Study was conducted at Insectary Biological Control Labs., IPMP, National Agricultural Research Centre (NARC) Islamabad under controlled environmental conditions at 26±1°C, 60±5% R.H. and 12:12 L: D photoperiod. Research experiments were done on whitefly Bemisia tabaci (Gennadius) which has been a serious pest in Pakistan for over the past two decades and its potential parasitoid Encarsia sophia (Girault & Dodd) as it is invasive, can be mass reared easily and able to parasitize more hosts. Results regarding host plant effect (cotton, brinjal and tomato) on B. tabaci biology concluded that development times, ovipositional preferences and survival of immature stages was strongly mediated by host plants selected for rearing. Among all the selected host plants, brinjal was considered as most suitable host in terms of developmental times (19.95±0.22 days) as compare to tomato (20.77±0.24 days) and cotton (23.05±0.23 days). Mean population density of eggs on cotton showed stronger preferences of B. tabaci for oviposition among three host plants with a mean number of (194.20±7.69) eggs laid/female with a mortality rate of (23.33%). According to the results, host plants selected for rearing of B. tabaci also showed significant effect on the biology of E. sophia. E. sophia developmental times (egg-adult) in B. tabaci instars was significantly shorter for parasitoids that emerged from brinjal (13.50±0.16 days) with highest percentage of parasitism (29.00±2.12) ranged from 15.00-55.00 and calculated higher emergence on brinjal 92.60±1.71% over other host plants. Results on population growth parameters also exposed brinjal as a good and effective host choice as parasitoid populations develops successfully on it with significantly higher net reproductive rate (Ro) 22.83 andintrinsic rate of natural increase (rm) 0.1043 with shorter mean generation time (Tc) 29.99 and doubling time (DT) 6.64 as compared to reared on cotton and tomato. The immature developmental stages observed for E. sophia were as eggs, 1st, 2nd and 3rd larval instars, pre-pupa and black pupae. Mean development duration from egg deposition till adult emergence was significantly different 12.61±0.13 days for female and 13.94±0.16 days for male. E. sophia preferred to oviposit in all four host instars of B. tabaci although preference differences were observed among host stages in terms of parasitism and feeding behaviours. Parasitism was observed higher on 3rd and 4th instars both in No-choice and Multiple- instar choice experiments but feeding was relatively higher in younger instars. Emergence was found both in light and dark conditions suggesting activity of parasitoid throughout 24h. Newly emerged parasitoid when placed under light source created ease for mating. Honey significantly prolonged the longevity of E. sophia adult. E. sophia fitness and efficacy in terms of fecundity, parasitism, developmental time, pupal survival and longevity was significantly altered by host instars. Almost all biological attributes of E. sophia were at its best developed on 3rd instars of B. tabaci with a faster developmental time (11.80±0.20 days), emerged more synchronously (86±2.66%) and lived longer (12.95±0.78 days) with maximum oviposition rate (62.50±1.69) eggs/female. Therefore it was concluded that larger hosts tended to produce better parasitoids than smaller hosts. The functional response studies determined that E. sophia have the ability to adjust itself at higher densities. E. sophia in relation to the density of host fitted the description of a type II functional response. Results commended a density of 30 instars as the highest critical density for parasitoid where parasitism rate and number of progeny was maximum with a minimum rate of super-parasitism. Host discrimination behaviour of E. sophia was also observed and it was concluded that newly emerged females NEF exhibit less ability to discriminate between the parasitised and unparasitised hosts over SHF and DEF. Females used to exploit minimum hosts when offered an unparasitized host while exploited maximum hosts when offered with a parasitized host as. Mean handling time for E. sophia was higher on unparasitized host while minimum for already parasitized host. The information will be useful in designing mass rearing protocols and in release trials of E. sophia for suppression of B. tabaci populations.