Temperature elevation broadens the dietary niche of an insect herbivore

Ambient temperature influences plant–herbivore interactions in multiple ways, through both direct effects on insects and indirect effects mediated by changes in host plant quality as a food source for herbivores. These effects may have important consequences for ecosystem structure and functions under climate warming.

We used the willow‐feeding leaf beetle Chrysomela lapponica to experimentally test the hypothesis that increasing air temperature changes the realised dietary niche breadth of insect herbivores.

We examined the direct effects of temperature on C. lapponica at 10, 15, 20 and 25°C, using leaves from one optimal host ( Salix myrsinifolia ) and two suboptimal hosts ( S. caprea and S. phylicifolia ) grown under the same thermal conditions. At 10°C (the average air temperature during host selection by beetles in nature), beetles exhibited a strong preference for and best survival on the optimal host, but the differences between optimal and suboptimal hosts diminished at higher temperatures. Correspondingly, the realised dietary niche was the narrowest at 10°C and broadened with increasing temperature.

Larval survival on suboptimal hosts grown under elevated temperatures in open‐top chambers approached the level observed on the optimal host. Consistently, bioassays conducted over 8 years using field‐collected willow foliage grown under different (year‐specific) thermal conditions showed that the differences in larval survival between optimal and suboptimal hosts decreased significantly with increasing June temperature.

Our findings demonstrate that elevated temperature broadens the realised dietary niche of C. lapponica through both direct and indirect (via host plants) effects. We conclude that climate warming may benefit insect herbivores by enhancing food resource availability through the increased use of suboptimal host plants, potentially leading to an increase in herbivory and the spread of insect damage to a wider range of plant species.