Heat-induced post-stress growth delay: a biological trait of many Metarhizium isolates reducing biocontrol efficacy?

The habitats of many pest insects have fluctuating climatic conditions. To function effectively, the pathogens of these pests must be capable of infecting and developing disease at a wide range of temperatures. The current study examines ten Metarhizium spp. isolates as to their ability to recover normal metabolic activity after exposure to high temperature for several hours daily; and whether such recovery, with at least some isolates, requires a temporary repair ("retooling") period. Fungal colonies were exposed to 40°C for 4h or 8h followed by 20h or 16h at 28°C, respectively, for three consecutive days. Growth rates during treatments were compared to control plates (constant 28°C) and to plates with growth stoppage by cold treatment (4h or 8h at 5°C per day). All ten isolates survived 3days of cycled heat treatment and resumed normal growth afterward; some isolates however, were considerably more negatively affected by heat-cycling than others. In fact, some isolates underwent greatly reduced growth not only during 8h heating, but also some hours after cessation of heat treatment. This phenomenon is labeled in the current study as "post-stress growth delay" (PSGD). In contrast, all isolates stopped growing during 8h cold treatments, but immediately recommenced growing on return to 28°C. The delay in recommencing growth of some isolates after heat treatment amplifies the effect of this stress. In addition to the studies on the effects of heat cycling on fungal cultures, the effects of imposing such temperature cycling on fungal infection of insects was documented in the laboratory. Three Metarhizium isolates were bioassayed using Galleria mellonella larvae. Treated insects were placed at daily temperature regimes matching those used for the in vitro fungus rate-of-growth study, and insect mortality recorded daily. For all three isolates the levels of insect mortality at the highest-heat dose (40°C at 8h daily) significantly reduced infection. Fluctuating temperatures are likely to be a factor in most pest-insect habitats; therefore, the presence and level of PSGD of each isolate should be a primary consideration in selecting field-appropriate fungal isolates.