India's Tea Research Association puts the country's annual crop loss to insect pests at 147 million kilograms of made tea, worth roughly 28.65 billion rupees (about $318 million) a year, according to the association's secretary, Joydeep Phukan. That figure predates the current run of warm years. It has only grown more relevant since. Researchers at India's and Bangladesh's national tea institutes now describe a specific mechanism: longer summers, delayed monsoons, and shorter winters are giving several of tea's worst pests more months a year in which to breed, and are letting some of them establish in places too cool for them a decade ago.

The mechanism is a longer breeding season, not a new pest

Tea has never lacked for insects that eat it. A 2009 review in the Annual Review of Entomology, drawing on decades of prior research, counted 1,031 species of arthropods associated with commercially grown tea worldwide and put the potential yield loss, if none of them were controlled at all, at 11 to 55 percent. Most of that pressure is kept in check by cold winters, dry spells, and the insects' own natural predators. What has changed, according to entomologists at the two institutes closest to the problem, is how much of the year those checks actually hold.

"Due to rising temperatures, pest infestation is increasing as well as pest status is changing," Mohammad Shameem Al Mamun, a principal scientific officer in the entomology division of the Bangladesh Tea Research Institute, told Mongabay in December 2025. Joydeep Phukan, of India's Tea Research Association, described the same shift on the Indian side of the border: "Given the temperature rise, pest infestations are increasing." In Sylhet, Bangladesh's main tea district, the pattern is not simple heat. Average daytime highs actually fell slightly between 2011 and 2024, from about 33.9 to 31.2 degrees Celsius (roughly 93 to 88 degrees Fahrenheit), according to the institute's own weather records, cited by Mongabay. But average night-time lows rose over the same span, from about 16 to 20.7 degrees Celsius (61 to 69 degrees Fahrenheit). A narrower gap between day and night, with fewer genuinely cold nights, is exactly the change that lets an insect population overwinter instead of dying back.

Where it is showing up on the ground

The damage is not evenly spread, and the regional pattern tracks the mechanism. In West Bengal's Dooars valley, home to hundreds of gardens on the Bhutan border, yields have fallen 10 to 25 percent in affected blocks such as Madarihat-Birpara, Kalchini, and Kumargram, Mongabay reported. Tamil Nadu's Valparai hills, in South India, saw output nearly halve, from about 30 million kilograms in 2009-10 to 16.7 million kilograms in 2021-22, a decline the United Planters' Association of Southern India attributes in part to the same pest and disease pressure. National production tells a smaller version of the same story: India's processed tea output fell from about 1.4 billion kilograms in 2023 to 1.3 billion kilograms in 2024, and Bangladesh's fell from 102.9 million to 93 million kilograms over the same period, though both figures reflect weather and pests together, not pests alone.

The specific insects doing the damage have also shifted. Bangladeshi entomologists now list red spider mites and thrips as major pests, both minor a decade ago, alongside a resurgence of the looper caterpillar and the spread of tea mosquito bugs, red coffee borers, and red slug caterpillars into estates that had not previously reported them. India's experience runs on the same short list: tea mosquito bugs and looper caterpillars are the two pests the Tea Research Association names as the costliest, alongside thrips and termites, across a belt that now runs from the Dooars and Assam's south bank into Cachar, Tripura, Arunachal Pradesh, and Darjeeling, a wider footprint than the same pests held a decade earlier.

What growers actually spend fighting it

The cost is not only the lost crop. Plant protection spending in the tea belts of northern West Bengal and India's northeast has climbed to 25,000 to 30,000 rupees per hectare (roughly $300 to $360, on a hectare of about 2.5 acres), the Tea Research Association told Krishi Jagran. That is a sharp rise over the past two decades. Growers' options for that money are also narrower than the figure suggests. India's Central Insecticides Board and Registration Committee has approved only seven pesticide products for use on tea. The short list is set largely by the residue limits export markets, especially the European Union, will accept on a food crop. A grower facing a new or resurgent pest cannot simply reach for a stronger chemical. The approved list does not grow as fast as the pest pressure does.

One episode put a rough number on what happens when spraying stops altogether. During India's pandemic lockdown, when normal field operations paused, major tea companies reported crop losses of 20 to 25 percent to pest infestation in the gap, according to Somnath Roy, an entomologist at the Tocklai Tea Research Institute in Jorhat, Assam, speaking to the Hindi-language outlet Gaon Connection. That is a single, disrupted season rather than a controlled trial, but it is a rare real-world data point on the gap between managed and unmanaged pest pressure on the same bushes.

The one producer with a different story

Sri Lanka's account, reported alongside Bangladesh's and India's in the same Mongabay piece, runs in a different direction. Roshan Rajadurai, of the Planters' Association of Ceylon and managing director of Kelani Valley Plantations, described his country's tea belt as facing short, heavy rain events rather than sustained heat. He said Sri Lankan growers deal with comparatively few pests as a result. Integrated pest management and soil and water conservation are still standard practice there, he said, with approved pesticides reserved for the specific pockets of high humidity and rainfall where pests do turn up. "That is why Sri Lanka is recognized as the source of the world's cleanest tea, with the lowest pesticide residues," Rajadurai said. The current reporting does not settle whether that is a durable structural advantage, or simply a producer whose particular climate stress has not yet arrived in insect form. It is one data point, from one industry figure, in a comparison that so far has only three countries in it.

The response is bugs against bugs, not more spray

With the approved chemical list fixed and export buyers tightening residue limits rather than loosening them, India's response has leaned toward biological control instead. Researchers at the Tocklai Tea Research Institute, under director Azariah Babu, have been rearing an assassin bug, Sycanus collaris, a reduviid predator, and releasing it into affected gardens to prey on looper caterpillars and tea mosquito bugs. "A single female can consume up to eight looper larvae daily," Roy said of the predator insect. The institute's parallel goal is more basic: restoring the soil and canopy conditions that let birds and other natural predators return to a garden on their own, since a chemically simplified garden has fewer of them to begin with. Separate academic modelling adds a disease dimension to the same warming trend: a 2021 study in Frontiers in Cellular and Infection Microbiology projected that as tea's climatically suitable growing area shifts over the coming decades, three major fungal pathogens (Colletotrichum acutatum, C. camelliae, and Exobasidium vexans) will still overlap with 10 to 44 percent of the newly suitable land, so a garden that migrates to escape one kind of climate stress does not fully escape the disease side of it.

The bottom line

None of this is a forecast this publication is making about future losses; it is a synthesis of what two national tea research institutes have already measured and said on the record. The figures line up in one direction: a longer window of mild, humid weather is letting tea's oldest pests do more of what they have always done, the crop loss and the pesticide bill are both rising in the two countries with the data to show it, and the chemical toolkit available to fight back is not expanding to match, which is why India's own research institute is now betting on a predator insect rather than a new spray. For what a warming climate does to yield and growing area more broadly, see How Much Does Climate Change Actually Cost the Tea Industry?; for what a single bad season does to the price at auction, see When the Rain Fails, What Actually Happens to the Tea Price?.