Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/17/2010
Publication Date: 5/15/2010
Citation: Castrillo, L.A., Griggs, M., Liu, H., Bauer, L.S., Vandenberg, J.D. 2010. Assessing deposition and persistence of Beauveria bassiana GHA (Ascomycota: Hypocreales) applied for control of emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), in a commercial tree nursery. Biological Control. 54:61-67. Interpretive Summary: The emerald ash borer (EAB) is an invasive insect that has killed millions of ash trees and has spread to most northeastern states. We evaluated the deposition and persistence of spores of the fungus Beauveria bassiana applied as a biological insecticide against EAB infesting ash trees. We used three methods for this: one to culture the fungus from samples of leaves and bark following spray applications, another to quantify the amount of fungus DNA on these samples, and a third to measure beetle mortality after exposure to fungus-sprayed leaves and bark. We found that fungus sprays covered upper and lower leaf surfaces equally well. We found a significant decline in the number of viable spores within a week after application. However, we observed that fungus spores remain viable longer on bark than on leaves, possibly due to spores in bark crevices being protected from sunlight and direct rainfall. We observed very high beetle mortality after their exposure to freshly-sprayed leaves and bark. Mortality was reduced for beetles exposed to leaves and bark treated one or two weeks prior. We found the fungus culture method to be a more reliable predictor of beetle mortality. These findings improve our understanding of the utility of fungus-based bioinsecticides for emerald ash borer control. The results will aid our design of studies to monitor the effects of control efforts on beetle populations and on the performance and survival of ash trees.
Technical Abstract: 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 30 min, 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 (CFUs) of B. bassiana within one 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. Persistence of sufficient conidia on leaves or bark may make pre-emergent sprays a practical means to target adults during emergence, pre-oviposition feeding, or oviposition.