlose or melibiose, sugars typically present in different types of hemipteran-honeydew. These results suggest that adult A. melinus use hemipteran-honeydew as a food source in the field.