Biological and physiological responses of Bradysia odoriphaga 4th instar larvae to long‐term mild heat stress

Abstract

The high summer temperature is a key barrier to Bradysia odoriphaga (Diptera: Sciaridae), a devastating soil pest in China. The 4th instar larvae possess the strongest thermal tolerance to high temperatures. To determine whether B. odoriphaga 4th instar larvae possessed other adaptations to summer mild heat stress than thermal plasticity, we evaluated the biological performance of 4th instar larvae under constant long‐term mild heat stress. The physiological responses were studied including energy storage, hormone level and related gene expression. Bioassay results indicated that 4th instar larvae survived, without pupation, for more than 40 days under long‐term mild heat stress (34°C). Long‐term mild heat stress pretreatment delayed larval development, reduced longevity and fecundity. A summer diapause‐like phenomenon was observed, including larval development stasis under mild heat exposure and prolonged recovery time after mild heat. Protective physiological responses were involved in regulating the larval stage, including fat and glycogen accumulation, a high level of juvenile hormone and a low level of moulting hormone. In addition, high expression levels of hsp70 and hsp90 were detected. The delayed development of 4th instar larvae appears to be a special adaptation strategy to withstand mild heat stress in summer that regulated by complex physiological processes.