Oviposition Preference of Homalodisca coagulata for Two Citrus limon Cultivars and Influence of Host Plant on Parasitism by Gonatocerus ashmeadi and G. triguttatus (Hymenoptera: Mymaridae)
Oviposition preference of Homalodisca coagulata (Say) and two of its mymarid egg parasitoids, Gonatocerus ashmeadi Girault and G. triguttatus Girault, for two Citrus limon L. cultivars (‘Eureka’ and ‘Lisbon’) was investigated. In laboratory oviposition choice tests, the number of leaves containing H. coagulata egg masses, the number of H. coagulata egg masses, and the total number of H. coagulata eggs were significantly higher at 187.2%, 204.2%, and 181.7%, respectively, on ‘Eureka’ versus ‘Lisbon’. In the field, there was no significant difference in the number of H. coagulata motiles counted in five-minute searches of foliage on ‘Eureka’ and ‘Lisbon’ trees, and numbers of leaves with old (emerged) and new (unemerged) H. coagulata egg masses were equivalent between field-planted cultivars. In the laboratory, parasitism of H. coagulata egg masses by G. ashmeadi and G. triguttatus was 18.6% and 23.2% higher, respectively, for eggs laid on ‘Eureka’ leaves compared to ‘Lisbon’, but these differences were not significant. Leaf surface morphology and thickness of leaf cell layers of both lemon cultivars were compared with scanning electron microscopy (SEM). SEM demonstrated that total leaf thickness and the thickness of the palisade layer was 19.2% and 38.6% higher, respectively, in ‘Eureka’ leaves compared to ‘Lisbon’, and that H. coagulata egg placement was between the lower epidermis and spongy parenchyma layer for both cultivars. Furthermore, ‘Lisbon’ leaves had a smooth underside, whereas ‘Eureka’ leaves had many small ridges. The thickness and rough surface of ‘Eureka’ leaves may be beneficial for H. coagulata oviposition. However, additional research is required to further investigate whether leaf characteristics or xylem chemistry are responsible for H. coagulata oviposition choice. For mass rearing programs with lemons as host plants, it is recommended that the ‘Eureka’ cultivar be used in preference to ‘Lisbon’ because H. coagulata prefers this cultivar for oviposition and parasitoid foraging is not adversely affected.