Temperature affects insect outbreak risk through tritrophic interactions mediated by plant secondary compounds

Global warming may affect population dynamics of herbivorous insects since the relative impact of bottom‐up and top‐down processes on herbivore survival is likely to be influenced by temperature. However, little is known about the mechanisms by which warming could affect regulation of populations, particularly when indirect effects across trophic levels are involved. We quantified larval survival of the needle‐feeding European pine sawfly, Neodiprion sertifer , either protected from (caged) or exposed to natural enemies at three geographically separated localities in Sweden. The study shows that larval survival is affected by temperature but the direction of the effect is influenced by plant secondary compounds (diterpenes). The results suggest that survival of exposed larvae feeding on needles with high diterpene concentrations will decrease with increasing temperature, while larval survival on low diterpene concentration is less predictable with either no change or an increase with temperature. This food quality dependent response to temperature is probably due to diterpenes having a double‐sided effect on larvae; both a negative toxic effect and a positive anti‐predator defense effect. Increased temperature had also consequences at the population level; an established population model parameterized using data from the study to evaluate the influence of temperature and plant secondary compounds on the regulation of the sawfly predict that, depending on food quality, outbreak risks could both decrease and increase in a warmer climate. If so, effects of plant secondary compounds will play an increasing role for larval survival in a future warmer climate and temperature will, via multitrophic effects on larval survival, strongly influence how sawfly and other insect populations are regulated.