Factors affecting yellow pan trap captures of the emerald ash borer biocontrol agents Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae) and Spathius galinae Belokobylskij & Strazanac (Hymenoptera: Braconidae): Implications for monitoring establishment and seasonal abundance

Abstract

Yellow pan traps are frequently used to monitor establishment of hymenopteran parasitoids introduced for biological control of the invasive ash pest, emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae). To evaluate and improve their efficacy as a monitoring tool, we conducted field studies during 2016–2019 in southern Michigan where the introduced hymenopteran larval parasitoids, Tetrastichus planipennisi Yang (Eulophidae) and Spathius galinae (Braconidae) Belokobylskij & Strazanac are established. Our objectives were to describe parasitoid seasonal abundance, identify host tree variables that might influence parasitoid capture numbers (i.e., number of adults captured) and capture rates (i.e., number of positive traps per collecting period), and compare parasitoid trap captures to parasitized EAB larval densities. Yellow pan trap capture numbers and rates of both species were low for most study sites and years. Captures of both parasitoid species occurred throughout the growing season and neither showed a consistent trend in seasonal abundance. Emerald ash borer infested trees with suppressed or intermediate crown classes had higher T. planipennisi capture numbers and rates than dominant or codominant crown classes. During some years, T. planipennisi capture rates increased with decreasing tree diameter at breast height (DBH) and T. planipennisi capture numbers increased with increasing numbers of epicormics sprouts on host trees. Spathius galinae captures had no significant relationship with host tree variables but low overall capture numbers, due to its more recent introduction, may have prevented detection of any relationships. Tetrastichus planipennisi adult capture numbers were positively related to previous year’s densities of T. planipennisi parasitized larvae dissected from trees. Spathius galinae capture numbers were not correlated with parasitized EAB larval densities. We discuss the relevance of our findings to improve yellow pan trap efficacy for monitoring introduced EAB larval parasitoids.