Sampling Trissolcus japonicus using yellow sticky traps: Does location in the tree canopy matter?

Samurai wasp, Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae), is an egg parasitoid of the invasive agricultural and nuisance pest, brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Adventive T. japonicus populations have been detected in North America since 2014, and yellow sticky cards (YSC) have proven effective for sampling to monitor its distribution and spread. In a previous study, destructive sampling of felled Tree of Heaven (Ailanthus altissima) yielded H. halys egg masses parasitized by T. japonicus from the mid- and upper canopy, and none from the lower canopy. However, most investigators have deployed YSC for T. japonicus from the ground in the lower canopy, raising questions about the effect of trap placement on T. japonicus captures. Five female tree of heaven were sampled for T. japonicus by deploying YSC on bamboo poles suspended from mid- and lower-canopy branches; trap elevation (m) in the lower and mid-canopy was 2.36 ± 0.44 and 5.82 ± 0.49, respectively. Traps were replaced weekly and assessed for known scelionid parasitoids of H. halys. In total, 190 T. japonicus were captured in 2020. Traps deployed in the lower-canopy captured significantly more T. japonicus (both sexes combined) than those in mid-canopy. This difference was also significant for female T. japonicus, but not for males. These results have implications for the effectiveness of sampling T. japonicus via YSC and challenge previous assumptions about the vertical distribution of T. japonicus in host trees. Additional sampling is needed to resolve the question of optimal trap placement.