Thermal tolerance and life history responses of Eupeodes corollae (Diptera: Syrphidae) to elevated temperatures.

Eupeodes corollae (Fabricius) (Diptera: Syrphidae) is a widespread aphid predator and pollinator whose biocontrol services may be altered by climate warming, yet most thermal assays used constant temperatures that do not reflect diurnal heat pulses. We quantified stage-specific developmental duration, survival, larval predation, adult longevity, and fecundity under a constant 25 °C control and 3 fluctuating regimes with daily 6 h peaks of 33, 35, and 37 °C. Survival was analyzed with a binomial generalized linear model (GLM), consumption with Welch's ANOVA, and Games-Howell tests, and adult survival with Kaplan-Meier, log-rank, and Cox models. Survival remained high at 25 to 33 °C, declined at 35 °C (first instars 29.0%; pupae 48.0%), and collapsed at 37 °C (pupal eclosion 0%; eggs 6.0%). Daily consumption of second instars declined at 33 °C, whereas first and third instars were stable at 33 to 35 °C and decreased only at 37 °C; lifetime consumption fell by 77% at 35 °C and to 40.05 ± 14.40 aphids at 37 °C. Adult longevity and fecundity were unaffected up to 35 °C but halved (8.25 d) and collapsed (43.30 eggs per female) at 37 °C, with a median survival time (LT50) of 7.0 d and Cox hazard ratio 7.25 vs. 25 °C. Late instars were the most heat-tolerant and eggs and pupae the most vulnerable, indicating 33 °C as the upper tolerable peak and 37 °C as a critical threshold. Fluctuating regimes should underpin predictions of hoverfly biocontrol; releases should prioritize second- and third-instar larvae and avoid peaks ≥37 °C.