Best of 2019: Pests love a warmer world

As climate change gathers pace, insects are proliferating across the world, which scientists say will threaten food production, particularly in bio-diverse countries like India

It is now becoming increasingly clear that climate change is affecting virtually all aspects of life, and one alarming dimension is the growth in the number of insect pests that are a threat to food security, particularly in developing economies such as India.

Vital to ecosystems and humans, insects pollinate crops and flowers, provide food for higher-level organisms, break down the detritus, maintain a balance in ecosystems by eating the leaves of plants and help recycle nutrients in the soil. However, they also consume somewhere between 18% and 26% of crops produced around the world, a loss valued at USD 470 billion.

The greater proportion of losses occurs in the field, before harvest, and this is heaviest in developing countries. As the world warms up, these losses are likely to grow. India is likely to be badly affected. Although the country has only 2% of the world’s land area, it has around 8% of the world’s species, including insects.

Climate change will affect agriculture, but predicting exactly how is complicated. However, researchers worldwide are of the opinion that agriculture and horticulture yields will decrease as growing-season temperatures increase. Insect pests are likely to exacerbate this effect.

Crop losses

Crop losses will be most acute in areas where warming increases both population growth and metabolic rates of insects. These conditions are centred primarily in temperate regions, where most grain is produced. While climate change directly affects insect growth, development, reproduction and survival, it also indirectly influences breakdown in temperature-sensitive host-plant resistance.

The insect world is vast, diverse and millions are yet to be discovered and identified. Until now, less than one million species of insects are known out of an estimated 30 million.  Of these, anthropods (ants, cockroaches, grasshoppers, beetles, crabs, spiders, scorpions, mites, millipedes, centipedes etc.) are the most dominant and constitute more than 90%.

“As insects are cold-blooded organisms, the temperature of their bodies is approximately same as that of the environment,” said V.V. Ramamurthy, Division of Entomology, Indian Agriculture Research Institute (IARI), New Delhi. “Due to global warming, when the temperature changes and also the relative humidity with more rainless humid days, the sucking pests, in particular, will increase. Also, due to changes in climate in hilly terrains, there will be an upward movement of insect populations. In this process, at least temporarily there will be extensions in range.”

Insects burn calories at a faster clip when their surroundings heat up, forcing them to eat more. Elevated levels of carbon dioxide can increase levels of simple sugars in leaves and lower their nitrogen content. These can increase the damage caused by many insects, which will consume more leaves to meet their metabolic requirements of nitrogen.

In India, there are cases available to show how several minor pests have become major due to change in agriculture practices and climatic variability. The brown plant hopper is a case in point. It was a minor pest in rice of no economic significance till its outbreaks in Kerala in 1973-74. The spotted bollworm completes its life cycle in 88 days when the temperature is 16 degrees Celsius, but at 25 degrees, the lifecycle reduces to 31 days, which means more pests.

Caterpillar’s hunger

Infestation of brown plant hopper in Basmati (aromatic) and non-Basmati rice in western Uttar Pradesh, Haryana and Delhi are recent examples. Incidents of thrips (a sucking pest) attack on crops like groundnut, cotton, chilli, roses, grapes, citrus and pomegranates are being reported. There are also more frequent attacks of diamondback moth in cabbage, and hoppers on mango. In layman’s terms, the appetite of the caterpillar is likely to rise as the earth gets warmer.

A team of researchers from various US universities — using a computer model focused on crop yield in relation to pest destruction — showed that the number of crops lost globally each year due to insects is likely to increase by 10% to 25% per degree of global surface warming.

Closer home, a study on the effect of elevated temperature on the development time of rice Yellow Stem Borer (YSB) by researchers of Coimbatore’s Tamil Nadu Agricultural University revealed that the number of eggs laid increased at higher temperatures, while egg hatching was reduced. Insects develop faster, which may be why they lay eggs early, and hence, the population is likely to grow earlier than expected.

Pest attacks

Increased temperature results in extension of the geographical range of pests and pathogens and sudden outbreak of some notorious pests, said R.R. Patil, Professor of Entomology at the Department of Agriculture Entomology of Dharwad-based University of Agriculture Sciences. “Pink bollworm and maize armyworm in recent times are live examples. Both have devastated large areas of cotton and maize in Karnataka and the neighbouring states of Andhra Pradesh and Maharashtra.”

Studies undertaken by Indian scientists on the trends of temperature rise, heat waves, droughts and floods, and sea level rise are in consonance with the findings of the Intergovernmental Panel on Climate Change (IPCC), though the magnitude of changes could differ. The mean temperature in India is projected to increase up to 1.7 degrees Celsius in the Kharif season (July to October) and up to 3.2 degrees during the Rabi season (November to March). Mean rainfall is expected to increase by 10% by 2070, a scenario leading to a situation affecting many familiar insects restricted to smaller habitats while new pests spread over wider areas.

According to India’s National Action Plan on Climate Change, crops such as rice, wheat, maize, sorghum, mustard, potato, cotton and coconut are likely to be adversely affected. Wheat production is likely to decrease by 6-23% by 2050. Maize production could fall by 18%; paddy may witness a fall by 4-6% by 2020; and potatoes would show a decline by 2.5% by 2020, 6% by 2050 and 11% by 2080. Soybean is, however, predicted to fare better in the future, with its production likely to increase by 8-13% from 2030 to 2080.

Chances of invasion

There is an increased risk of invasion by migrant pests due to global warming. The most recent case is that of the dreaded fall armyworm crop pest (Spodoptera frugiperda), which mainly devours maize in its caterpillar phase, but can feed on more than 80 plant species, including other key crops like rice, sorghum, cotton and vegetables.

First spotted in Africa in 2016, it has spread rapidly through the sub-Saharan belt and was spotted in Karnataka. According to Nigel Andrew, Professor of Entomology, the University of New England and Fellow of the Royal Entomological Society, as quarantine regulations are diluted due to free-trade agreements, more pests are likely to get into areas previously free of them.

As pests are part of natural ecosystems and compete with humans for their existence, crop protection technologies are likely to be less effective and need to be customised to suit the changes. “Alternatives are being developed in some crops like wheat, rice, cotton and some selected horticultural crops and in hotspots of rain-dependent agriculture under the NICRA (National Initiative on Climate Resilient Agriculture) of the ICAR (Indian Council of Agricultural Research) during the last few years,” said Ramamurthy, who also is chief editor of the Entomological Society of India journal. “As the modified technologies have started emerging recently it will take more time and investments to become the established alternatives.”

Protection put to test

As temperatures rise, we are looking at a future when the effectiveness of traditional crop protection technologies will be put to test. In such a situation, pest forewarnings are likely to provide lead time for impending attacks and thus minimise crop loss and optimise pest control leading to reduced cost of cultivation.

Agreeing that new farming strategies to adapt to a rapidly changing climate will be difficult to implement, Andrew said that integrated pest management methods will have to become the norm.

“Cultural methods of insect control will need to be re-introduced and used more extensively. Areas of alternative habitat will need to be replanted on to farms to enable predators and parasitoids to be active,” he said. “Methods such as sterile insect techniques will also need to be used as alternatives to chemical controls.  Chemical controls will need to be more specialised and used more intelligently.”