Pyrophilic ground beetle rearing study

Many insects are drawn to the heat, ash, and smoke produced by forest fires and arrive in large numbers at recent burns, often while the fire is still active. Some of these insects are pyrophilic and reproduce exclusively in the immediate aftermath of fire but are rarely, if ever, collected from unburnt habitats. Numerous observations made at active fires note an apparent preference among some pyrophilic insects to oviposit exclusively in the burnt portions of trees, raising broader questions about the adaptive benefits of reproduction in the post-burn environment. Here, we tested whether the reproductive output of pyrophilic ground-beetles (i.e., Sericoda spp.) increased in heat-sterilized soils. In the first experiment, eggs of Sericoda quadripunctata were reared in three types of soil collected from burnt forests: recently burnt soil (collected 1-2 weeks after the fire), soil collected one year after burn, and soil from an unburnt patch of forest adjacent to the fire. Daily monitoring through a dissecting microscope documented extensive predation of eggs by soil microarthropods (mites, springtails, and nematodes), especially in 1-year old and unburnt soil treatments. This led to a second experiment that included the same three treatments and an additional fourth soil treatment: recently burnt soil reheated to 100 °C for 2 hours (i.e., reheated soil). In this experiment, male and female pairs (n = 100) of Sericoda obsoleta were reared for 14 days in jars containing 90 g of soil corresponding to each of the four soil treatments. Reproductive output, measured as the number of larvae produced by each breeding pair, was significantly higher in the reheated soil, suggesting that heat-sterilization and removal of soil-dwelling predators improved egg survival. Our findings suggest that pyrophily in insects may have evolved as a means of increasing reproductive output in the post-burn environment through access to heat-sterilized ovipositing substrates. Furthermore, the abrupt disappearance of Sericoda and other pyrophilic insects following fire may be explained by the sub-optimal reproductive conditions as the burn is colonized by other organisms and by local emigration in favor of other recent burns on the landscape.

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