Savage mode: In a warmer, wetter world, pests are multiplying faster and damaging crops severely

In April, tea planters from southern India sought urgent government intervention, saying that the tea mosquito bug is causing havoc in the most unlikely places. The bug, recognised as a serious pest of fruits and tea plantations across the world, is usually confined to low elevation areas.

Now, it is spreading to plantations in high elevation areas. The United Planters Association of Southern India (UPASI) has issued a press release that states the tea mosquito bug (species of Helopeltis) is affecting tea production in both low and high elevation plantations in northern and southern states.

The extent of damage can be seen in Tamil Nadu’s Valparai hills, where tea production has declined by 50 per cent—from 30 million kg in 2009-10 to 16.73 million kg in 2021-22. Furthermore, affected tea estates are spending over R12,000 per hectare (ha) on pesticide application every year, but to no avail.

While UPASI has sought government approval for the use of some new pesticides in tea plantations to curb the spread of the bug, the measure can, at best, be a myopic solution. The fact is that the tea mosquito bug and several other plant pests are expanding their territory and becoming more infectious in a warming world.

Helopeltis theivora, the most predominant tea mosquito bug species in India, is spreading in an alarming form in tea plantations of Tamil Nadu’s Anaimalai and Valparai hills. Sikkim has recorded the first infestation of H theivora in red cherry pepper, write researchers from Bidhan Chandra Krishi Viswavidyalaya, West Bengal, in the Journal of Plant Health Issues in June 2020.

H bradyi, which has so far been restricted to Peninsular India, was reported for the first time on the high altitude Tura region of Meghalaya. H antonii, which causes damage to cashew crops, is now being reported in Gujarat, Chhattisgarh and Odisha, the authors note.

In Rajasthan’s Alwar district, farmers are troubled by both old and new foes. Tidda (leafhoppers) and silverleaf whitefly (Bemisia tabaci) are not new to Ramgarh village, where farmers primarily grow wheat, millets, mustard, vegetables and pulses. But they have become more menacing in recent years.

“The pests destroyed 40 per cent of my pearl millet in just 15 days,” says Mansingh, who owns a little over 6 ha. “Last year, there were six attacks during the whole season. This year, there were four attacks till February,” he says, adding that aphids (Aphidoidea), shoot borer (Leucinodes orbonalis) and termites have also caused widespread damage in recent years.

“Until about a decade ago, we were able to get rid of pests using natural techniques such as organic pest deterrents. We never used pesticides until 2012. Now every farmer has to spray at least three rounds of pesticides,” he says.

About 5 km from Ramgarh is Nuagaon village. When Down To Earth (DTE) visited the region in February 2023, Roshan Lal, a farmer in Pithu Ramgarh hamlet, pointed to some black insects on his mustard crop.

“I have never seen this insect in my life,” claimed the 59-year-old. A quick image search on the internet indicates that the insect is a black aphid, likely Aphis fabae.

“Since farmers are unable to identify the pests, they are not able to take any remedial action in time,” says Subhash Yadav, head of the Krishi Vigyan Kendra in Naugaon, adding that pests which were never seen before in the region are being spotted now because of climate change.

Scientists at the University of Agricultural Sciences, Gandhi Krishi Vignan Kendra, Bengaluru, corroborate Yadav’s observation.

In a 2013 study, published in Knowledge Systems of Societies for Adaptation and Mitigation of Impacts of Climate Change, the scientists write that in India, such warming is likely to result in pest attacks on staple crops such as cotton, wheat, barley, oats, rice, pulse crops, maize, sorghum, oilseed and vegetables, on which a majority of the population relies for daily nutrition.

They have identified pests such as mealy bug, whitefly, tobacco caterpillar, cereal aphids and plant hoppers as the major threats to crops, along with leaf folders, Lepidopterous pod borers, spider mites and other kinds of aphids.

In a warming world, changes in both temperature and rainfall are the two big drivers of shifts in how and where pests and diseases spread. This trend will be exacerbated by increasing concentration of carbon dioxide in the atmosphere.

Warming, a multiplier

The impact of warming will profoundly affect aphids—soft-bodied insects that suck into plant saps to deform leaves and flowers. About 250 aphid species are recognised as crops pests for agriculture, as they can affect multiple crops of different plant families.

