Effect of alternative host species across phenological shifts and temperature
On average, single-species treatments, with D. sulfurigaster(probability = 0.53 (95% CI: 0.43, 0.63)) and D. birchii(probability = 0.17 (95% CI: 0.12, 0.24)) had the highest and lowest probabilities of host survival respectively, while the treatment with both species present showed an intermediate probability of survival (Probability = 0.29 (95% CI: 0.22, 0.38)) (Fig. S5). Parasitism rates showed similar patterns across single (D. sulfurigasterprobability = 0.03 (95% CI: 0.02, 0.05%) and D. birchiiprobability = 0.04 (95% CI: 0.03, 0.07)) and multiple host species combinations (sulfurigaster-birchii probability = 0.05 (95% CI: 0.03, 0.07)), with average parasitism rates being intermediate when multiple host species were present. While average host survival rates differed significantly among host species combinations (P < 0.05), average parasitism rates showed no differences. Thus, the average development time, host survival, and parasitism rates of thesulfurigaster-birchii combination were comparable to the pooled mean values of the two host species in isolation. However, the probability of host survival varied across host species combinations and this effect varied with temperature and phenological delay (Table S4; Fig. S5 & Fig S6). Interestingly, each species in isolation showed significant differences in survival across temperatures at each relative delay, but when an alternative host species was present, host survival rates did not significantly differ across temperatures at any relative delay (P > 0.05). This is likely due to opposing responses to phenological delay, temperatures, and resource competition betweenD. birchii and D. sulfurigaster. These results suggest that within a single generation the presence of alternative host species dampened differences in the probability of host survival and parasitism rates across both temperatures and levels of resource competition, but not phenological shifts.