Morphological markers in embryonic development of Drosophila suzukii (Diptera: Drosophilidae): Prerequisites for cryopreservation

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) is an invasive pest recently reported and spreading in Europe; the insect infestations caused severe economic damage to many agricultural crops, in particular small fruits and cherries. There are serious concerns about the currently available chemical insecticides because of their low efficacy in controlling the species and their environmental impact; so, several studies have focused on environmentally safe strategies. The sterile insect technique (SIT) could be evaluated in pest management of this species. This technique requires colony maintenance in laboratory and production of large numbers of live animals; therefore the ability to cryobiologically preserve such stocks would be of substantial value. Important prerequisites for long-term cryopreservation are determination of the embryonic stages, identification of specific embryonic stages, and knowledge of development time.

The present study is the first research on the embryonic development of D. suzukii aimed at identifying characteristic markers of the different stages easily recognizable in live embryos. Moreover, the length of time-temperature combination for each stage was determined to simplify the storage handling of embryos. Eggs were collected on blueberry juice-agar plates with a spot of yeast paste in embryo collection cages. Five hundred eggs were collected for observations of embryonic development; short collection intervals (30 min) were used for more precise timing of development. The entire embryonic development was observed using live eggs treated with Halocarbon Oil 27 to make the chorion transparent. D. suzukii embryogenesis lasts 23–25 h at 25°C and can be divided into 17 stages defined by specific morphological markers. The processes of cleavage-blastoderm, gastrulation, germband elongation, germband retraction, dorsal closure and final morphogenesis were reached in 4.00, 4.20, 7.40, 11.40, 15.00 and 23.30 hours, respectively. Particular attention has to be paid to the variability of the egg retention times in the oviduct, which may affect the overall efficiency of the procedure up to 25% of laid eggs.