To answer this question, many people would tell you that using pesticides is unnatural and therefore the spirit of the leaf gets corrupted. But I’m an agricultural engineer by training and I'll give you arguments founded in science.
The alternative to using pesticides is to rely on garden design to limit the damage caused by pests. A common technique in Yunnan is to plant shade trees that will attract spiders. The spiders will weave webs all around the trees and prey on the things that want to eat your leaves. It won’t work 100%, some species will find their way around the nets, but they might attract other natural predators, such as praying mantises or assassin bugs. If you plant shade trees, you’ll get a thriving ecosystem in your gardens, eventually, some birds will come and prey on the larger insects.
But if you spray insecticides on such a garden, you’ll kill all the bugs, including the natural predators and your now clean garden will be fertile ground for further incursions of pests, once the protective effect of the pesticide has subsided. If you want to keep protecting your garden, you’ll have to spray pesticides again.
Either you choose to work with biodiversity, relying on natural predators for a partial protection, or you get rid of the insect biodiversity entirely. The second option will allow you better protection, you’ll have beautiful leaves, with no signs of damage at all, but it will require extra work, you’ll need to pay for the pesticides and it will generate chemical pollution in and around your garden, affecting both nature and the workers who spray.
As a result, spraying pesticides is only viable if you’re aiming for a maximum yield, if the look of the leaves needs to be perfect (something more important in green tea production, and generally in cheaper teas). You can now understand why quality tea can’t be grown with the use of pesticides. If you’re aiming at high quality, you don’t care that much about reaching the maximum yield. If your tea grows slowly, they will have higher amounts of polyphenols and that will provide extra protection against the pest and disease.
Transitioning from a pesticide management model to a natural tea garden is a challenge. In the first years without pesticides, the tea trees are very vulnerable to pest attacks and you’ll suffer heavy losses. Interestingly, the tea trees become more resistant as time goes. This could be due to the fact that stopping spraying pesticides also means changing the fertilization plan. If the tea is less fertilized it will grow more slowly and the leaves will have more time to build up chemical defenses, making them less vulnerable to pest attacks.
In Yunnan, quality tea is generally grown at high altitude, the colder weather and dryer air provides more resilience against fungi and bacteria. If you don’t spray fungicide, you’ll see all sorts of spots on the leaves at some times in the year, especially during the rainy season. Those might affect the yield in summer and autumn tea. You can mitigate the damage by ensuring the garden is well drained, this is the case in sandy soils, and you find this type of soil on most of the higher slopes in the mountains.
Too many shade trees in the garden can keep the moisture high and facilitate the propagation of disease, therefore there is a balance to strike between pest protection and disease protection when establishing shade trees. You want to attract natural predators, but also keep the garden dry enough.
You’ll find most high quality tea produced at high altitude on poor soils. Reaching the maximum yield is not a concern, you want to sell your tea at a high price because you’re aiming at the tea enthusiast market and can accept some losses due to pest and disease, especially since most of their income comes from the spring harvest, which is not too affected by pest and disease due to the favorable weather.
The plantations located at low altitude and growing on rich clay soils will have a harder time producing good tea. They are more likely to sell to the mass market at a base price. For such gardens, the use of pesticides makes more sense because they need to produce as much as they can, their buyers care about the leaf aspect, the summer and autumn harvests are a large part of their income, and their customers are not concerned about pesticide levels or damage to the environment.
Certified organic large tea estates are in an uncomfortable middle ground. They cater to the mass market and cannot ask for a large premium on organic tea. They need to have a high yield and harvest almost all year round. Very often, they use a similar approach to the conventional management, they just have to buy more expensive certified-organic pesticides, which generate less chemical pollution but affect the garden in the same way. They can also use UV lights and sticky papers to reduce the amount of insects in the gardens, workers can pluck the cocoons of certain pests, increased monitoring can allow for a more rapid response to an invasion. But these methods mean they have higher costs than conventional plantations, more workers are needed to attend the gardens and these plantations have a harder time generating a profit.
As a conclusion, the management method used depends on the environment you’re in, high altitude makes it easier to not use pesticides. Eventually, it comes down to the willingness of the consumer to pay. The tea enthusiast world can enjoy teas grown without pesticides because they can pay ten times the mass market price for leaves from low-yield gardens. A lot of the tea nowadays is not sold to the consumer as leaves, think of takeaway bubble tea, bottled ice tea or tea served at a restaurant. The mass market industry will always look at ways to reduce the costs and if we want to protect the environment on a large scale, we must rely on government regulations to set a minimum standard for the industry. If you live in a democracy, your vote matters.