Surface coating aids survival of Serratia entomophila (Enterobacteriaceae) in granules for surface application

The New Zealand grass grub (Costelytra zealandica) is one of New Zealand’s major pasture pests that may be controlled using the non-spore forming bacterium Serratia entomophila. However, S. entomophila is sensitive to environmental stress and must be formulated to improve survival. Existing formulations require subsurface application limiting the area that can be treated. Formulations that allow delivery by broadcast methods are desirable to reduce application costs and increase the potential for aerial application to inaccessible areas. The objective of this work was to determine if formulation could improve delivery of S. entomophila to soil and, in particular whether formulation could meet the challenge of delivering beneficial bacteria with the need for subsurface application. Two formulations were prepared for use in experiments examining the persistence and movement of inoculum through soil. Bacteria were coated onto a zeolite core and then covered with secondary coatings of minerals and starch designated as either thin-coat or thick-coat granules. When granules were applied to the soil surface, bacterial survival was negligible in the uncoated core, but improved with increasing thickness of the coating. The thin-coat granule was further tested after incorporation into the soil (drill formulation) while the thick-coat granule was tested on the soil surface (broadcast formulation). Both survival of bacteria and release into the soil were influenced by soil moisture content. Granules at <12% soil moisture showed high bacterial mortality and reduced delivery to the soil, while at 28% soil moisture most bacteria were released to the soil. There was a high level of survival of the applied bacteria within granules at 20 and 28% soil moisture. Both coating thicknesses maintained viability of S. entomophila in granules stored under ambient conditions for more than 6 months. In laboratory and field tests, application of granules caused disease in the target grass grub larvae, whether application was applied to the surface or subsurface. In field trials, broadcast applied granules could produce equivalent disease to thin-coat granules drilled into the soil, but these levels of disease were associated with the occurrence of precipitation shortly after application. The timing and intensity of these events requires further investigation. These trials demonstrate successful delivery of S. entomophila to the soil profile when applied by broadcast methods, increasing the potential use of this biopesticide through reduced application cost and expanding application to areas previously untreatable. There is further potential for other gram negative species using these formulations.