**3.2 Effect of carbon dioxide (CO2) on insect pest and plants**

Higher concentrations of CO2 with the rise in temperatures in the atmosphere have direct effects on plant metabolism and affect the distribution, abundance and productivity of insects that feed on plants (**Figure 1**). The behaviour of phloemfeeding insects, when supplied with plants grown under increased CO2, increases compared to leaf chewing insects [16]. When leaf chewing insects like grasshoppers and caterpillar larvae feed on plants that are grown under higher CO2 levels, more leaf area is eaten than they actually eat [17]. *Spodoptera litura* has been reported to grow under higher levels of CO2 as a serious pest [18]. The larvae of *Helicoverpa,* grown under high CO2 ate much more leaf tissue than those under ambient CO2. However, under elevated CO2, adult moths increased and lived longer and laid considerably few eggs [19].

The change in CO2 concentration also influences the plant biochemistry, along with the synthesis of secondary metabolites [20]. The higher concentration of CO2 is subjected to increased ratio of carbon to nitrogen in plants. Insects are allowed to consume more in order to achieve sufficient dietary nitrogen, resulting in slower larval growth and increased mortality. Phytophagous insects can become more susceptible to changes in atmospheric CO2 concentration by CO2 cascading effects on plant biochemistry, as certain plant feeding insect species produce their pheromone molecules on the basis of compounds taken from the host plants [21]. Example: Bark beetles use the mevalonate pathway to generate pheromones, where certain components of aggregation pheromones originate from the hydroxylation of secondary metabolites derived from tree [22]. Besides affecting the plant biochemistry, along with the synthesis of secondary metabolites changes in CO2 concentration could also affect the plant yield. Example: [23], estimated a yield loss in wheat, maize and cotton of 36 to 40 per cent in a scenario of low CO2 emissions, and between 63 to 70 percent in a scenario of high CO2 emissions.

#### **3.3 Effect of precipitation on insect pest and plants**

More frequent and extreme precipitation events during climate change are expected to have detrimental effects on the population of insect pests. It is one of the weather factor that acts upon the activities of several insects by means of soil moisture or directly when exposed. Increased summer rainfall encourages a rapid rise in the soil dwelling wireworms, *Agriotes lineatus* population and larvae of root chewing insects, *Agriotes lineatus* [24]. Soil moisture kills insects by means of submerging in water, or affects the soil texture by preventing the emergence of insects. It is also harmful mainly to the insects that are free living in the soil as eggs or as newly-hatched larvae or nymphs.

The effect of the intense raindrops or water in the leaf axils will dislodge or drown small insects such as aphids, or newly-hatched larvae or nymphs from the plants. High proportions of cabbageworm young larvae, *Pieris rapae* and

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

diamondback moth, *Plutella xylostella* on cabbage, are killed by high precipitation. Intense precipitation also has a catastrophic effect on the boring insect eggs and newly-hatched larvae such as the European corn borer, *Ostrinia nubilalis*, before boring into the plants. It also destroys aestivating adults of the black cutworm larva, *Agrotis ipsilon* and results in drowning of larvae in low-lying areas. Changes in pattern of rainfall are tracked by desert locust, *Schistocerca gregaria* migratory patterns in Sub-Saharan Africa [25]. Precipitation also has a positive association with plant height, total area of the leaves, number of plants and number of leaves, nitrogen and chlorophyll content of the leaves, which has a direct or indirect impact on the population of insect pests.

### **3.4 Effect of climate change on interaction between insect pests and plants**

Climate change can directly affect insect-plant interactions and alter the functioning of both insect pests and plants. The development of secondary metabolites of the plants and other phytochemicals is also affected (**Table 1**). Both plant and herbivore structures can be modified by increasing temperature, CO2, precipitation, etc. Rise in global temperature, atmospheric CO2, and the duration of dry season are all likely to have consequences for tropical plant/herbivore interactions, with significant implications on food security and natural habitats. It will increase


#### **Table 1.**

*Effects (+,* −*, 0) of GEC drivers on plant chemical traits that mediate plant–herbivore and plant–pollinator interactions [26].*

the effect of pests benefiting from reduced host defences due to stress resulted from the lack of adaptation to sub-optimal conditions of climate. Climate change could support non-resistant crops or cultivars, contributing to greater insect pest's infestation [27]. But, plants grown under increased temperatures or CO2 would be less nutritious, as indicated by many researchers and longer larval period and increased mortality of insects is observed upon the insects feeding on them [28]. The defence mechanism of plants against insect pests is diminished by climate change, thereby rendering them susceptible to attack. For example: Early initiation of *H. armigera* infestation in cotton and pulses in Northern India [29]. It has also been found that CO2 decreases the plant defences towards insect pests. For example, under increased levels of CO2 in soybeans, the plant defence pathway signalling mediated by jasmonic acid (JA) does not work [30]. Plants become susceptible to insect pests like Japanese beetle, *Popillia japonica* and western corn rootworm, *Diabrotica virgifera* due to reduced production of defensive cysteine proteinase inhibitors (CystPIs). Additionally, the herbivore-induced plant volatiles (HIPVs) are influenced by higher temperatures and CO2 [31].

## **3.5 Effect of climate change on plant volatile compounds**

The production and release of plant volatile organic compounds (VOCs) can be influenced by changes in abiotic factors and are expected to influence how insects recognise and make use of plant VOCs in intra- and inter-specific interactions [32]. VOCs involved in a number of insect-plant interactions, ranging from positive (e.g., pollination and seed dispersal) to negative (e.g., herbivore defences). The atmosphere could be made more fragrant by global climate, due to release of higher levels of fragrant chemicals in a changing environment by plants. This, in turn, would affect how plants communicate with each other through competitive and allelopathic processes and how they protect themselves from pests, like insects, viruses and pathogens. Few major studies have been conducted to address the effect of changing temperature and gas concentration on VOCs metabolism and expression. Plants are required to develop increased concentrations of VOCs for extended time periods under higher temperatures, thus altering their ecological role in interactions of insects and plants. For example, monoterpene emissions are highly temperaturesensitive exhibiting a 3 fold increase for every 10°C increase in temperature [33]. Therefore, future herbivorous rates are reduced by the development and emission of higher concentrations of VOCs like methyl jasmonate or methyl salicylate that act as plant signalling molecules against insect attack. On the other hand, if a more fragrant atmosphere, confuses pollinators and seed dispersers, beneficial relations may also be interrupted, causing plant reproduction and fitness to be reduced.

VOCs are expected to increase at high CO2 concentrations because of the positive relationship in between the carbon supply and VOCs production. On the basis of the resource allocation hypothesis, increased CO2 concentrations are hypothesised to increase emissions of monoterpenes and sesquiterpenes into the atmosphere [34]. As per this theory, when there is an abundance of carbon availability relative to what is required for plant growth, increases the production of C-based plant secondary compounds. In conifers and cultivated plants, the development of certain C-based VOCs increases under high CO2 conditions [35]. Higher temperature and CO2 affects the emission of herbivorous mediated plant volatile organic compound (HIPVs) [36]. Any changes made to HIPVs would have a direct impact on the effectiveness of biological control. The olfactory perception of the volatiles will be diminished by change in temperatures, thereby affecting the host position capacity of the natural enemies. Higher CO2 concentrations would also modify the levels of oxalic and malic acids in chickpea, affecting its herbivorous resistance [37].
