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.