The evolution of precision agriculture has been significantly accelerated by the integration of Unmanned Aerial Vehicles (UAVs), or agricultural drones, for crop protection. This technology offers unparalleled advantages in operational efficiency, the ability to treat difficult terrain, and reduced soil compaction compared to traditional ground-based sprayers. However, the shift from high-volume ground application to the low-volume, fine-droplet spectra typical of drone spraying presents unique physicochemical challenges. The core challenge lies in balancing efficacy with environmental stewardship: achieving sufficient pest control while minimizing off-target drift and evaporation losses inherent to small spray droplets. This is where the convergence of novel pesticide formulations and specialized spray additives becomes critical.
Nanoscale pesticides represent a frontier in formulation science. By engineering active ingredients into particles with dimensions often below 100 nm, these formulations offer enhanced solubility, stability, and biological activity due to their high surface-area-to-volume ratio. For agricultural drone operations, which often use concentrated tank mixes and ultra-low application volumes, the superior dispersion and reduced risk of nozzle clogging offered by nano-formulations are highly desirable. Yet, their very strength—small particle size—can be a liability during aerial application. The fine droplets necessary for good canopy coverage are highly susceptible to evaporation and wind drift, potentially reducing the amount of active ingredient that successfully deposits on the target foliage.
This is where spray adjuvants play a transformative role. Adjuvants are substances added to the spray tank to modify the properties of the spray solution or the target surface, aiming to enhance pesticide performance. In the context of applications involving an agricultural drone, adjuvants are not merely additives but essential components for success. Their functions can include reducing the dynamic surface tension to improve droplet spread and adhesion, increasing droplet viscosity or size to mitigate evaporation and drift, and enhancing rainfastness. The synergy between a nanoscale pesticide’s inherent bioavailability and an adjuvant’s ability to ensure its precise delivery and retention on target could unlock true precision in crop protection, enabling potential reductions in chemical input without compromising efficacy.
This investigation was therefore conceived to systematically evaluate this synergy. We focused on a nanoscale formulation targeting a pervasive pest in a major crop system: the cotton aphid (Aphis gossypii) in cotton fields. The central hypothesis was that by incorporating specific adjuvants into a reduced-rate nanoscale pesticide tank mix for an agricultural drone, we could fundamentally improve the spray solution’s physicochemical properties, thereby enhancing droplet deposition characteristics and ultimately achieving pest control efficacy comparable to, or exceeding, that of full-rate conventional applications.
Materials and Experimental Framework
Formulations and Additives: The core active ingredient was a composite nanoscale formulation containing dinotefuran and dinotefuran. A conventional pesticide blend, serving as a benchmark, was prepared by tank-mixing commercially available formulations of the same active ingredients. Three distinct adjuvants were selected based on their common use and differing chemistries: Youruopai (a blended alcohol-ester type), Jiexiaofeng (a vegetable oil-based adjuvant), and Aero-mate 320 (another vegetable oil-based adjuvant).
Experimental Design: The study was structured into two main phases: a comprehensive laboratory analysis of physicochemical properties and a controlled field efficacy trial. The core treatments for the agricultural drone application are summarized in Table 1.
| Treatment Code | Description | Formulation Rate | Adjuvant & Concentration (v/v) | Spray Volume (L/ha) |
|---|---|---|---|---|
| NF-Full | Nanoscale Pesticide – Full Rate | 100% of recommended (1200 g/ha) | None | 22.5 |
| NF-Red | Nanoscale Pesticide – Reduced Rate | 80% of recommended (960 g/ha) | None | |
| NF-Red + YRP | Reduced Rate + Adjuvant 1 | 80% of recommended | Youruopai @ 0.1% | |
| NF-Red + JXF | Reduced Rate + Adjuvant 2 | 80% of recommended | Jiexiaofeng @ 0.1% | |
| NF-Red + AM320 | Reduced Rate + Adjuvant 3 | 80% of recommended | Aero-mate 320 @ 0.6% | |
| Conv-Ground | Conventional Pesticide – Ground | Full AI equivalent | None (Standard Mix) | 450 |
Laboratory Physicochemical Characterization: All spray solutions were prepared with deionized water according to the field dosage. Key properties were measured:
1. Particle Size: Using dynamic light scattering (DLS) to confirm the nanoscale nature of the formulation.
2. Surface Tension: Both static (plate method) and dynamic (maximum bubble pressure method over 15-5000 ms) surface tension were quantified. The dynamic measurement is crucial for agricultural drone applications as it reflects the tension at the time of droplet formation and impact.
