Pests in orchards are mainly controlled through the use of chemical pesticides, which decrease fruit loss by 66% to 90% ^[1][2][3][4][1,2,3,4]. Orchard air-assisted spraying technology is recommended as highly effective by the Food and Agriculture Organization (FAO) of the United Nations, and this method has been widely used for orchard pest control. Traditional orchard air-assisted spraying methods involve spraying a pesticide solution in a continuous and uniform manner. This not only requires a large amount of pesticide, but also causes environmental pollution due to the drift of excess spray into the air and onto the ground ^[5][6][7][8][5,6,7,8]. To address this problem, orchard air-assisted target-oriented spraying systems with various sensors have been developed, thus enabling variable-rate spraying based on information such as tree location, canopy profile and leaf density, and significantly reducing the amount of pesticides used ^[9][10][11][9,10,11].

Currently, orchard air-assisted target-oriented spraying systems mainly use photoelectric, ultrasonic and light detection and ranging (LiDAR) sensors. Among them, photoelectric sensors are the easiest to use in terms of applying target-oriented pesticide spraying in orchards. They allow control of the nozzle opening by detecting the position of the tree trunk or tree canopy to achieve target-oriented spraying ^{[12][13][14][15]}[12,13,14,15]. Based on the photoelectric target-oriented detection technique, He et al. [16] developed a precise target-oriented spraying control system for orchards. This system has photoelectric sensors mounted at levels matching the upper, middle and lower portions of canopies to detect trees at different heights, and this system can decrease pesticide use by 50%–75%. Zhai et al. [17] located the canopy by detecting the tree trunk and used this information to design a photoelectric-based target-oriented controller for young trees. Zou et al. [18] used photoelectric sensors to detect fruit canopy positions in real time, and target-oriented spraying was realised by controlling the opening time of the nozzle according to the position of the fruit tree canopy. Because target-oriented spraying with a photoelectric sensor can only selectively spray based on the presence or absence of a tree and cannot perform variable-rate spraying based on information such as canopy profile and denseness, a target-oriented spraying system equipped with an ultrasonic sensor has been developed ^{[11][19][20][21]}[11,19,20,21]. Maghsoudi et al. [22] acquired the volume of fruit tree canopies in real time through ultrasonic sensors and achieved variable-rate spraying according to the change in canopy volume, cutting pesticide use by 34.5% on average to achieve spraying effectiveness similar to that of conventional spraying methods. Gail et al. [23] developed an orchard sprayer equipped with ultrasonic sensors capable of variable-rate spraying according to changes in the volume of fruit tree canopies, and this sprayer reduced pesticide usage by 21.9%. Petrović et al. [24] examined the effects of a conventional spraying system and a target-oriented spraying system equipped with ultrasonic sensors on spray deposition and drift on a pear tree canopy. They found that in comparison to the conventional spraying system, the target-oriented system reduced ground drift by 48.74% and air drift by 59.16%. Compared with photoelectric and ultrasonic sensors, LiDAR sensors can obtain more information about the characteristics of fruit trees, but LiDAR point cloud data processing requires high controller performance. Osterman et al. [25] designed a target-oriented spraying system equipped with a LiDAR sensor that enables real-time sensing of the canopy shape at different tree heights and achieves form spraying by controlling the angles and positions of the upper, middle and lower spraying nozzle arms. Li et al. [26] designed a target-oriented spraying system equipped with a LiDAR sensor capable of variable-rate spraying according to the change in the volume of fruit tree canopies. Compared to traditional air-assisted sprayers and directional air-assisted sprayers, this sprayer decreased droplet drift by 23.2% and 42.7%, respectively, and ground loss by 67.4% and 58.8%, respectively. Zhu et al. [27] developed a laser-guided variable-rate sprayer for managing insects in ornamental nurseries, and field tests in the three studied nurseries with three different insect pests showed that the sprayer reduced spray volume rates by 25% to 80%.

Currently, compared with LIDAR sensors, photoelectric and ultrasonic sensors are low in cost and more mature in their application in orchard target-oriented spraying systems. However, different types of sensors detect the characteristics of fruit tree canopies in different ways, which affects spray deposition and drift on the canopy and ground. The objectives of the research were to evaluate a developed photoelectric-based target-oriented spraying system and a developed ultrasonic-based target-oriented spraying system and to compare pesticide off-target deposition and drift from traditional spraying with those from the two target-oriented spraying systems. Meanwhile, the target-oriented detection accuracy of the sensor-based detection methods for different target detection positions is affected by fruit tree types and canopy structure. The photoelectric-based target-oriented spraying system enables target-oriented spraying by locating the canopy position through the detection of tree trunks. Further, the ultrasonic-based target-oriented spraying system enables target-oriented spraying by directly detecting the canopy position. Young cherry trees and adult apple trees were selected to verify the influence of the target detection position (trunk-based detection and canopy-based detection) on the target-oriented detection effect for different fruit tree types and canopy structures.

Two target-oriented spraying systems, one based on photoelectric sensors and one based on ultrasonic sensors were developed in this study. Three spraying treatments were applied to young cherry trees and adult apple trees and revealed that the two target-oriented spraying systems can greatly reduce the ground deposition caused by off-target spraying. Compared with that from the commercial sprayer without any target-oriented spraying system, the ground deposition from the sprayers with the photoelectric-based target-oriented spraying system (trunk-based target-oriented detection) and the ultrasonic-based target-oriented spraying system (canopy-based target-oriented detection) was decreased by 50.63% and 38.74%, respectively, for the young fruit trees and by 21.66% and 29.87%, respectively, for the adult fruit trees. The trunk-based target-oriented detection method can be considered more suitable for young trees, whereas the canopy-based target-oriented detection method can be considered more suitable for adult trees. Along the spray pathway, as the distance from the sprayer nozzle increased, the ground deposition first increased and then decreased, peaking 1.5 m from the tree trunk. The highest deposition occurred at the back of the canopy and was caused by the high airflow at the air outlet of the sprayer. To reduce ground deposition and improve pesticide deposition in fruit tree canopies, it is necessary to control the sprayer to provide a suitable air speed and air volume.