Parthenogenesis in Bees
Because the "Y chromosome" has nothing to do with sex determination in bees.  Instead, the number of X chromosomes determines the sex.  With only one X chromosome, the egg will develop into a male.  This is called a haplodiploid system of sex determination (the male bees are haploid, having only half the number of chromosomes).

EDIT: I should add that there are many systems of sex determination, not just XY or the haplodiploid system.  See the Z-W system (males are ZZ and females are ZW), the X0 system (the males are diploid for all but the sex chromosome), and temperature-dependent system of sex determination.