Author：Chen Ying-Cheng/Associate Research Fellow, Tainan District Agricultural Research and Extension Station

For most lotus farmers nowadays, the timing of pesticide applications against Scirtothrips dorsalis Hood, commonly known as chilli thrips, is based on the growth stages of lotus plant. Previous studies have shown that thrips begin to appear during the floating leaf stage and gradually increase in population density, eventually causing severe damage during the flowering stage. However, once the lotus plants rise above the water surface—often taller than the average height of a person— manual pesticide application becomes extremely difficult. The dense and humid environment of the paddy fields, with the difficulty of even walking through muddy terrain, significantly increases the physical burden on farmers and the risk of inhaling pesticide droplets. As a result, farmers are less inclined to apply pesticides, making effective chilli thrips control even more challenging.

To date, no domestic studies have investigated the use of unmanned aerial vehicles (UAVs) for the control of chilli thrips in lotus fields. Most existing control strategies have relied on pressurized hose sprayers or backpack sprayers to assess efficacy. However, since chilli thrips primarily inhabits the underside of lotus leaves, effective control requires either direct contact with the leaf underside or the systemic action from the pesticide.

Factors such as droplet size and coverage density can significantly affect pesticide efficacy. Research suggests that fine droplets can greatly improve control results. Droplets sized 10–50 μm are ideal for controlling flying insects, while those between 30–150 μm are more suitable for controlling crawling larvae on leaf surfaces. The TR8001 nozzles used in this study’s UAV setup produce droplets ranging from 100–136 μm, making them suitable for targeting chilli thrips larvae.

With the rapid development of UAVs, their application in pest control has become increasingly common—covering insects on crops such as rice, Chinese cabbage, and tea (specifically the tea green leafhopper). Farmers have gradually accepted UAVs as a viable spraying tool. However, as chilli thrips primarily lays eggs and remains active on the underside of leaves during its early stages, this study explores whether UAVs can effectively deliver pesticide droplets to the underside of leaves via airflow, thereby enabling contact with the chilli thrips to achieve successful control. The performance of UAV spraying was compared with that of traditional manual spraying, aiming to establish a UAV-based control strategy for chilli thrips in lotus cultivation.

The field test began in the sixth week after the leaf stage. The test field was divided into four areas: A, B, C, and CK (Figure 1). CK served as an untreated area. The other areas received treatment once a week for two consecutive weeks starting from the sixth week. The control treatments in this study were initiated during the sixth week after the leaf stage. Areas A and B were treated using an unmanned aerial vehicle (UAV; Droxo DX-10) equipped with eight hollow cone nozzles (TR8001). The UAV operated at a height of 3 meters above the lotus canopy, flying at a speed of 3 meters per second in a S-shaped pattern over the treatment zones. In contrast, Area C was treated manually using a high-pressure motorized sprayer with a single nozzle and hose. The operator walked along the field embankments surrounding the treatment area, swinging the nozzle side to side, while spraying into the plot against the wind direction. The first pesticide application used 20% Formetanate soluble powder. The UAV-treated Areas A and B showed better control efficacy than the manually treated Area C, with significantly lower thrips densities of 75.1 and 201.4 individuals, respectively, corresponding to control rates of 77.1% and 78.2%. In contrast, the chilli thrips density in Area C increased to 827.3 individuals, with a control rate of only 9.8%. After the second application, the chilli thrips densities decreased across all plots. However, Areas A and B still maintained significantly lower chilli thrips densities than Area C, with 20.3 and 44.3 individuals, and control rates of 74.5% and 80.8%, respectively. The chilli thrips density in Area C declined to 164.0 individuals, with a control rate of only 26.3%. By the fourth week after the two treatment cycles, the chilli thrips densities in Areas A and B remained below an average of 50 individuals, whereas Area C still recorded a density of 143.7 individuals. These results indicate that applying pesticide using UAVs during the 4th to 6th week after the leaf stage provides more effective and sustained control of chilli thrips compared to manual spraying.

To provide reference of pesticide dosage in future pesticide application using UAVs, the minimum recommended application rates per hectare for 20% Formetanate and 10% Chlorfenapyr, as announced by the government, were used to calculate the effective spray concentrations based on a 10-liter UAV tank. The recommended application rate for 20% Formetanate is 3 – 4.5 kg per hectare, while that for 10% Chlorfenapyr is 0.6 – 2.5 L per hectare. When applying pesticides with a UAV in lotus fields for the control of chilli thrips, the estimated water volume used per hectare is approximately 80 liters. Accordingly, the minimum recommended spray concentrations per hectare were calculated as follows:
• For 20% Formetanate: (20% × 106) / (80 / 3) = 7,499.1 ppm
• For 10% Chlorfenapyr: (10% × 106) / (80 / 0.6) = 750.0 ppm

In this study, the lethal concentration study revealed that the LC50 and LC90 values for Formetanate were 223.3 ppm and 1,335.1 ppm, respectively, and for Chlorfenapyr, 63.7 ppm and 143.6 ppm, respectively. These values are significantly lower than the minimum recommended application concentrations per hectare. These findings provide a useful reference for optimizing pesticide concentrations in UAV-based applications and support the goal of reducing pesticide usage in the field.