A changing climate will affect plant growth, but what will it do to plant defences? Liu and colleagues at Henan University examined two plants and their herbivores. The study, in Journal of Plant Ecology, examined how a native species and an invasive species differed in their response to warming. If they responded in contrasting ways, the scientists believed this could also affect herbivory. They found they were right, and rising temperatures did change leaf chemistry. They also found that the two plants responded in different ways. Their findings highlight some of the unpredictable effects that global warming will have on the planet.
Alternanthera sessilis is a plant native to Southeast Asia and goes by many names, including joyweed. It’s an aquatic plant that provides forage for bees. In contrast, A. philoxeroides, alligatorweed, is an invasive species in the region. It creates dense mats of vegetation in the water, out-competing other plants for light. It also disrupts water flow, causing problems for wildlife. It does have natural enemies.
Spodoptera litura is a polyphagous species, meaning it’s happy eating many things (poly = many, phagous = eat). It’s called tobacco cutworm or cotton leafworm, but it’s happy eating other plants too. Cassida piperata – tortoise beetle – is oligophagous, meaning it eats only a few things. Liu and colleagues ran experiments to see how these herbivores reacted when the plants grew in warmer conditions.
The team compared plants grown at 2°C compared to a control sample. Raising the temperature reduced the nitrogen in A. sessilis leaves as well as increasing flavonoid and phenol concentration. The higher temperature also decreased the nitrogen content of the invasive A. philoxeroides. For S. litura the effect was to reduce the weight of the larvae. For both species’ larvae, the warmer A. sessilis reduced weight and also prolonged larvae development time.
“These findings might help us understand how climate warming influences the interactions of invasive and native plants with insects, and suggest that elevated temperature may shift the interaction of native herbivores, native plants, and invasive plants,” write Liu and colleagues.
The authors note that while the two species are in the same genus and have similar morphology, the different responses impact plant fitness.
“The most novel finding of our study was that warming shifted the interactions of native herbivores with the native plant more strongly than invasive plant, as indicated by C. piperata pupal weight, S. litura larval weight and larval development time,” write Liu and colleagues. “This finding supports the prevailing view that changes in chemistry in host plants will further affect insect performance. Our results showed that the weight of C. piperata pupal reared on A. sessilis decreased under warming conditions, even though C. piperata showed higher larval survival rate on A. sessilis than on A. philoxeroides. Moreover, warming treatment significantly prolonged S. litura larval development time and such negative effects only occurred in A. sessilis. These results suggest that the indirect impact of warming on the insect is stronger via the native plant than the invader.”
One suggested reason for the dissimilarity is the different stresses the species face. If invasive species has trouble with herbivores, it can allocate more resources to growing. When warming happens, the plant chemistry reacts in different ways. A combination of reduced nutrition and increased defences shows that warming will have an indirect effect on insects as well as the direct effects on physiology. For botanists, it shows that the impact of herbivory under warming could work with or against effects on physiology when looking at a plant’s fitness in a warmer climate.