Assessing deposition and persistence of Beauveria bassiana GHA (Ascomycota: Hypocreales) applied for control of the emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), in a commercial tree nursery

Determining the deposition and field persistence of mycoinsecticides is essential in the development of effective and economical application strategies, including specifically the timing and frequency of spray applications. In this study we used three methods to evaluate the persistence of Beauveria bassiana strain GHA applied for control of the emerald ash borer (EAB), Agrilus planipennis, an invasive pest attacking ash trees, Fraxinus spp., in North America. Fungal inocula present on ash bark and leaves, collected at ⩽1, 7 and 14 days after spraying, were quantified by use of molecular (real-time PCR assay) and culture-based methods (semi-selective wheat germ dodine agar). We also assayed fungus-sprayed leaves and bark against adult beetles to determine whether the level of inocula persisting in the field was sufficient to affect beetle survival. Our data quantified deposition of B. bassiana and documented fungal persistence. We observed significant decline in recovery of colony-forming units of B. bassiana within 1 week of application. The decline was more pronounced on leaves than on bark, and was also evident in loss of virulence of treated substrates to adult EAB with increasing time after application. However, sufficient inocula persisted on bark and leaves for 7--14 days to cause 40--57% and 17--38% mortality, respectively, making pre-emergent sprays a practical means to target adults during emergence, pre-oviposition feeding, or oviposition.