3. Contact Angle & Wettability: The contact angle of a 4 µL droplet on a cotton leaf surface was tracked over 340 seconds. The final spread area of a 3 µL droplet was also measured to assess practical wettability.
4. Evaporation Dynamics: The evaporation rate of a 5 µL droplet was monitored under controlled conditions (33°C, ~60% RH). The evaporation inhibition rate (R) was calculated as:
$$ R = \frac{V_0 – V_i}{V_0} \times 100\% $$
where $V_0$ is the volume change for the non-adjuvant solution and $V_i$ is the volume change for the adjuvant-containing solution.
Field Evaluation Protocol: The field trial was conducted in a cotton field. A commercially available multi-rotor agricultural drone equipped with standard hydraulic nozzles was used for all UAV treatments, flying at a height of 1.5 m above the canopy at 5 m/s. The conventional treatment was applied using a standard knapsack sprayer. Droplet deposition was assessed using water-sensitive papers placed in the upper and lower canopy. The median volumetric diameter (DV0.5) and droplet density (droplets/cm²) were analyzed from these cards. Aphid populations on predefined leaves were counted before application and at 1, 3, and 7 days after treatment (DAT). Control efficacy (EC) was calculated using Henderson-Tilton’s formula:
$$ EC = \left[ 1 – \frac{n_1 \times N_2}{n_2 \times N_1} \right] \times 100\% $$
where $n_1, n_2$ are pest counts in the control plot pre- and post-treatment, and $N_1, N_2$ are counts in the treated plot pre- and post-treatment.
Results: Modifying Physicochemical and Biological Performance
1. Confirmation of Nanoscale Properties and Adjuvant Impact on Solution Characteristics:
Dynamic light scattering analysis confirmed the fundamental difference between the formulations. The conventional blend formed aggregates with an average size of $348.78 \pm 5.32$ nm, while the nanoscale formulation had a mean particle diameter of only $10.90 \pm 0.04$ nm. This order-of-magnitude difference underscores the “nano” advantage for solubility and dispersion.
The addition of adjuvants to the reduced-rate nanoscale solution significantly altered its interfacial properties. As shown in Table 2, the static surface tension (SST) was lowered from 36.43 mN/m (NF-Red) to between 31.82 and 34.99 mN/m. More importantly, dynamic surface tension (DST) measurements revealed how quickly these adjuvants acted. At very short surface ages (relevant for droplet shattering from an agricultural drone nozzle), Youruopai facilitated the fastest reduction in DST, while Aero-mate 320 provided the lowest overall DST at equilibrium-like conditions.
| Treatment | Static Surface Tension (mN/m) | Final Contact Angle (°) | Wetting Area (mm²) | Evaporation Rate (µL/min) | Evaporation Inhibition Rate (%) |
|---|---|---|---|---|---|
| NF-Full | 34.77 ± 0.21 c | 15.12 ± 0.45 b | 13.88 ± 0.37 c | 0.31 ± 0.01 a | – |
| NF-Red | 36.43 ± 0.18 a | 19.28 ± 0.67 a | 14.03 ± 0.23 c | 0.36 ± 0.03 ab | – |
| NF-Red + YRP | 34.99 ± 0.25 bc | 12.25 ± 0.33 d | 17.13 ± 0.26 b | 0.09 ± 0.02 d | 73.40 |
| NF-Red + JXF | 33.15 ± 0.31 b | 14.85 ± 0.51 c | 18.74 ± 0.28 a | 0.21 ± 0.01 c | 40.74 |
| NF-Red + AM320 | 31.82 ± 0.29 c | 13.81 ± 0.42 cd | 17.18 ± 0.83 b | 0.26 ± 0.02 bc | 27.72 |
2. Enhancing Droplet-Target Interactions:
The improved surface activity directly translated to superior interactions with the cotton leaf surface. The contact angle for the NF-Red solution decreased over 340 seconds to 19.28°. All adjuvants accelerated and enhanced this wetting process, with NF-Red+YRP achieving the lowest final contact angle of 12.25°, a 36.5% improvement. Consequently, the practical wetting area increased significantly, by 22.1% to 33.6%, for adjuvant-containing solutions compared to NF-Red.
