Effects of global warming on pest status in agriculture

Introduction

Climate change has been an important criterion in abundance and distribution of species. Global warming is the increase in temperature which is now a major issue for mankind. It possess threat to both survival and distribution of insect pests. For food security, increasing crop yield would be a rather sustainable approach than clearing crop field. Modern approaches are focused on impact of climate change which includes rising global temperature, carbon dioxide concentrations, heat waves, flooding, intense storms, droughts etc. Hence more attention has been given to above abiotic factors. However, along with these abiotic factors some biotic factors also play a major role in impacting crop yield. Among all the biotic factors, the major factor is the pest population which are also affected by weather disturbances. Increase in temperature affects reproduction, survival, and population dynamics of pest.

Impact of increase in temperature

Effect of temperature on insects largely suppresses the effects of other environmental factors (Bale et al. 2002) [1] . Temperature is the most important factor which governs the survival, reproduction, distribution and lifecycle of pests. Out of different temperature range, low temperature is having more detrimental effect than high temperature in determing the geographical distribution of pests. Rising temperature on insect can be direct, through the influence of climatic factors on the insects' physiology and behavior or indirect, as mediated by host plants, competitors or natural enemies (Thomson et al., 2010) [2] . Helicoverpa zea in North America (EPA, 1989) and H. armigera in North India has shown the impact of temperature on pest status successfully (Sharma et al. 2010) [3] . Rise in temperature has a great influence on the habitat shifting of pests. Viability and incubation period of H. armigera eggs is strongly influenced by the fluctuation in temperature. Today, many butterfly species are depleting due to decrease of the natural vegetation for developmental activities (Sidhu and Mehta, 2008) [4] . Around 99.9% of all species have become extinct. Due to climate change the current extinction rates are 100 to 1,000 times greater than what has happened earlier, and nearly 45 to 275 species are becoming extinct everyday (Sharma, 2010) [3] . Rise in temperature influences the distribution of insects. Around 1ºC rise in temperature increases the speed 200 km northwards (in northern hemisphere) or 40 m upward (in altitude). Due to fluctuation in temperature unfavourable areas may become favourable for pests. Early adult emergence and an presence of migratory species of aphid have been reported in UK (Harrington et al., 2007) [5] .

Diapause is a period of suspended developmental activities, which is controlled by temperature, humidity and photoperiod. Insects that experience winter diapause are likely to experience the most significant changes in their thermal environment. Increased temperatures will speed up the development cycle of insects which results in more cycles of generations per year. Hence insects can complete more generations per year (Petzoldt and Seaman, 2010) [6] . Climate change has increased the frequency and intensity of of insect-pests populations. Occurrence of of sugarcane woolly aphid Ceratovacuna lanigera Zehntner in sugarcane growing areas of Karnataka and Maharashtra during 2002-03 resulted in 30% yield losses (Joshi and Virakamath, 2004) [7] . Climate change can cause incidence of insect transmitted plant diseases through multiplication of insect vectors (Sharma et al., 2005) [8] .

Increased CO2 levels

Increased CO 2 level can have a significant effect on pest population status. Increased CO 2 level can increase the carbon nitrogen ratio which ultimately dilutes the nitrogen feeding by protein limited insects (Coviella et al., 1999) [9] . Showed in soybean a low level of deterrent phytochemicals under high ambient CO 2 allows Japanese beetle to feed heavily on the plants. High levels of this gas increases the carbohydrate content of which affects the growth of aphids (Chen et al., 2004) [10] . Phytophagous insects may also develop adaptations to overcome higher carbon to nitrogen ratios. Lindroth et al. (1993) [11] reported that performance of saturniid caterpillars is only marginally affected when the carbon to nitrogen ratio increased by 13-28%. Biochemical analysis of the foliage showed that plants grown under the elevated CO 2 levels had lower N-content, and higher Ccontent, C/N ratio and polyphenols. Larvae grown under higher concentration of CO 2 had shown higher digestibility to leaves than larvae grown under standard condition. Rao et al. (2009) [19] have demonstrated feeding behaviour with two foliage feeding insect species, A. janata and S. litura using castor plants as sample under three concentrations viz., 700 ppm, 550 ppm, 350 ppm (ambient) of CO 2 . Insects grown under 700 ppm and 550 ppm concentrations had shown higher digestibility over insects under ambient CO 2 concentration. Even though relative consumption rate increased, the efficiency of digestibility decreased. Soyabean grown under elevated CO 2 condition have shown 57% more damage to Japanese beetle, Potato leaf hopper and Mexican beetle.

Effects of rainfall

Change in frequency of rainfall may affect the distribution and abundance of pests. Under change in climatic conditions frequency of rainfall will decrease while intensity will increase. Under such circumstances sucking pests like aphids, jassids, whiteflies will reduce as they get washed away with heavy rainfall (Pathak et al., 2012) [12] . Due to climate change, average rainfall will decrease in several regions which leads to occurrence of summer droughts in many areas. Aphid population on wheat and other crops was adversely affected by rainfall and sprinkler irrigation (Daebeler and Hinz, 1977) [13] . Masters et al., (1998) [14] have carried out novel manipulations of local climate to investigate how warmer winters with either wetter or drier summers would affect the homopteran insects a major component of the insect fauna of grasslands.

Impact of climate change on Pests Breakdown of Host Plant Resistance

Host Plant Resistance is one of the most important criteria for managing the pest population. Efficiency of HPR largely depends on climatic factors like temperature, rainfall and carbon dioxide concentration. Changes in these climatic factors may alter the interactions between insect pests and their host plants (Sharma et al., 2010) [3] . Under stressful environment plants become more susceptible to attack by the pest due to failure in its own defensive mechanism. Due to rise in temperature and water stress conditions sorghum plant may show susceptibility to sorghum midge and spotted stem borer (Sharma et al., 2005) [8] . High CO 2 causes changes in nutritional quality and secondary metabolites of the host plants. Severe drought benefits Bark beetles, wood borers, and sap sucking insects, while Spodoptera exigua (Hub.) exhibited a reduced ability to feed on droughtstressed tomato leaf tissue. CO 2 enriched atmosphere reduces nutritional quality of leaves due to dilution in nitrogen content by 10-30% (Coley and Markham, 1998) [15] .

Transgenic crops

High temperature, elevated CO 2 levels, or drought, leading to decreased resistance to insect pests due to reduction in formation of toxic protein. Cotton bollworm, Heliothis virescens destroyed Bt cottons due to high temperatures in USA (Kaiser, 1996) [16] . Reasons for the failure of insect control may be due to inadequate production of the toxin protein, environment stress, locally resistant insect populations, and development of resistance due to inadequate management (Sharma and Ortiz, 2000) [17] .

Natural Enemies

Global warming changes the relationship between pest and natural enemies. Changes in temperature will also alter the timing of diurnal activity patterns of different groups of insects. Temperature affects not only rate of development in insects but also the fecundity and sex ratio. Oriental armyworm, populations increased during extended periods of drought followed by heavy rainfall. High concentration of CO 2 and temperature increases aphid population, however, the parasitism rates remain unchanged in elevated CO 2 (Triltsch, 1996) [18] .

Conclusion

Climate change is now a reason of great concern. It has a great impact on diversity, distribution, incidence, reproduction, growth, development, voltisim and phenology of insect pests. Climate change affects the host plant resistance and natural enemy efficiency. Dealing with the climate change bears uncertainty, unpredictability over time and place. Effects of climate change on pest population should be given proper attention to have successful pest management strategies. Hence, It is a need to have a proper look at the likely effects of climate change on crop protection, and devise appropriate measures to mitigate the effects of climate change on food security.

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