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Nystrom Santacruz et al.: Cold tolerance of T. japonicus and T. cultratus 1 Biological Control Cold tolerance of Trissolcus japonicus and T. cultratus, potential biological control agents of Halyomorpha halys, the brown marmorated stink bug Nystrom Santacruz et al.: Cold tolerance of T. japonicus and T. cultratus 2 ABSTRACT Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) is native to Asia and has become a severe agricultural and nuisance pest in the U.S. Therefore, foreign exploration was conducted in Asia to identify potential classical biological control agents. Several Trissolcus spp. (Hymenoptera:Scelionidae) parasitize H. halys eggs in Asia and are being evaluated for potential release in the U.S. Since H. halys has invaded regions that experience sub-zero winter temperatures, cold tolerance is important for evaluation of Trissolcus spp. Our study compared the cold tolerance of populations of T. japonicus and T. cultratus, in order to assess relative suitability of the populations for release. We used thermocouple thermometry to determine the supercooling point and lower lethal temperature after brief exposure to cold temperature for each population. In addition, we subjected adult T. japonicus to a short photoperiod and low temperature regime, which increases cold tolerance in H. halys, to observe whether these conditions cause a change in cold tolerance in the parasitoid. We found that populations of both species froze and survived at colder temperatures than those reported for H. halys. In addition, there were no ecologically relevant differences in the temperature at which freezing or survival occurred among populations of either species, indicating that these populations are equally cold tolerant and suitable for introduction. Finally, T. japonicus does not acclimate by increasing cold tolerance in response to conditions that increase cold tolerance in H. halys, suggesting that the above-mentioned measures of cold tolerance are ecologically relevant.