Effects of phenological shifts across temperatures
The effect of parasitoids on average host survival depended on the
relative emergence time of parasitoids, and its interaction with warming
(Fig. 2A, & Fig. S4B; Table S4; note that warming alone did not alter
average host survival). Probability of host survival was at or near zero
when host and parasitoids emerged simultaneously and increased
significantly through the first four days of delay, followed by a
plateau between four and six days (P > 0.05) of parasitoid
delay (Fig. 2A). Increased development rates in elevated temperatures
magnified differences in host survival across phenological delays for
the first four days of phenological delays: the largest increase in host
survival occurred between 2- and 4-days
(OR = 22.36, P < 0.001)
in ambient temperatures, compared to 0- and 2-days (OR = 31.11, P
< 0.001) in warming (Fig. 2A). Interestingly, even with late
phenological delays, parasitoids continued to inflict mortality on
hosts, as rates of host survival rarely exceeded 80% of what was
attained in trials without parasitoids.
Like host survival, parasitism rates differed significantly across
phenological delays, with rates of parasitism declining as phenological
delay increased (P < 0.05 for all phenological delays).
Warming significantly reduced parasitism rates, but the decline in
parasitism was not coupled with increased host survival. When emergence
phenologies matched, host survival was at or near 0% in both
temperature treatments, but rates of successful parasitism ultimately
exceeded 50% in ambient temperature, compared to a maximum of 10%
under warming. Consequently, even though hosts were developing faster in
elevated temperatures, differences in parasitism rates across
phenological delays were diminished (Fig. 2B & Fig. S4A; Table S5),
since the chance of parasitism was already less than 5% with just a
two-day delay. Together, these results suggest that warming reduced the
effects of phenological shifts on parasitism rates due to significant
declines in parasitoid performance and, to a lesser extent, because of
increased development rates in hosts.