Conditioning the biocontrol agent Trissolcus japonicus on rearing hosts and plant-herbivore microcosms affects its foraging behaviour

• Alternative host rearing and adult conditioning affect T. japonicus female behaviour. • Adult conditioning improves attraction to V. faba – H. halys microcosm. • Conditioning not necessarily improves attraction to V. faba – D. baccarum microcosm. • Regardless of the rearing host, T. japonicus preferred H. halys adult traces. • T. japonicus can be mass-reared on D. baccarum prior to H. halys arrival. Pre-adult conditioning on rearing hosts and adult conditioning on plant-herbivore cues can induce phenotypic plasticity in parasitoids. This is particularly important in pre-emptive biocontrol, when parasitoids may be mass-reared on alternative hosts prior to the arrival of the target pest. We evaluated how rearing the egg parasitoid Trissolcus japonicus (Ashmed) (Hymenoptera: Scelionidae) on its exotic target host, Halyomorpha halys (Stål), or on the alternative host, Dolycoris baccarum L. (both Hemiptera: Pentatomidae), affects its foraging behaviour. Parasitoid females were tested naïve (without prior exposure to plant-host cues), or after one week of conditioning on a microcosm consisting of a Vicia faba L. plant continuously exposed to feeding by either stink bug. In the olfactometer, parasitoids reared on H. halys did not respond innately to either microcosm, but conditioned parasitoids were attracted to the same type of microcosm used for conditioning. Parasitoids reared on D. baccarum exhibited innate attraction to the corresponding microcosm, whereas conditioning induced positive response only to H. halys . In arrestment arena, T. japonicus preferred H. halys adult traces, regardless of the rearing host. Further studies should evaluate parasitoid responses in different ecologically relevant scenarios, also considering plants bearing stink-bug eggs and the potential oviposition reward for parasitoids during conditioning. Our results suggest that, once pre-release approval is granted, T, japonicus can be reared on the alternative host in preparation for its release at the first detection of the target host, and that prior conditioning on host-associated cues would likely ensure parasitoid long-distance location of the target population.