Provided is an arthropod male germline gene expression system suitable for conditional expression of an effector gene in an Arthropod male germline. The system comprises a first expression unit comprising an effector gene and a promoter therefor operably linked thereto; and a second expression unit. Said second unit comprises a coding sequence for a transcription factor and an upstream regulatory element operably linked thereto, the transcription factor being capable of acting upon the promoter in the first expression unit to drive expression of the effector gene. The upstream regulatory element includes a promoter for the transcription factor; and a 5' UTR adjacent a start site for the transcription factor coding sequence. The upstream regulatory element driving sufficient expression of the transcription factor such that the transcription factor protein in turn drives transcription of the effector gene before meiosis. Also provided are uses of the system for instance in methods of biocontrol and quality cont
