No abstract

An appraisal of the regulatory role of natural enemies on target pests requires the identification of the mechanisms/traits that enhance the ability of an organism to control the density of its prey/host. After stinging herbivore hosts with their ovipositor, hymenopteran parasitoids tend to reject them without ovipositing or host-feeding. Termed pseudoparasitism, the frequency and consequences of this type of attack (hereafter oversting) have been largely disregarded in the hymenopteran parasitoid literature. We choose the parasitoids Aphytis melinus and A. chrysomphali and their common host, Aonidiella aurantii as a model system to study this behaviour. Using field and lab observations, we showed that overstinging is a common behavior in the wild. Under controlled conditions, overstinging occurred more frequently than hostfeeding, a behavioral trait that is used to evaluate the potential of parasitoids as biological control agents. Oversting reduced the fecundity and survival of the herbivore host. When we compared between parasitoid species that attack the same host species, the virulence and frequency of this behavior depended on parasitoid species. These results demonstrate that overstinging should be incorporated in the models of host-parasitoid interactions to analyze population dynamics as well as in the future selection of parasitoids for biological control.

Herbivore suitability and quality for hymenopteran parasitoids is dynamic, varying with host development. Generally, within the same host species, large hosts (i.e. older instars) are considered of higher quality for progeny development. Studies of interespecific competition between parasitoids have considered the effect of host instar on indirect competition but its effect on interference competition remains unknown. Here, we report the first results on whether host instars (of different quality for immature development) might dictate the outcome of interference competition between sympatric parasitoids of the genus Aphytis Howard (Hymenoptera: Aphelinidae) when they attack low (second) and high quality (third) instars of their common host Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae). Oviposition behavior (host acceptance and clutch size) in low and high quality host instars was similar for both Aphytis species in the absence of competition. When they found heterospecific parasitized hosts of high quality, Aphytis melinus DeBach laid more eggs and accepted significantly more hosts than A. chrysomphali (Mercet), whereas there were no significant differences in the low quality instar. This result suggests that interference competition is mediated by host quality.However, the progeny proportion of both parasitoids in multiparasitized hosts (outcome of competition) was independent of host quality and A. melinus always emerged at higher rates.Therefore, the final result of interference competition between these sympatric parasitoids was not affected by host quality and this competition will contribute to the displacement of the native A. chrysomphali by the introduced A. melinus, as it has been observed in some areas of the Mediterranean basin.

Parasitoids are among the most important and successful groups of natural enemies used in the biological control of insect pests. In most systems, several parasitoid species can parasitize the same pest. The coexistence of parasitoids in agroecosystems and their efficacy as biological control agents may be disrupted by global warming. An increase of approximately 3°C is predicted by the end of the twenty-first century in the Mediterranean basin (IPCC, 2014). In this context, we compared the present and future performance of two sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae), which control the armoured scale Aonidiella aurantii (Hemiptera: Diaspididae) in Mediterranean citrus, either alone or in combination. The net reproductive rate (R0) of the introduced Aphytis melinus DeBach was higher than that of its competitor, the native Aphytis chrysomphali (Mercet), under current conditions. The two parasitoids responded differently to higher temperature and competition. The R0 of A. chrysomphali decreased by 50% when both parasitoids competed in the same patch, but was not affected by the temperature increase. The R0 of A. melinus decreased approximately 40% with the increase in temperature because the proportion of females was reduced. However, the presence of A. chrysomphali competing in the same patch mitigated the negative effect of the increase in temperature on A. melinus (R0 decreased by only 20%). Overall, our results suggest that global warming will have a negative effect on the biological control of A. aurantii and that this effect will be higher in areas, such as southern Spain, where A. melinus has displaced A. chrysomphali.