Technical Abstract: Sensors functioning reliably under harsh field conditions are needed for the development of variable-rate sprayers to apply pest control agents for tree liners in ornamental nurseries. Two ultrasonic sensors were tested to determine how their durability and detection stability would be influenced by the changes in temperature, wind, dust, travel speed and spray cloud. One of the sensors did not perform satisfactorily. It had a 0.1% change in root mean square (RMS) error of the detecting distance before and after exposing to the outdoor cold weather conditions. Mean RMS error was 8 % under the dusty condition, 1.5–1.8 % under the windy condition, and 12.3 % to 23 % for the travel speed ranging from 0.8 to 3.0 m/s. It also showed that increasing ambient temperature from 16.7 to 41.6 °C reduced the detection distance by 4.0 %. Detecting through spray cloud caused the RMS error to vary from 1.2 to 61.4 %, the largest measurement error among the six variables tested. To reduce the RMS errors, optimal configurations between the sensor and spray nozzles were identified. In addition, strategies of synchronized multiple sensors were tested for improving the measurement stability and accuracy of the sensor while detecting targets.