Incidence of pest and diseases is most severe in tropical regions due to favorable climate/weather conditions, multiple cropping and availability of alternate pests throughout the year. Therefore, in the south Asia, pests and diseases deleteriously affecting the crop yields are prevalent. Climatetors are the causative agents in determining the population fluctuations of pests. They influence plant disease establishment, progression and severity. In fact, a clear understanding of population dynamics, as influenced by abiotic and biotic parameters of environment, is of much help in pest forecasting and to formulate control measures.
Indicators of climate change can be a few of the crop species, rhythm/migratory behavior of specific insects/birds, etc. The global warming may affect growth and development of all organisms including insect-pests themselves. Among all the abiotic factors, temperature is the most important one affecting insect distribution and abundance in time and space, since these are cold-blooded animals. The insects cannot regulate their body temperature and thereby, ambient temperature influences their survival, growth, development and reproduction.
The swarms of locust produced in the Middle East usually fly eastward into Pakistan and India during summer season and they lay eggs during monsoon period. The swarms as a result of this breeding, return during autumn to the area of winter rainfall, flying to all parts of India and influencing kharif crops (Rao and Rao 1996). Changes in rainfall, temperature and wind speed may influence the migratory behaviour of locust.
Diseases are often hurdles in increasing rice productivity. The rice blast, caused by Pyricularia grisea, is most prominent disease across the eco-systems. In the past, rice blast, brown spot and stem rot, were the serious diseases. Consequent to the adoption of high yielding varieties and associated agronomic practices during 1970's, diseases like bacterial leaf blight, sheath blight, sheath rot, tungro virus (transmitted by Nephotettix spp.) and bacterial leaf streak, have gained importance over the traditionally known diseases, especially stem rot and brown spot. False smut and discolouration of rice grain, caused by several fungi, have been of minor significance with occasional concern in certain regions only. While analyzing the effect of climatic variability and change on disease status, the interaction of land use and land cover change should also be taken into consideration.
Climate and weather selectively induce specific diseases to develop. The mono-cyclic diseases, such as stem rot, sheath rot and false smut, are less influenced by the ambient weather conditions. Epidemics of monocyclic diseases are relatively rare in the sense of an explosive increase in their population. In contrast, the poly-cyclic diseases, such as blast, brown spot, bacterial leaf blight and rice tungro virus that invade the aerial parts of the plants, are subjected to constant interaction with weather. They easily attain epidemic proportions to cause heavy losses (Abrol and Gadgil 1999).
Forecasting of aphids (Lipaphis erysine Kalt) on mustard crop, grown during winter season in northern part of India based on the movement of western disturbances, has been established (Ramana Rao et al. 1994). Western disturbances bring in cold and humid air from the Mediterranean region, resulting in cloudy and favourable weather conditions for occurrence of aphids on mustard crop. It was observed that there was a sharp increase in the population of aphids when the mean daily temperature ranged from 10 to 14°C, with relative humidity of 67-85% and cloudiness greater than 5 octas.
For every insect species, there is a range of temperature within which it remains active from egg to adult stage. Lower value of this range is called threshold of development or developmental zero. Within favourable range, there is an optimum temperature where most of the individuals of a species complete their development. Exposure to temperature on either side exerts an adverse impact on the insect by slowing down the speed of development (Pradhan 1946).
The studies have shown that insects remain active within temperature range from 15 to 32°C (Phadke and Ghai 1994). In case of red cotton bug, at constant temperature of 20, 25 and 30°C, the average duration of life cycle was found to be 61.3, 38.3 and 37.6 days, respectively, while at 12.5 and 35°C, the pest did not show any development (Bhatia and Kaul 1966).
A maximum temperature ranging from 19 to 24°C with a mean of 12-15°C for mustard aphid, Lipaphis erysimi; maximum temperature between 26.9 and 28.2°C with a relative humidity of 80.6-82.1% for rice stink bug; temperature from 20 to 28°C for rice green leafhopper, temperature from 24.8 to 28.6°C for brown plant hopper; mean temperature around 27.5-28.5°C for aphids, thrips and leaf weevil on green gram and maximum temperature from 23 to 27.8°C for gram pod borer, have been found most congenial for their development (Phadke and Ghai 1994).
With the increase in temperature, the rate of development of insects may also increase, if temperature still lies within the optimal range for the pests. As a consequence, they could complete more number of generations for inflicting more loss to our crops. Crop-pest interaction needs to be evaluated in relation to climate change in order to assess the crop losses.
Development of diseases and pests is strongly dependent upon the temperature and humidity. Any change in them, depending upon their base value, they can significantly alter the scenario, which ultimately may result in yield loss. Any small change in temperature can result in changed virulence as well as appearance of new pests in a region. Likewise, crop-weed competition may be affected, depending upon their growth behaviour.
The following scenarios can be visualized regarding impact of climate change on pest dynamics in agriculture.
• With an increase in concentration of carbon dioxide, the nutritional status of crop will change, and the net effect on agricultural production will depend upon interaction between pests and crops.
• Gradual climate warming will lead to changes in the composition of pest fauna in different areas. The high population growth rate of many species will ensure changes in pest distribution.
• If the rise in winter temperature takes place, the duration of hibernation of pests may decrease, thus increasing their activity.
• Uncongenial areas for pests due to low temperature at present may become suitable due to rise in temperature.
However, we should not forget that insects could adapt to slow changes in the environment and with increase in temperature, their favorable range of temperature may also shift.
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