Effects of Climate Change on Natural Control of Insect Pests

Natural enemies of insects are likely to undergo diverse effects due to changes in atmospheric CO2 levels, increase in temperatures and shifts in precipitation. Plants respond to elevated levels of CO2 with higher biomass. As a result there would be a dilution effect on nitrogen levels and those chemical constituents that require nitrogen. Lower nutritional value of plants adversely impacts insects that feed on them directly and also their parasitoids and predators indirectly. Increased temperatures can alter both plant and herbivore phenology with likely impact on synchronization between the two again indirectly influencing the activity of natural enemies and the effectiveness of their natural control. Hymenopteran wasp parasitoids which are relatively host-specific are likely to be influenced more than tachnid flies which generally survive by feeding on a variety of insects. Higher minimum temperatures in temperate regions can lead to expansion of geographical range of insect pests which are currently intolerant to low temperatures. This may result in pest outbreaks in the newer areas if natural enemies fail to track and follow their hosts. Variability in rainfall reportedly has an adverse influence on parasitism levels of several caterpillar pests. Sucking pests like cereal aphids are less susceptible to climate change effects. In case of mealybugs, parasitism is reduced under conditions of water stress associated with drought conditions apparently due to improved immune response. The effects of climate change on natural enemies mediated by CO2, temperature and moisture effects on plants could be complex and unlikely to be predicted easily due to interactions between these effects. Adaptation and mitigation practices to combat climate change such as conservation agriculture practices are likely to have a positive effect on parasitoid and predator abundance with resultant benefits on natural pest control. Much of the climate change research has been conducted in the temperate countries. While referring to these available results, an attempt has been made in this review to illustrate examples of natural regulation of insect pests in India in the context of climate change and variability.