Effect of soil environment on the efficacy of fungal pathogens against scarab grubs in laboratory bioassays

The effect of soil temperature and water on fungus-induced mortality of scarab grubs was investigated in the laboratory. Soil applications of dry mycelial particles of Metarhizium anisopliae (Metsch.) Sorokin were tested against the Japanese beetle, Popillia japonica Newman, and particles of Beauveria brongniartii (Saccardo) Petch were tested against the European chafer, Rhizotrogus majalis (Razoumowsky). Japanese beetle mortality occurred fastest in soil at 27°C and 11% water content and slowest in soil at 21°C and 17.5% water content. Japanese beetle mortality at Week 5 of the bioassay was affected by both soil temperature and water when a high dosage of fungus was used. The proportion of Japanese beetle cadavers with sporulating fungus was influenced by soil water, dose, and a soil temperature and dose interaction. In contrast, European chafer mortality was affected only by soil temperature. Mortality rates for B. brongniartii -killed European chafers and the proportion of cadavers supporting sporulating fungus were both significantly higher in soil at 21°C than at 27°C. Survival of M. anisopliae and B. brongniartii was affected by soil water; concentrations of both fungi were consistently higher in soil at low water content.