December 6, 2022

1. Pain Points in the Aerial Protection of Lychee Trees

Lychee, along with longan, bananas and pineapples, are known as the “Four Great Fruits of Southern China”. In 2020, lychee was grown on approximately 540,000 hectares of land in China, distributed mainly in provinces such as Guangdong, Fujian, Guangxi and Hainan. Guangdong alone has about 273,000 hectares of lychee orchards, which accounts for over 50% of the total cultivation area in the country.

In lychee orchards, crop protection operations are typically carried out 14-16 times on average a year (the number of spraying rounds vary depending on the region, crop variety, and management method, from 6-8 times to 20 times a year). In cases where crops need to be sprayed frequently, the combined factors of limited time windows and labor shortage drive up the operational cost.

Agricultural drones, the Phantom 4 RTK, the Phantom 4 Multispectral and other advanced tools and technology are constantly being innovated and evolving to reshape people’s production methods and lifestyles. The replacement of labor with machines addresses the issues of labor shortage and high production costs faced by fruit farmers.

Firstly, lychee (or longan) trees typically live longer than citrus and mango trees (those in some lychee orchards in Maoming are 20 to 30 years old). The size of lychee trees and their canopy diameter (usually six to eight meters tall, and canopy diameter four to six meters) are both larger than those of citrus and mango trees. Lychee plants enjoy sunlight with most of their leaves growing in the outer layers while the foliage closer to the center is sparse. In comparison, the leaves of citrus and mango trees are denser in their canopy. This presents a great challenge to agricultural drones, whereby the pilot must ensure the droplets can penetrate the plants while not harming the flowers and fruits with the strong draft from the aircraft. As a result, we had to conduct a large number of spraying tests within the growth cycle of lychee plants, to confirm the operating parameters suited for lychee spraying.

Lychee trees

Canopy and inner space density of a lychee tree

Secondly, most of the lychees in Guangdong are planted on hill slopes. The varying heights of the trees, altitude difference of the ground, and intricate power lines in these locations are particularly challenging for terrain-follow flights and obstacle avoidance by drones.

For the T30 agricultural drone, you would first need to generate a 3D map of the orchard and plan a 3D flight path using the Phantom 4 RTK. For the T40 or T20P, if the terrain’s incline is less than 45 degrees, the area can be mapped and a 3D route generated with the agricultural drone directly, which reduces the difficulty and cost of the operation.

In the lychee orchard

The lychee orchard with its steep incline

2. Aerial Protection Solution for Lychee Trees

To tackle challenges of aerial fruit tree spraying, DJI Agriculture has been innovating its technology constantly to offer optimal solutions. For large orchards with steep slopes and complex terrain, the Phantom 4 RTK is used to survey the area, with a 3D map and flight routes then generated with DJI Terra or DJI Agras Intelligent Cloud, and then imported into the agricultural drone for it to carry out the spraying operation. For smaller orchards with flat terrain, the T40 or T20P can be used to map the area and plan a 3D flight operation directly.

DJI Agras Intelligent Cloud

The Fruit Tree Mode has two spraying and four planning modes. The two spraying modes are Continuous Spraying and Spot Spraying. The four planning modes are Distance Interval (Auto), Tree Crown Center (Auto), Semi-automated, and Manual.

Multiple Spraying Methods

[Continuous spraying]: The drone sprays evenly on the fruit trees throughout the flight. This method has highly efficient and suited for orchards with evenly spaced trees.

[Spot spraying]: The drone only sprays while hovering or circling over the top of each tree. This method is less efficient and suited for orchards with scattered trees or where only tree crown centers need to be sprayed.

The Two Spraying Modes

Multiple Route-planning Methods:

[Distance Interval (Auto)]: Adds field and boundary points, and automatically generates a zig-zag route with the same line spacing, similar to flight paths for field operations.

[Tree Crown Center (Auto)]: Adds field and boundary points, and automatically generates a route including all the tree crown centers.

[Semi-automated]: Adds waypoints along the fruit trees, which automatically connect to form a route. The route is automatically adjusted to link to the surrounding tree crown centers, generating a final operation route.

[Manual]: Adds waypoints along the fruit trees, which automatically connect to form an operation route.

The Four Planning Modes  (From left to right, Auto Distance Interval, Auto Tree Crown Center, Semi-automated, Manual)

For better results and efficiency, the “Continuous Spraying – Distance Interval (Auto)” and “Continuous Spraying – Semi-automated” modes are typically used, depending on the terrain of the orchard. Due to the tall heights and dense canopies of lychee trees, the height and row spacing applied in operations would be slightly smaller than those for citrus and mango trees.

3. Setting Operating Parameters

The operating parameters for fruit trees are determined based on many factors, such as their growth momentum (e.g. age, height, canopy diameter, etc.), growth phase (e.g. flowering, fruiting, shoot controlling or other phases), the growth phase of the targeted pests or pathogens (e.g. eggs, larvae, or adult worms), pesticide types, and weather. By taking into account these conditions together with the drone’s performance, you could then decide the best parameters to operate under. Lychees are susceptible to fewer pests and diseases. They include Litchi stem borers, eriophyid mites, and geometer moths, as well as fungi such as Litchi downy blight and Litchi anthracnose.

The following parameters are based on operations carried out on lychee orchards in Maoming. They may vary from location to location and are thus for reference only.

Note that the more optimal parameters had been tested and verified continuously during the growth cycle of lychee trees. As it stands, the parameter categories for which we have operated the most are lychee trees around four to six meters tall and canopy diameter around four to five meters long. For specific pesticides, the above parameters deliver comparable results to those of manual spraying. The other parameters had been tested with smaller sample sizes and therefore should be set with more care and discretion before they are applied.

Jenny