Bio-assays are mostly applied to larvae and engorged female.This is because of the importance of female in spreading the diseases by laying eggs, kindly read on ftp://ftp.fao.org/docrep/fao/010/ag014e/ag014e05.pdf and Biocontrol of pigeon tick Argas reflexus (Acari: Argasidae) by entomopathogenic fungus Metarhizium Anisopliae (Ascomycota: Hypocreales) by Tavassoli et al. hope it helps