Lycoriella auripila Winnertz (Diptera: Sciaridae) is one of the most serious pests of mushroom cultures, Agaricus bisporus (Lange) Imbach. Damage of this pest can result in substantial yield losses directly and indirectly. Gaeolaelaps aculeifer Canestrini as a predator suppresses population of soil-dwelling pests e.g. mites, nematodes and fungi, and todays it has been considered as a commercial biological control agent against eggs and larvae of sciarid flies. Prey preference of natural enemies is one of the components of predation that should be taken into account to choose the exact releasing time against appropriate prey stages. In this study, the prey preference and predation rate of G. aculeifer were investigated at 23±1 °C, 60 ± 5% R.H. and a photoperiod of 0: 24 (L: D) h under laboratory conditions. Results showed that life stages of prey influenced prey preference, as females of G. aculeifer killed 5.66 ± 0.33 and 2.33 ± 0.21 of 2nd and 3rd instar larvae of L. auripila respectively. Accordingly, Manly's index of preference was 0.697 ± 0.0377 and 0.303 ± 0.0377, respectively. This difference might be attributed to prey size difference between 2nd (1.36 ± 0.08 mm) and 3rd (2.52 ± 0.10mm) sciarid larval instars. Moreover, in terms of obtained results, proper timing of releasing predatory mite reduces sciarid fly population efficiently. It has been recommended that releasing of G. aculeifer females prior to emergence of 3rd instar larvae of L. auripila would suppress the sciarid flies population densities in mushroom production.
