Compounds of the general formula (1) or an isomeric form thereof:(FORMULA) wherein R1 is a halo group; or a lower alkyl, lower alkoxy or lower alkylthio group, in each of which the alkyl group may be substituted with one or more halo groups; R2 is hydrogen or a halo or halomethyl group; or R1 and R2 together form a methylenedioxy, or a difluoro- methylenedioxy group or, R1 and R2 together with the