They also known to transmit plant viruses. Aphids are found across the world and have the ability to migrate up to 1,300 km, typically during spring and autumn seasons. Since they are sensitive to changes in ambient temperatures, this will translate to changes in their lifecycle.

A March 2020 study published by Pubmed Central shows that early higher spring temperatures enable cotton aphids to hatch sooner and attack crops. Warming of climate prompts aphid Elatobium abietinum to take early flight before the season.

Another 2020 study in the journal Insects says that in southern Poland, warmer early winter in 2013-14 led to extension of the four-month-long life of female adult aphids of Stomaphis species.

Using data from 1964 to 2014, the study also says that under the influence of climate change, the first migration of 55 aphid species in the UK came earlier than usual and was significantly extended.

Abhishek Pareek, research officer at department of entomology in Rajasthan’s agriculture department, tells DTE, “Aphids have 45 generations in a year.

However, due to mutation and their ability to survive in extreme conditions, the generations can extend further,” he says adding, “Unlike humans who have a single generation of about 60 years, insects with shorter life cycles tend to adapt and mutate their DNA at a faster rate. It is estimated that the insect’s generation may increase by one to five for every 2°C rise in temperature.”

Pareek led a 2017 study published in the book Climate Change and Sustainable Agriculture which claims increasing temperature will affect almost all insects. And the impact will be at multiple levels, wherein every small rise in global temperatures will reduce the lifecycle of insects, resulting in higher population of pests, increasing generations, extension of geographical range and development season, high risks of invasions by migrant pests and overwintering.

In India, which is home to 6.83 per cent of the world’s insect species, 1°C rise in temperature would enable them to expand in presence about 200 km northwards and 40 metres upward in terms of altitude. “The areas that are not favourable at present due to low temperature may become conducive with rise in temperature,” it states.

The increase in temperature will thus allow pests a greater ability to overwinter (wait out the winter season when normal insect activity is difficult) at higher altitudes thereby shifting pest intensity from South to North.

Besides this, say the scientists, the insects will attain accelerated metabolic rates, leading to faster consumption of nutrients, and shorten the time of diapause—when insects display reduced food intake due to overwintering.

Conducive rainfall

Most attacks by locusts in India are restricted to Rajasthan. The swarms usually enter India in May and return by November. The record rainfall in June 2019 extended the stay of locusts in India till 2021 by providing optimum conditions not only for their survival but also for breeding.

The Centre said locust attacks damaged over 200,000 ha between 2019-21, destroying crops in states like Rajasthan, Punjab, Gujarat, Haryana, Uttarakhand, Bihar, Uttar Pradesh, Madhya Pradesh, Chhattisgarh and Maharashtra.

Roshan Lal of Nuagaon says earlier, aphids were a minor concern, that too only during monsoons. Green aphids would appear in cloudy conditions and light showers. But heavy rains would wash them away. However, for the past three to four years, their destruction has been extending till winters.

“Last winter, my gooseberry crop was attacked by aphids and production was reduced from 60 tonnes to 15 tonnes,” he says. Farmers in Nuagaon and nearby villages whom DTE spoke to note that pod borers have started causing heavy damage to chickpeas in recent years.

This change in pest behaviour, say farmers, is because of a shift in rainfall patterns. As per the India Meteorological Department, between 2010 and 2018, average rainfall in eastern Rajasthan, where Alwar is located, has increased by 14 per cent.

Other factors that farmers gauge could be leading to pest attacks are changes in soil conditions. Apart from mustard and wheat, farmers in Pithu Ramgarh grow vegetable crops like carrot, radish, cauliflower, tomato and cucumber.

“However, lowering of groundwater table due to overexploitation of ground resources has reduced moisture in the soil, making it suitable for pests,” says Lal. This has, since 2020, led to more serious attacks by whitefly, leaf hoppers and aphids, right before harvest, ruining nearly 80 per cent of all farmers’ crops, he says.

CO2 aids propagation

An important factor in climate change is the release of greenhouse gases. This release, and in particular the increase in carbon dioxide concentrations in the atmosphere, will aggravate the abundance and behaviour of herbivorous insects, says a 2021 study published in the Insects.