A critical finding for agricultural drone applications was the dramatic effect on evaporation. The adjuvants substantially suppressed the evaporation rate of suspended droplets. Youruopai was particularly effective, reducing the evaporation rate by 73.4%, followed by Jiexiaofeng (40.7%) and Aero-mate 320 (27.7%). This property is vital for ensuring droplets retain their volume and reach the canopy before volatilizing.
3. Field Deposition and Droplet Spectrum:
The field measurements confirmed the laboratory predictions. While droplet density (deposits/cm²) on the upper and lower canopy was not statistically altered by the adjuvants, a profound and critical change occurred in droplet size spectrum. As presented in Table 3, the addition of any adjuvant significantly increased the DV0.5 of droplets collected on water-sensitive papers.
| Treatment | Droplet Density (droplets/cm²) | DV0.5 (µm) | ||
|---|---|---|---|---|
| Upper Canopy | Lower Canopy | Upper Canopy | Lower Canopy | |
| NF-Red | 14.7 ± 1.2 a | 5.0 ± 0.8 a | 122.4 ± 3.5 b | 106.7 ± 4.1 c |
| NF-Red + YRP | 12.3 ± 1.0 a | 4.1 ± 0.6 a | 157.6 ± 4.8 a | 152.3 ± 5.2 a |
| NF-Red + JXF | 14.0 ± 1.3 a | 5.6 ± 0.9 a | 151.7 ± 4.1 a | 136.5 ± 4.7 b |
| NF-Red + AM320 | 13.8 ± 1.1 a | 4.5 ± 0.7 a | 153.5 ± 5.0 a | 157.2 ± 5.5 a |
For the NF-Red treatment, DV0.5 was 122.4 µm (upper) and 106.7 µm (lower). With adjuvants, these values increased to a range of 151.7–157.6 µm in the upper canopy and 136.5–157.2 µm in the lower canopy. This shift towards a larger droplet spectrum is a direct indicator of reduced drift potential and improved droplet settling velocity, key metrics for efficient agricultural drone spraying.
4. Biological Efficacy Against Cotton Aphid:
The culmination of these physicochemical improvements was reflected in the control of cotton aphids. At 1 DAT, all treatments showed moderate and statistically similar initial efficacy (~22-35%). By 3 DAT, a divergence emerged. The efficacy of the non-adjuvant, reduced-rate nanoscale treatment (NF-Red) remained low at 26.1%, while all adjuvant-containing treatments showed significantly higher control, reaching 48.7–51.5%, which was comparable to the conventional ground application (52.8%).
The most compelling results were observed at 7 DAT (Table 4). The NF-Red treatment’s efficacy plateaued at only 47.6%. In stark contrast, incorporating adjuvants into the reduced-rate nanoscale mix boosted efficacy to 63.2–73.3%. Critically, the performance of NF-Red+AM320 (73.3%) and NF-Red+JXF (65.0%) was statistically on par with both the full-rate nanoscale drone application (NF-Full, 71.8%) and the conventional, high-volume ground application (Conv-Ground, 74.9%).
| Treatment | Application Method | Control Efficacy (%) at 7 DAT |
|---|---|---|
| NF-Full | Agricultural Drone | 71.8 ± 2.1 a |
| NF-Red | Agricultural Drone | 47.6 ± 1.8 c |
| NF-Red + YRP | Agricultural Drone | 63.2 ± 2.4 b |
| NF-Red + JXF | Agricultural Drone | 65.0 ± 2.0 ab |
| NF-Red + AM320 | Agricultural Drone | 73.3 ± 1.9 a |
| Conv-Ground | Knapsack Sprayer | 74.9 ± 2.3 a |
Discussion and Mechanistic Insights
The results unequivocally demonstrate that the strategic use of adjuvants is a powerful tool to optimize the performance of nanoscale pesticides applied via agricultural drone. The 20% reduction in the active ingredient rate of the nanoscale formulation led to a predictable decrease in efficacy, likely due to a combination of factors including less favorable spreading and higher susceptibility of the fine droplets to off-target losses. The adjuvants counteracted these deficiencies through a multi-mechanism approach.
