**4.6 Impact of climate change on insect-plant interactions**

Insects and plants are affected by climate change and severe weather actions and the direct impact of anthropogenic climate change has been reported on each and every continent, ocean, and in many main taxonomic groups. Plants experience new environmental problems like higher CO2 and O3 levels, increased temperature and UV radiation, and changes in rainfall pattern across the seasons as a result of recent activities of human and their influence on global climate. Insects constitute

#### **Figure 3.**

*Effects of CO2 induced photosynthesis and stomata conductance on plant growth responses [61].*

nearly half of the biodiversity and are vital for the structure and function of ecosystem. Because of their close relationship with host plants, through the changes undergone by their host plants, herbivorous insects are likely to experience climatic change direct and indirect consequences. In many ways, global climate changes are reported to influence the interactions between insects and plants. They could directly influence insects, through changes in parameters of physiology, behavioural and life history, as well as indirectly, by means of change in their morphology, biochemistry, physiology and patterns of richness, diversity and abundance experienced by host plants [63]. By functioning as herbivores, pollinators, predators and parasitoids, insects play major roles in ecosystem services and by altering their abundance and diversity, have attained the capacity to modify the services they offer [64]. Over past 20 years, the studies documenting the impacts of climate change on insects have risen exponentially.

#### *4.6.1 Increased temperature*

In many global change scenarios meant for plants and insect herbivores, the ecological-niche models use revealed a definite spatial mismatch among the monophagous butterfly, *Boloria titania*, and its larval host plant, *Polygonum bistorta* due to each species expressing differential range expansion in response to changes in climate and land use [65]. These findings indicate that, because of species-specific responses to climate change problems, temperature increase and other altered factors by humans have the capacity to disturb the insect- plant interactions at trophic level. Another example of the impacts of rising temperatures on the generation of asynchrony between insects and their food sources is the winter moth, *Operophtera brumata*. The outbreaks of climate-dependent psyllid, *Cardiaspina* sp. and their effects on the *Eucalyptus* dieback across thousands of hectares of Western Sydney's seriously endangered Cumberland Plain Woodlands (CPW) are due to the effect of change in temperatures. Summer heat waves (maximum above 46°C) combined with resource shortages due to defoliation triggered the *Cardiaspina* sp. outbreak in 2013 and in the CPW it became unnoticeable [66]. Conversely, by mid-2015, population levels grew and large parts of the CPW were defoliated again until a heat wave led to extreme decline in populations of psyllid in early 2017 (up to 46°C maximum).

While most of the studies, have concentrated on negative interactions involving insects on the effect of global warming on trophic interactions. Memmott et al. [67] have discussed that how climate change can interrupt or even eradicate mutual interactions like pollination and dispersion of seed in between organisms. By means of simulations based on a real network of interactions between 1,419 pollinating insect species with 429 species of plants, they showed that 17 to 50 per cent of all pollinators studied would suffer a decrease in the supply of food with phenological progress of their floral resources by two weeks. For specialist pollinators, this reduction would be even more extreme. Data on the impact of climate change on the synchrony of host-parasitoid interactions are not as widespread as interactions between plant-herbivores and predator–prey [68] but recent studies have proven that parasitoid and host asynchrony affects of climate change can be direct or indirect through changes in host plant.

### *4.6.2 Enriched atmospheric CO2*

It affects the physiology of plants, with significant implications on plant growth and biochemical composition. Plant chemical composition influences both positive and negative trophic interactions and decomposition, which will then react to

*Climate Change and Its Potential Impacts on Insect-Plant Interactions DOI: http://dx.doi.org/10.5772/intechopen.98203*

atmospheric CO2 concentrations [69]. Even though the impacts of increased CO2 on plants are erratic and not uniform, increased activity of photosynthetic, production and leaf area/biomass are often exhibited by plants grown under high CO2 conditions. Higher CO2 levels could change the primary and secondary metabolism of plants as well. The increase in the supply of carbon for tissues of plant and the subsequent C/N ratio changes influence the amount of nitrogen in plant tissues, triggering a "nitrogen dilution effect". This lower nitrogen concentration, combined with higher C/N ratio with possible influence on the plants secondary metabolism, suggests lower leaf protein concentration and thus reduces the nutritional value of herbivores. Increased CO2 usually raises the concentration of leaf carbohydrates and reduces the amount of nitrogen (N) in combination with elevated temperatures. Higher CO2 exposure depresses the jasmonic acid (JA), a plant defence hormone while stimulating salicylic acid (SA) production. This results in increased vulnerability to chewing insects and increased tolerance to pathogens.

In addition to higher CO2, elevated ozone (O3) concentrations in the troposphere also affect plants and insects indirectly. In North America and Europe, tropospheric ozone layer is known as main hazardous and well-known pollutant affecting the ecosystems of agriculture and forests. Since the pre-industrial period, O3 concentrations have increased by almost 40 per cent and are reported to affect directly the plant species and affect herbivorous insects indirectly. O3 in plants triggers a cascade of adverse physiological effects, disrupting the process of photosynthesis and reducing the carbohydrates supply in the plant [69]. While higher CO2 concentrations stimulate the productivity and development of plants, O3 tends to have detrimental impacts on plants, usually leading to reduced growth and lower quality of nutrition in the leaves. This modification in plants quality resulted in the increased rate of herbivory due to overcompensation by insects because of lower nutritional features of tissues. Plants grown under increased O3 conditions generally display lower photosynthetic rates, reduced leaf area, premature leaf abscission and damaged branch and root growth. Increased O3 concentrations are expected to have indirect effects on insect and would depend on the extent of change in the condition of host plant (bottom-up factors) or the influence of natural enemies (top-down factors). Elevated O3 may alter the population of natural enemies by making changes in their diversity, number and prey quality or by changing the behaviour of natural enemies [64].