It suggests that increased levels of atmospheric carbon dioxide will have a severe impact on wheat, rice and cotton crops compared to corn and sorghum.

Manoj Oak, scientist at the department of genetics and plant breeding at Agharkar Research Institute, Pune, says, “Carbon dioxide emissions enable increase in photosynthesis, leading to higher accumulation of sugar and starches on leaves. This may favour insect attacks and their propagation.”

In a 2005 study published in New Phytologist, scientists find that feeding in Japanese beetle, Popillia japonica, a major pest of soybean, was prolonged by 8.25 per cent under increased carbon dioxide levels and females laid twice as many eggs compared to feeding under balanced conditions.

Soybean grown under higher carbon dioxide levels caused damage of up to 57 per cent with Japanese beetle, western corn rootworm, Mexican bean beetle and potato leafhopper. Experts also noted a higher fecundity in aphids due to higher carbohydrate levels in food plants grown under higher carbon dioxide levels.

Adaoting to climate 

As climate change disrupts the ecological balance, it will lead to changes in the pests’ spread, size, density and genetic composition. Further, insects are likely to expand their host plants for survival, says Pareek.

“For instance, cotton bollworm (Helicoverpa armigera) was earlier considered a minor pest for chilli, pearl millet and maize. But it has become a major threat for these plants over recent years,” he says.

P R Shashank, scientist at division of entomology, India Agricultural Research Institute (IARI), New Delhi, concurs with these findings, and adds that the change in the insects’ behaviour and life cycles also makes it difficult for their natural enemies to interact with them.

“Usually, insects have predators as a part of the food chain. However, the pests growing in large numbers and at a much earlier stage than their natural predators do not allow their population to be regulated by their natural enemies, resulting in such destructive scenarios,” he tells DTE.

At the same time, rise in temperatures directly affects volatilisation of pesticides and chemical fertilisers, reducing their impact on pests, says Mewalal Kewat, principal scientist, department of agronomy, college of agriculture, Jawaharlal Nehru Krishi Vishwavidyalaya, Jabalpur.

“The dosage requirement of these chemicals is expected to double. For instance, at present glyphosate dosage is recommended at 2.5 kg to control disease. But increase in volatility will increase it to 3-3.5 kg per dosage. Urea’s efficiency stands at 50 per cent and high temperatures will demand more dosages for increasing the dosages,” he says.

Kewat says that such exercises will make the land more toxic, degrade fertility, increase input costs for farmers and still deliver poor quality of yield.

The UN Food and Agriculture Organization (FAO) estimates that 15 plant pests have spread and may expand further owing to climate change. The increase in agricultural pests due to climate change directly attacks FAO’s target of achieving 50 per cent food production by 2050 to feed an estimated population of 9.1 billion.

As per a 2011 paper in Plant Pathology, global food production may drop significantly as plant pests and disease could deprive humans of 82 per cent of the potential yield in terms of cotton and 50 per cent for major crops.

Manzoor Shah, professor and botanist at the University of Kashmir, says many research papers and studies clearly suggest that rise in temperature due to climate change will lead to increase in pest outbreaks.

If such attacks increase, it will affect the critical food supply and overall yield. Citing an example of wheat blast, a disease outbreak in Bangladesh, India and Mexico, Shah says, “In 2019, the nations had to introduce a crop holiday, which means banning growing of the wheat crop, for three years. Such instances may increase and force us to change cropping patterns.”

“Pest attacks are a global threat and there has been a lot of brainstorming and global treaties and agreements to address the aspects. One of the topics being discussed widely is to adopt climate-resilient crops,” says Shah.

“The International Rice Research Institute is already distributing climate-resilient varieties throughout the world for testing them across universities. The aim is to check their productivity across different climatic conditions and see where they fare better or worse.”

Experts claim that effective pest management and use of organic and chemical methods hold a key in controlling pests, but suggest monitoring, awareness and evolving pest management methods is the need of the hour.

Subhash Chander, scientist at the Indian Council for Agricultural Research-National Research Centre for Integrated Pest Management (ICAR-NCIPM), says pest forewarning systems need to be placed.

This was first published in the 1-15 August, 2023 print edition of Down To Earth

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