Derivatives of 3,6-DI-substituted-1,2,4,5-tetrazine, 3- (2-bromo-phenyl) -6- (2,6-DI-fluoro-phenyl) -1,2,4, 5-tetrazine, 3,6-bis- (2,6-DI.fluoro.fenyl) -1,2,4,5-tetrazine, 3- (2-chloro-phenyl) -6- (2,6-DI -fluoro-phenyl) -1,2,4,5-tetrazine, process for the preparation of derivatives of 3,6-DI-substituted-1,2,4,5-tetrazine, acaricidal, larvicide and ovidically active composition, composition translaminar, oviciously active, composition with transovarian effect and process for reducing the number of mites and their larvae and eggs, process for the preparation of an acaricidal, larvicide and oviciously active composition