Effect of parasitoid competition, ant exclusion and carbohydrate sources on biological control fo Saissetia oleae on olive trees

Résumé

The black scale, Saissetia oleae (Olivier), is considered a secondary pest of olive tree in Portugal, where it is attacked by a complex of natural enemies that control its population. Parasitoids of genus Metaphycus (Hymenoptera: Encyrtidae) and Coccophagus (Hymenoptera: Aphelinidae) are among the main natural enemies. In this thesis, the several aspects of the successful natural biological control of this scale are analyzed in the field and laboratory. In detail, we analyzed: (i) whether the facultative autoparasitoid Coccophagus lycimnia (Walker) can adversely affect the population densities of Metaphycus spp. parasitoids and disrupt the biological control of S. oleae; (ii) the effect of ant-exclusion on the biological control of S. oleae and its parasitoid complex; and (iii) the effect of the carbohydrate sources available in the field (nectar and honeydew) on the survival of parasitoids of both genus. For the development of the first objective, the population densities of the scale, the primary parasitoids of the genus Metaphycus and the autoparasitoid C. lycimnia were recorded for three consecutive seasons in 28 olive groves. The populations of the parasitoids followed opposite patterns. At the end of the life cycle of the S. oleae (May), the number of Metaphycus spp. observed was practically nil whereas the number of C. lycimnia was five times greater than at the beginning of the assay. In September and November, Metaphycus spp. parasitoids parasitized smaller scales than C. lycimnia, demonstrating that Metaphycus spp. parasitoids are superior at exploitative competition. Metaphycus spp. and C. lycimnia sex ratio was similar in September, but in November the sexual proportion of Metaphycus spp. was higher than that of C. lycimnia. The facultative autoparasitoid C. lycimnia tended to lay male eggs on Metaphycus spp. females. The density of S. oleae decreased from September to May, when the ratio S. oleae-C. lycimnia reached 2:1. Altogether, these results demonstrate that the facultative autoparasitoid C. lycimnia is a superior parasitoid and it can successfully control the scale under the climatic conditions of northern Portugal. The effect of ant exclusion on the parasitoid complex of S. oleae, as well as on scale density, was studied in an olive grove during two seasons (spring and fall). Crematogaster auberti (Emery) was the most abundant ant species in both seasons but it was more active in spring than in fall. Ant exclusion did not affect the number of parasitoids recovered in fall, the beginning of the life cycle of the scale. The parasitoid complex was, then, by Coccophagus spp. parasitoids, which can lay their eggs faster than Metaphycus spp.. In late spring, the number of parasitoids recovered from S. oleae adults was higher in ant-excluded trees than in ant-allowed trees, being Metaphycus lounsburyi (Howard) the main parasitoid species recovered, especially in ant-allowed trees. Ant exclusion, however, did not affect S. oleae density in any season. These data altogether, suggest that the biological control of S. oleae can be affected by ants only at the end of its life cycle when C. auberti is the main ant present in olive groves. To study the effect of the main carbohydrate sources available in olive groves from northern Portugal, the survival of three S. oleae parasitoids: Metaphycus lounsburyi, Coccophagus semicircularis (Förster) and C. lycimnia was tested. The carbohydrates were the nectar of six species of flowering spontaneous plants and honeydew excreted by two hemipterans: S. oleae and Euphyllura olivina (Costa). The honeydew of S. oleae was the best food source for the three parasitoids, as they survived, on average, about seven times more when fed on this honeydew than on nectar from the provided flowers. Euphyllura olivina honeydew, however, was a poorer carbohydrate source. Parasitoids fed on this honeydew lived longer than unfed parasitoids, but lived much less than those fed on S. oleae honeydew. Taking into account that the honeydew of S. oleae is present throughout the year and its high quality for parasitoids, carbohydrates should not be considered a limiting factor in the biological control of S. oleae in olive groves. Future studies should analyze S. oleae honeydew to detect the components responsible for its high quality, as honeydew is usually a poorer carbohydrate source than nectar for natural enemies.