Page "Hymenoptera" Paragraph 9

Among most or all hymenopterans, sex is determined by the number of chromosomes an individual possesses.

Fertilized eggs get two sets of chromosomes ( one from each parent's respective gametes ), and so develop into diploid females, while unfertilized eggs only contain one set ( from the mother ), and so develop into haploid males ; the act of fertilization is under the voluntary control of the egg-laying female.

This phenomenon is called haplodiploidy.