Modifying effects of competition across phenological shifts and temperature
The effects of temperature and relative emergence time on host survival presented above did not vary with levels of resource competition (Table S4). Alongside shifts in relative emergence time, resource competition had the strongest effect on host survival (Table S4), where hosts had a 22% (95% CI: 0.16, 0.30%) chance of survival at high levels of resource competition, compared to 42% (95% CI: 0.33, 0.51%) at low levels of competition (OR = 0.40, P < 0.001). Importantly, the relationship between phenological shift and survival varied significantly across competition treatments. For example, delaying parasitoid emergence by two days (relative to 0-day delay) increased survival by 9% in high competition compared to a 34% increase at low competition levels.
In contrast, the effects of temperature and phenological shifts on parasitism rates depended on levels of resource competition (Table S5). Parasitism rates became significantly greater in high competition treatments at the 2-day delay (OR = 2.17, P = 0.032) in elevated temperatures, compared to the 6-day delay in ambient temperatures (OR = 2.61, P = 0.01). Notably, competition was unable to increase parasitoid recruitment with delays longer than 2-days under warming. Thus, even though competition slowed development rates, including in higher temperatures, host species did not remain susceptible to parasitism across a broader range of phenological delays under warming. Together, we revealed that host survival was determined primarily by shifts in resource competition, while elevated temperatures were primarily responsible for difference in parasitism rates.