INTRODUCTION Acerophagus papayae Noyes and Schauff, 2003 (Hymenoptera: Chalcidoidea: Encyrtidae) is a koinobiont endoparasitoid of papaya mealybug, Paracoccus marginatus Williams and Granara de Willink 1992 (Hemiptera: Pseudococcidae). It is native to same of Mexico from where P. marginatus originated (Noyes and Schauff, 2003). In 2008, a team of IPM CRSP researchers first time reported P. marginatus from Indonesia and India, causing serious damage to papaya and warned about its potential presence and spreading in the neighboring countries (Muniappan et al., 2008). However, (Mastoi et al., 2011) first time confirmed the presence of P. marginatus in papaya orchards of Malaysia along with its parasitoids i.e., A. papayae, Chartocerus sp. (Signophoridae: Hymenoptera), Marietta leopardine (Aphelinidae: Hymenoptera) and Cheiloneurus sp. (Encyrtidae: Hymenoptera).During last decades, A. papayae is widely introduced in many countries of the world i.e., Guam, Palau, Puerto Rico, Sri Lanka, Dominican Republic and India to manage the populations of invasive P. marginatus (Walker et al., 2003; Meyerdirk et al., 2004; Muniappan et al., 2006; Shylesha et al., 2010; Galanihe et al., 2010). However, for continuous and successful augmentative biological control programme of P. marginatus requires mass rearing of A. papaya in large enough populations to suppress mealybug outbreaks. Accordingly, knowledge of the most suitable host stage to support growth and multiplication of the parasitoid is vital for the mass rearing (Rehman and Powell, 2010). Large hosts are considered of better quality as they contain more food resources to support many parasitoid offspring, whereas only a single parasitoid can survive in a small host (Vinson, 1976). Moreover, a female parasitoid from the Hymenoptera has the ability to influence offspring sex ratio at oviposition considering the size of host as larger hosts are supposed to support a female biased offspring ratio (King, 1987). Although mealybug biological stages usually overlap in the field, data on host stage preference of A. papayae and dependent sex-ratios will ensure synchronization with the most preferable host stage availability/abundance at the time of release and thus the optimum parasitoid offspring fitness. Previous studies on parasitism of A. papaya on P. marginatus were conducted without differentiating the male and female mealybug instar nymphs (Amarasekare et al., 2009, 2010, 2012). Thus, no information is available on the relative parasitism and gregarious behaviour of A. papayaeon male instar nymphs of P. marginatus except Mastoi et al. (2014a), who studied percent parasitism and sex ratio of A. papaya on various male and female stages of P. marginatus. Better Pak. J. Agri. Sci., Vol. 55(2), 375-379; 2018 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 DOI: 10.21162/PAKJAS/18.5600 http://www.pakjas.com.pk EFFICIENCY OF Acerophagus papayae ON DIFFERENT HOST STAGE COMBINATIONS OF PAPAYA MEALYBUG, Paracoccus marginatus Muhammad Ishaque Mastoi 1,* , Nur Azura Adam 2 , Rita Muhamad 2 , Idris Abd Ghani 3 , Arfan Ahmed Gilal 4 , Javed Khan 1 , Abdul Rauf Bhatti 1 , Ahmed Zia 1 and Jam Ghulam Mustafa Sahito 4 1 Department of Plant and Environmental Protection, NARC Park Road Islamabad, Pakistan; 2 Department of Plant Protection, Faculty of Agriculture, Universiti Putra Malaysia; 3 Faculty of Science and Technology, University Kebangsaan Malaysia; 4 Faculty of Crop Protection, Sindh Agriculture University Tandojam, Pakistan * Corresponding author’s e-mail: ishaqnscri@yahoo.com Acerophagus papayae is a koinobiont endoparasitoid of the invasive papaya mealybug, Paracoccus marginatus and has been introduced as a classical bio-control agent of the mealybug in many countries. Considering the importance, parasitism efficiency of A. papayae against various life stages of P. marginatus was conducted in choice experiments. Results suggested that A. papayae showed higher parasitism efficiency towards third instar female nymphs and adult female P. marginatus in comparison to second instar male. However, second instar male nymphs were more preferred in comparison to second instar female nymphs. Acerophagus papayae exhibited a highly significant sex-biased development ratio as a female dominant progeny emerged while feeding on female hosts and vice versa. No difference was recorded in the developmental time of male and female A. papayae, however, females matured one day later than males. The only gregarious behaviour of A. papayae was recorded on adult female P. marginatus as more than one adult parassitoid emerged from a single host. Study results suggested that A. papayae has a tremendous potential to be utilized in classical biological control programs against P. marginatus as it showed more preference towards female hosts; hence not only reducing available pest population but also will affect the pest population of next generation. Keywords: Acerophagu spapayae, papaya, mealybug, Paracoccus marginatus, parasitism