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Abstract:
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.