First, by lowering both dynamic and static surface tension, the adjuvants improved the immediate wetting and spreading of the droplet upon impact with the waxy cotton leaf surface. The reduction in contact angle and increase in wetting area, as quantified in our laboratory tests, ensure a larger and more intimate interface between the pesticide solution and the pest’s habitat, potentially enhancing uptake. The relationship between surface tension ($\gamma$), contact angle ($\theta$), and spreading can be conceptualized through the work of adhesion ($W_a$):
$$ W_a = \gamma_{lv} (1 + \cos\theta) $$
where $\gamma_{lv}$ is the liquid-vapor surface tension. A lower $\gamma_{lv}$ and $\theta$ increases $W_a$, promoting better adhesion and spread.
Second, and paramount for agricultural drone applications, was the modification of the droplet spectrum and the suppression of evaporation. The significant increase in DV0.5 measured in the field is a direct consequence of adjuvant chemistry, likely through effects on viscosity and sheet disintegration during atomization. This shift moves the droplet spectrum away from the drift-prone size range (<150 µm), thereby conserving more of the applied volume within the target zone. Concurrently, the dramatic reduction in evaporation rate, particularly with the alcohol-ester adjuvant (Youruopai), ensures that droplets retain their mass and active ingredient concentration during their descent. The evaporation of a droplet can be modeled simplistically as a function of its surface area and ambient conditions. By forming a film or altering vapor pressure at the droplet surface, adjuvants effectively reduce the evaporation constant ($k$) in the equation:
$$ \frac{dV}{dt} = -k \cdot A \cdot \Delta P $$
where $dV/dt$ is the volumetric evaporation rate, $A$ is surface area, and $\Delta P$ is the vapor pressure gradient.
The synergy of these physical modifications—better deposition, less drift, reduced evaporation—directly translated to sustained biological efficacy. The adjuvant-enabled, reduced-rate nanoscale treatments closed the performance gap with the full-rate treatments. This demonstrates that the role of an adjuvant in an agricultural drone spray regime is not merely additive but multiplicative, unlocking the full potential of advanced pesticide formulations. It effectively decouples efficacy from sheer chemical load, aligning with the principles of sustainable intensification.
It is noteworthy that the three adjuvants, despite differing in composition and optimal concentration, all provided significant benefits, though with varying strengths. The vegetable oil-based adjuvants (JXF, AM320) excelled in enhancing wetting area and providing robust efficacy, while the blended adjuvant (YRP) was exceptional in suppressing evaporation. This indicates that adjuvant selection can be tailored based on the primary environmental challenge (e.g., high temperature vs. difficult-to-wet foliage) expected during the agricultural drone operation.
Conclusion
This study provides a comprehensive, evidence-based framework for integrating adjuvant technology with nanoscale pesticide formulations for use in agricultural drone systems. We have quantitatively shown that adjuvants are critical for mitigating the inherent challenges of low-volume aerial application. They transform the spray solution by:
1. Enhancing surface activity to promote droplet spread and adhesion on target leaves.
2. Increasing droplet size to minimize wind drift, a major concern for agricultural drone operators.
3. Significantly inhibiting droplet evaporation, ensuring delivery of the intended dose to the canopy.
The collective outcome of these physicochemical improvements is the restoration of high pest control efficacy even when the application rate of the advanced nanoscale pesticide is reduced by 20%. This synergy makes a compelling case for the adoption of “tank-mix optimization” as a standard protocol in agricultural drone pest management programs. By doing so, we can leverage the efficiency of drones and the potency of nanotechnology to achieve precise, effective, and reduced-input crop protection, marking a significant step forward in sustainable aerial agronomy.