3 Results

Humidity, duration of heat waves, larval stage, and their interactions did not affect adult emergence in unparasitized hosts (binomial GLM, allp > 0.05). However, in parasitized hosts, the relative success of host and parasitoid adults was significantly affected by experimental treatments (Fig. 2). Longer heat waves increased the probability of host adult emergence in parasitized hosts (Table 1), and experiencing 72 hours of heatwave in the early stage resulted in complete mortality of parasitoids, regardless humidity levels. Increased humidity decreased the probability of host adult emergence (Table 1). Larval stage when the heatwaves were applied did not affect the relative success of hosts and parasitoids, but it positively interacted with humidity, and negatively interacted with duration of heat waves to affect the probability of host adult emergence (Table 1). The effect of the interaction between duration of heat waves and larval stage on host adult emergence depended on humidity levels (three-way interaction, Table 1). Humidity, duration of heat waves, and larval stage did not affect the probability of no adult emergence (Table 1, Fig 2), indicating that the mortality of hosts was not affected by the treatments, but depended on the survival of parasitoids.
The juvenile development time of unparasitized hosts increased when experiencing longer heat waves (log-link Gamma GLM, estimates = 0.036, t = 4.110, p <0.001, Fig. 3a), and decreased in high humid conditions (log-link gamma GLM, estimate = -0.290, t = -7.429,p <0.001, Fig. 3a). Humidity positively interacted with duration of heat waves to affect host juvenile development time (log-link gamma GLM, estimate = 0.038, t = 2.976, p = 0.003). For parasitoids, experiencing heat waves at a later stage decreased juvenile development time (log-link Gamma GLM, estimate = -0.068, t = -2.123,p = 0.036, Fig. 3b), if they survived from heat stress. Neither humidity nor duration of heat waves affected the juvenile development time of parasitoids individually, however, increasing heat wave durations in the 5th instar stage decreased juvenile development time (log-link Gamma GLM, estimate = 0.035, t = 2.177,p = 0.031, Fig. 3b). The mid femur length of unparasitized hosts increased if hosts developed in high humidity (log-link gaussian, estimate = 0.189, t = 5.100, p < 0.001, Fig. 4a) or experiencing heat waves at later stage (log-link gaussian, estimate = 0.081, t = 2.206, p =0.029, Fig. 4a). Moreover, humidity negatively interacted with duration of heat waves (log-link gaussian, estimate = -0.142, t = -2.871, p =0.005), and larval stage (log-link gaussian, estimate = -0.050, t = -3.901, p < 0.001, Fig. 4a) to affect the mid femur length of unparasitized hosts. The hind tibia length of parasitoids increased if hosts developed in high humidity (log-link gaussian, estimate = 0.144, t = 3.032, p= 0.003, Fig. 4b), or experienced heat waves during the 5th instar stage (log-link gaussian, estimate = 0.082, t = 2.246, p = 0.026, Fig. 4b). Larval stage negatively interacted with duration of heat waves to affect the hind tibia length of parasitoids (log-link gaussian, estimate = -0.048, t = -2.577,p = 0.011).
Larval stage and duration of heat waves directly affected the survival time of hosts, but not the size of parasitoids (Fig. 5). Instead, the hind tibia length of parasitoids was negatively correlated with host survival time (Fig. 5), suggesting an indirect effect of larval stage and duration of heat waves on the size of parasitoids. All the paths to parasitoids were constrained to a global estimation (p> 0.05, more in Appendix Table S1), indicating that the effect of heat waves and hosts did not depend on humidity. However, the unconstrained paths to hosts indicated that the effect of heat waves on host survival time depended on humidity levels (p<0.05, Appendix Table S1 and S2). In low humidity conditions, host survival time was affected by larval stage and duration of heat waves, whilst it was only affected by duration of heat waves in high humidity conditions.