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.

诸多昆虫会被森林火灾产生的热量、灰烬与烟雾所吸引，常于火情仍在持续时便大量涌入新近过火区域。其中部分昆虫为嗜火昆虫（pyrophilic），仅能在火灾刚结束后的时段完成繁殖，且几乎从未在未过火生境中被采集到。诸多针对活跃火情的观测显示，部分嗜火昆虫会优先选择在树木的过火部位产卵，这引发了学界对过火环境下繁殖的适应性优势的广泛探讨。本研究针对嗜火步甲属（Sericoda spp.）的繁殖输出是否会在经高温灭菌的土壤中提升展开了实验验证。首轮实验中，研究人员将四星嗜火步甲（Sericoda quadripunctata）的卵放置于三种取自过火林的土壤中饲养：分别为火灾后1-2周采集的新近过火土壤、火灾后1年采集的过火土壤，以及紧邻火场的未过火林土壤。通过体视显微镜（dissecting microscope）每日观测发现，土壤微型节肢动物（soil microarthropods，包括螨类、弹尾虫与线虫）对卵存在大量捕食现象，该现象在1年生过火土壤与未过火土壤处理组中尤为显著。这一结果促使研究人员开展了第二轮实验，该实验保留了前述三种土壤处理组，并新增了第四组：将新近过火土壤重新加热至100℃并保温2小时的复热土壤。本轮实验中，研究人员将100对暗褐嗜火步甲（Sericoda obsoleta）雌雄成虫分别置于盛有90g对应四种土壤的饲养罐中，饲养时长为14天。以每对繁殖成虫产出的幼虫数作为繁殖输出指标，结果显示复热土壤组的繁殖输出显著更高，这表明高温灭菌及去除土壤栖息捕食者可提升卵的存活率。本研究结果表明，昆虫的嗜火习性或许是通过利用高温灭菌的产卵基质，提升过火环境下繁殖输出的适应性演化策略。此外，火灾后嗜火步甲属与其他嗜火昆虫的突然消失，可通过两方面得到解释：其一为过火区域被其他生物定植后，繁殖条件不再适宜；其二为当地种群向外扩散，转而利用景观中其他新近过火区域。