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

Abstract

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.

Complex responses of global insect pests to climate warming

“…Observations suggesting that responses to climate change differ among trophic levels, translating into shifts in the relative importance of bottom‐up and top‐down population processes, must be examined in greater depth, as even relatively small changes could result in large effects when multiple interactions are affected simultaneously (Kollberg et al . ).…”

Adverse climatic periods precede and amplify defoliator‐induced tree mortality in eastern boreal North America

“…Although such climatic events are important drivers of tree mortality, other factors (e.g., fire and insect damage) also generate widespread mortality, and their behaviour is often influenced by climate (Boulanger, Gauthier, & Burton, ; Kolb et al., ; MacLean, ). For instance, climatic variables are known to have a direct influence on insect population dynamics and the initiation of outbreaks (Bentz et al., ; Boggs & Inouye, ; Bouchard, Régnière, & Therrien, ; Kollberg et al., ). Climate may also indirectly affect insect population dynamics by reducing the ability of host trees to invest in defensive compounds, thus rendering them more vulnerable to insect damage (Anderegg et al., ; Bentz et al., ; Jactel et al., ; Kolb et al., ).…”

Differential drivers of chemical defenses in three widespread pine species along a 1500-m elevational gradient: a test of the elevational gradient in plant defense hypothesis

“…Resource and climatic gradients have also been shown to influence tree defenses in some species (O'Neill et al 2002;Moreira et al 2015;Ferrenberg 2016). At the same time, warming temperatures can interact with conifer defense chemistry to affect the relative risk of forest insect pest outbreaks (Kollberg et al 2015). A greater understanding of the underlying drivers of spatiotemporal patterns in tree defenses is necessary to improve predictions of forest pest dynamics in relation to global change pressures (Holdenrieder et al 2004;Ferrenberg 2016).…”